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SCons 4.7.0

SCons 4.7.0

User Guide

The SCons Development Team

Version 4.7.0

Released: Mon, 19 Nov 2023 17:52:53 -0700


Table of Contents

Preface
1. SCons Principles
2. How to Use this Guide
3. A Caveat About This Guide's Completeness
4. Acknowledgements
5. Contact
1. Building and Installing SCons
1.1. Installing Python
1.2. Installing SCons
1.3. Using SCons Without Installing
1.4. Running Multiple Versions of SCons Side-by-Side
2. Simple Builds
2.1. Building Simple C / C++ Programs
2.2. Building Object Files
2.3. Simple Java Builds
2.4. Cleaning Up After a Build
2.5. The SConstruct File
2.5.1. SConstruct Files Are Python Scripts
2.5.2. SCons Builders Are Order-Independent
2.6. Making the SCons Output Less Verbose
3. Less Simple Things to Do With Builds
3.1. Specifying the Name of the Target (Output) File
3.2. Compiling Multiple Source Files
3.3. Making a list of files with Glob
3.4. Specifying Single Files Vs. Lists of Files
3.5. Making Lists of Files Easier to Read
3.6. Keyword Arguments
3.7. Compiling Multiple Programs
3.8. Sharing Source Files Between Multiple Programs
4. Building and Linking with Libraries
4.1. Building Libraries
4.1.1. Building Libraries From Source Code or Object Files
4.1.2. Building Static Libraries Explicitly: the StaticLibrary Builder
4.1.3. Building Shared (DLL) Libraries: the SharedLibrary Builder
4.2. Linking with Libraries
4.3. Finding Libraries: the $LIBPATH Construction Variable
5. Node Objects
5.1. Builder Methods Return Lists of Target Nodes
5.2. Explicitly Creating File and Directory Nodes
5.3. Printing Node File Names
5.4. Using a Node's File Name as a String
5.5. GetBuildPath: Getting the Path From a Node or String
6. Dependencies
6.1. Deciding When an Input File Has Changed: the Decider Function
6.1.1. Using Content Signatures to Decide if a File Has Changed
6.1.2. Using Time Stamps to Decide If a File Has Changed
6.1.3. Deciding If a File Has Changed Using Both MD Signatures and Time Stamps
6.1.4. Extending SCons: Writing Your Own Custom Decider Function
6.1.5. Mixing Different Ways of Deciding If a File Has Changed
6.2. Implicit Dependencies: The $CPPPATH Construction Variable
6.3. Caching Implicit Dependencies
6.3.1. The --implicit-deps-changed Option
6.3.2. The --implicit-deps-unchanged Option
6.4. Explicit Dependencies: the Depends Function
6.5. Dependencies From External Files: the ParseDepends Function
6.6. Ignoring Dependencies: the Ignore Function
6.7. Order-Only Dependencies: the Requires Function
6.8. The AlwaysBuild Function
7. Environments
7.1. Using Values From the External Environment
7.2. Construction Environments
7.2.1. Creating a Construction Environment: the Environment Function
7.2.2. Fetching Values From a Construction Environment
7.2.3. Expanding Values From a Construction Environment: the subst Method
7.2.4. Handling Problems With Value Expansion
7.2.5. Controlling the Default Construction Environment: the DefaultEnvironment Function
7.2.6. Multiple Construction Environments
7.2.7. Making Copies of Construction Environments: the Clone Method
7.2.8. Replacing Values: the Replace Method
7.2.9. Setting Values Only If They're Not Already Defined: the SetDefault Method
7.2.10. Appending to the End of Values: the Append Method
7.2.11. Appending Unique Values: the AppendUnique Method
7.2.12. Prepending to the Beginning of Values: the Prepend Method
7.2.13. Prepending Unique Values: the PrependUnique Method
7.2.14. Overriding Construction Variable Settings
7.3. Controlling the Execution Environment for Issued Commands
7.3.1. Propagating PATH From the External Environment
7.3.2. Adding to PATH Values in the Execution Environment
7.4. Using the toolpath for external Tools
7.4.1. The default tool search path
7.4.2. Providing an external directory to toolpath
7.4.3. Nested Tools within a toolpath
7.4.4. Using sys.path within the toolpath
7.4.5. Using the PyPackageDir function to add to the toolpath
8. Automatically Putting Command-line Options into their Construction Variables
8.1. Merging Options into the Environment: the MergeFlags Function
8.2. Merging Options While Creating Environment: the parse_flags Parameter
8.3. Separating Compile Arguments into their Variables: the ParseFlags Function
8.4. Finding Installed Library Information: the ParseConfig Function
9. Controlling Build Output
9.1. Providing Build Help: the Help Function
9.2. Controlling How SCons Prints Build Commands: the $*COMSTR Variables
9.3. Providing Build Progress Output: the Progress Function
9.4. Printing Detailed Build Status: the GetBuildFailures Function
10. Controlling a Build From the Command Line
10.1. Command-Line Options
10.1.1. Not Having to Specify Command-Line Options Each Time: the SCONSFLAGS Environment Variable
10.1.2. Getting Values Set by Command-Line Options: the GetOption Function
10.1.3. Setting Values of Command-Line Options: the SetOption Function
10.1.4. Strings for Getting or Setting Values of SCons Command-Line Options
10.1.5. Adding Custom Command-Line Options: the AddOption Function
10.2. Command-Line variable=value Build Variables
10.2.1. Controlling Command-Line Build Variables
10.2.2. Providing Help for Command-Line Build Variables
10.2.3. Reading Build Variables From a File
10.2.4. Pre-Defined Build Variable Functions
10.2.5. Adding Multiple Command-Line Build Variables at Once
10.2.6. Handling Unknown Command-Line Build Variables: the UnknownVariables Function
10.3. Command-Line Targets
10.3.1. Fetching Command-Line Targets: the COMMAND_LINE_TARGETS Variable
10.3.2. Controlling the Default Targets: the Default Function
10.3.3. Fetching the List of Build Targets, Regardless of Origin: the BUILD_TARGETS Variable
11. Installing Files in Other Directories: the Install Builder
11.1. Installing Multiple Files in a Directory
11.2. Installing a File Under a Different Name
11.3. Installing Multiple Files Under Different Names
11.4. Installing a Shared Library
12. Platform-Independent File System Manipulation
12.1. Copying Files or Directories: The Copy Factory
12.2. Deleting Files or Directories: The Delete Factory
12.3. Moving (Renaming) Files or Directories: The Move Factory
12.4. Updating the Modification Time of a File: The Touch Factory
12.5. Creating a Directory: The Mkdir Factory
12.6. Changing File or Directory Permissions: The Chmod Factory
12.7. Executing an action immediately: the Execute Function
13. Controlling Removal of Targets
13.1. Preventing target removal during build: the Precious Function
13.2. Preventing target removal during clean: the NoClean Function
13.3. Removing additional files during clean: the Clean Function
14. Hierarchical Builds
14.1. SConscript Files
14.2. Path Names Are Relative to the SConscript Directory
14.3. Top-Relative Path Names in Subsidiary SConscript Files
14.4. Absolute Path Names
14.5. Sharing Environments (and Other Variables) Between SConscript Files
14.5.1. Exporting Variables
14.5.2. Importing Variables
14.5.3. Returning Values From an SConscript File
15. Separating Source and Build Trees: Variant Directories
15.1. Specifying a Variant Directory Tree as Part of an SConscript Call
15.2. Why SCons Duplicates Source Files in a Variant Directory Tree
15.3. Telling SCons to Not Duplicate Source Files in the Variant Directory Tree
15.4. The VariantDir Function
15.5. Using VariantDir With an SConscript File
15.6. Using Glob with VariantDir
15.7. Variant Build Examples
16. Building From Code Repositories
16.1. The Repository Method
16.2. Finding source files in repositories
16.3. Finding #include files in repositories
16.3.1. Limitations on #include files in repositories
16.4. Finding the SConstruct file in repositories
16.5. Finding derived files in repositories
16.6. Guaranteeing local copies of files
17. Extending SCons: Writing Your Own Builders
17.1. Writing Builders That Execute External Commands
17.2. Attaching a Builder to a Construction Environment
17.3. Letting SCons Handle The File Suffixes
17.4. Builders That Execute Python Functions
17.5. Builders That Create Actions Using a Generator
17.6. Builders That Modify the Target or Source Lists Using an Emitter
17.7. Modifying a Builder by adding an Emitter
17.8. Where To Put Your Custom Builders and Tools
18. Not Writing a Builder: the Command Builder
19. Extending SCons: Pseudo-Builders and the AddMethod function
20. Extending SCons: Writing Your Own Scanners
20.1. A Simple Scanner Example
20.2. Adding a search path to a Scanner: FindPathDirs
20.3. Using scanners with Builders
21. Multi-Platform Configuration (Autoconf Functionality)
21.1. Configure Contexts
21.2. Checking for the Existence of Header Files
21.3. Checking for the Availability of a Function
21.4. Checking for the Availability of a Library
21.5. Checking for the Availability of a typedef
21.6. Checking the size of a datatype
21.7. Checking for the Presence of a program
21.8. Extending SCons: Adding Your Own Custom Checks
21.9. Not Configuring When Cleaning Targets
22. Caching Built Files
22.1. Specifying the Derived-File Cache Directory
22.2. Keeping Build Output Consistent
22.3. Not Using the Derived-File Cache for Specific Files
22.4. Disabling the Derived-File Cache
22.5. Populating a Derived-File Cache With Already-Built Files
22.6. Minimizing Cache Contention: the --random Option
22.7. Using a Custom CacheDir Class
23. Alias Targets
24. Java Builds
24.1. Building Java Class Files: the Java Builder
24.2. How SCons Handles Java Dependencies
24.3. Building Java Archive (.jar) Files: the Jar Builder
24.4. Building C Header and Stub Files: the JavaH Builder
24.5. Building RMI Stub and Skeleton Class Files: the RMIC Builder
25. Internationalization and localization with gettext
25.1. Prerequisites
25.2. Simple project
26. Miscellaneous Functionality
26.1. Verifying the Python Version: the EnsurePythonVersion Function
26.2. Verifying the SCons Version: the EnsureSConsVersion Function
26.3. Explicitly Terminating SCons While Reading SConscript Files: the Exit Function
26.4. Searching for Files: the FindFile Function
26.5. Handling Nested Lists: the Flatten Function
26.6. Finding the Invocation Directory: the GetLaunchDir Function
26.7. Declaring Additional Outputs: the SideEffect Function
26.8. Virtual environments (virtualenvs)
27. Using SCons with other build tools
27.1. Creating a Compilation Database
27.2. Ninja Build Generator
28. Troubleshooting
28.1. Why is That Target Being Rebuilt? the --debug=explain Option
28.2. What's in That Construction Environment? the Dump Method
28.3. What Dependencies Does SCons Know About? the --tree Option
28.4. How is SCons Constructing the Command Lines It Executes? the --debug=presub Option
28.5. Where is SCons Searching for Libraries? the --debug=findlibs Option
28.6. Where is SCons Blowing Up? the --debug=stacktrace Option
28.7. How is SCons Making Its Decisions? the --taskmastertrace Option
28.8. Watch SCons prepare targets for building: the --debug=prepare Option
28.9. Why is a file disappearing? the --debug=duplicate Option
28.10. Keep it simple
A. Construction Variables
B. Builders
C. Tools
D. Functions and Environment Methods
E. Handling Common Tasks

List of Examples

E.1. Wildcard globbing to create a list of filenames
E.2. Filename extension substitution
E.3. Appending a path prefix to a list of filenames
E.4. Substituting a path prefix with another one
E.5. Filtering a filename list to exclude/retain only a specific set of extensions
E.6. The "backtick function": run a shell command and capture the output
E.7. Generating source code: how code can be generated and used by SCons

Thank you for taking the time to read about SCons. SCons is a modern software construction tool - a software utility for building software (or other files) and keeping built software up-to-date whenever the underlying input files change.

The most distinctive thing about SCons is that its configuration files are actually scripts, written in the Python programming language. This is in contrast to most alternative build tools, which typically invent a new language to configure the build. SCons still has a learning curve, of course, because you have to know what functions to call to set up your build properly, but the underlying syntax used should be familiar to anyone who has ever looked at a Python script.

Paradoxically, using Python as the configuration file format makes SCons easier for non-programmers to learn than the cryptic languages of other build tools, which are usually invented by programmers for other programmers. This is in no small part due to the consistency and readability that are hallmarks of Python. It just so happens that making a real, live scripting language the basis for the configuration files makes it a snap for more accomplished programmers to do more complicated things with builds, as necessary.

SCons would not exist without a lot of help from a lot of people, many of whom may not even be aware that they helped or served as inspiration. So in no particular order, and at the risk of leaving out someone:

First and foremost, SCons owes a tremendous debt to Bob Sidebotham, the original author of the classic Perl-based Cons tool which Bob first released to the world back around 1996. Bob's work on Cons classic provided the underlying architecture and model of specifying a build configuration using a real scripting language. My real-world experience working on Cons informed many of the design decisions in SCons, including the improved parallel build support, making Builder objects easily definable by users, and separating the build engine from the wrapping interface.

Greg Wilson was instrumental in getting SCons started as a real project when he initiated the Software Carpentry design competition in February 2000. Without that nudge, marrying the advantages of the Cons classic architecture with the readability of Python might have just stayed no more than a nice idea.

The entire SCons team have been absolutely wonderful to work with, and SCons would be nowhere near as useful a tool without the energy, enthusiasm and time people have contributed over the past few years. The "core team" of Chad Austin, Anthony Roach, Bill Deegan, Charles Crain, Steve Leblanc, Greg Noel, Gary Oberbrunner, Greg Spencer and Christoph Wiedemann have been great about reviewing my (and other) changes and catching problems before they get in the code base. Of particular technical note: Anthony's outstanding and innovative work on the tasking engine has given SCons a vastly superior parallel build model; Charles has been the master of the crucial Node infrastructure; Christoph's work on the Configure infrastructure has added crucial Autoconf-like functionality; and Greg has provided excellent support for Microsoft Visual Studio.

Special thanks to David Snopek for contributing his underlying "Autoscons" code that formed the basis of Christoph's work with the Configure functionality. David was extremely generous in making this code available to SCons, given that he initially released it under the GPL and SCons is released under a less-restrictive MIT-style license.

Thanks to Peter Miller for his splendid change management system, Aegis, which has provided the SCons project with a robust development methodology from day one, and which showed me how you could integrate incremental regression tests into a practical development cycle (years before eXtreme Programming arrived on the scene).

And last, thanks to Guido van Rossum for his elegant scripting language, which is the basis not only for the SCons implementation, but for the interface itself.

The best way to contact people involved with SCons, is through the SCons mailing lists.

If you want to ask general questions about how to use SCons send email to .

If you want to contact the SCons development community directly, send email to .

For quicker, informal questions, discussion, etc. the project operated a Discord server at https://discord.gg/bXVpWAy and a Libera.chat IRC channel at https://web.libera.chat/#scons (the former channel at irc.freenode.net is now unused). Certain discussions may also be moved by administrators from mailing list or chat to GitHub Discussions for greater permanence and easier finding.

This chapter will take you through the basic steps of installing SCons so you can use it for your projects. Before that, however, this chapter will also describe the basic steps involved in installing Python on your system, in case that is necessary. Fortunately, both SCons and Python are easy to install on almost any system, and Python already comes installed on many systems.

Because SCons is written in the Python programming language, you need to have a Python interpreter available on your system to use SCons. Before you try to install Python, check to see if Python is already available on your system by typing python -V (capital 'V') or python --version at your system's command-line prompt. For Linux/Unix/MacOS/BSD type systems this looks like:

$ python -V
Python 3.9.15
    

If you get a version like 2.7.x, you may need to try using the name python3 - current SCons no longer works with Python 2.

Note to Windows users: there are a number of different ways Python can be installed or invoked on Windows, it is beyond the scope of this guide to unravel all of them. Some have an additional program called the Python launcher (described, somewhat technically, in PEP 397): try using the command name py instead of python, if that is not available drop back to trying python

C:\>py -V
Python 3.9.15
    

If Python is not installed on your system, or is not findable in the current search path, you will see an error message stating something like "command not found" (on UNIX or Linux) or "'python' is not recognized as an internal or external command, operable progam or batch file" (on Windows cmd). In that case, you need to either install Python or fix the search path before you can install SCons.

The link for downloading Python installers (Windows and Mac) from the project's own website is: https://www.python.org/download. There are useful system-specific entries on setup and usage to be found at: https://docs.python.org/3/using

For Linux systems, Python is almost certainly available as a supported package, probably installed by default; this is often preferred over installing by other means as the system package will be built with carefully chosen optimizations, and will be kept up to date with bug fixes and security patches. In fact, the Python project itself does not build installers for Linux for this reason. Many such systems have separate packages for Python 2 and Python 3 - make sure the Python 3 package is installed, as the latest SCons requires it. Building from source may still be a useful option if you need a specific version that is not offered by the distribution you are using.

Recent versions of the Mac no longer come with Python pre-installed; older versions came with a rather out of date version (based on Python 2.7) which is insufficient to run current SCons. The python.org installer can be used on the Mac, but there are also other sources such as MacPorts and Homebrew. The Anaconda installation also comes with a bundled Python.

Windows has even more choices. The Python.org installer is a traditional .exe style; the same software is also released as a Windows application through the Microsoft Store. Several alternative builds also exist such as Chocolatey and ActiveState, and, again, a version of Python comes with Anaconda.

SCons will work with Python 3.6 or later. If you need to install Python and have a choice, we recommend using the most recent Python version available. Newer Python versions have significant improvements that help speed up the performance of SCons.

The recommended way to install SCons is from the Python Package Index (PyPI):

% python -m pip install scons
    

If you prefer not to install to the Python system location, or do not have privileges to do so, you can add a flag to install to a location specific to your own account and Python version:

% python -m pip install --user scons
    

For those users using Anaconda or Miniconda, use the conda installer instead, so the scons install location will match the version of Python that system will be using. For example:

% conda install -c conda-forge scons
    

If you need a specific version of SCons that is different from the current version, pip has a version option (e.g. python -m pip install scons==3.1.2), or you can follow the instructions in the following sections.

SCons does comes pre-packaged for installation on many Linux systems. Check your package installation system to see if there is an up-to-date SCons package available. Many people prefer to install distribution-native packages if available, as they provide a central point for management and updating; however not all distributions update in a timely fashion. During the still-ongoing Python 2 to 3 transition, some distributions may still have two SCons packages available, one which uses Python 2 and one which uses Python 3. Since the latest scons only runs on Python 3, to get the current version you should choose the Python 3 package.

You don't actually need to "install" SCons to use it. Nor do you need to "build" it, unless you are interested in producing the SCons documentation, which does use several tools to produce HTML, PDF and other output formats from files in the source tree. All you need to do is call the scons.py driver script in a location that contains an SCons tree, and it will figure out the rest. You can test that like this:

$ python /path/to/unpacked/scripts/scons.py --version
    

To make use of an uninstalled SCons, the first step is to download either the scons-4.7.0.tar.gz or scons-4.7.0.zip, which are available from the SCons download page at https://scons.org/pages/download.html. There is also a scons-local bundle you can make use of. It is arranged a little bit differently, with the idea that you can include it with your own project if you want people to be able to do builds without having to download or install SCons. Finally, you can also use a checkout of the git tree from GitHub at a location to point to.

Unpack the archive you downloaded, using a utility like tar on Linux or UNIX, or WinZip on Windows. This will create a directory called scons-4.7.0, usually in your local directory. The driver script will be in a subdirectory named scripts, unless you are using scons-local, in which case it will be in the top directory. Now you only need to call scons.py by giving a full or relative path to it in order to use that SCons version.

Note that instructions for older versions may have suggested running python setup.py install to "build and install" SCons. This is no longer recommended (in fact, it is not recommended by the wider Python packaging community for any end-user installations of Python software). There is a setup.py file, but it is only tested and used for the automated procedure which prepares an SCons bundle for making a release on PyPI, and even that is not guaranteed to work in future.

In some cases you may need several versions of SCons present on a system at the same time - perhaps you have an older project to build that has not yet been "ported" to a newer SCons version, or maybe you want to test a new SCons release side-by-side with a previous one before switching over. The use of an "uninstalled" package as described in the previous section can be of use for this purpose.

Another approach to multiple versions is to create Python virtualenvs, and install different SCons versions in each. A Python virtual environment is a directory with an isolated set of Python packages, where packages you install/upgrade/remove inside the environment do not affect anything outside it, and those you install/upgrade/remove outside of it do not affect anything inside it. In other words, anything you do with pip in the environment stays in that environment. The Python standard library provides a module called venv for creating these (https://docs.python.org/e/library/venv.html), although there are also other tools which provide more precise control of the setup.

Using a virtualenv can be useful even for a single version of SCons, to gain the advantages of having an isolated environment. It also gets around the problem of not having administrative privileges on a particular system to install a distribution package or use pip to install to a system location, as the virtualenv is completely under your control.

The following outline shows how this could be set up on a Linux/POSIX system (the syntax will be a bit different on Windows):

$ create virtualenv named scons3
$ create virtualenv named scons4
$ source scons3/bin/activate
$ pip install scons==3.1.2
$ deactivate
$ source scons4/bin/activate
$ pip install scons
$ deactivate
$ activate a virtualenv and run 'scons' to use that version
      

The single most important thing you do when writing a build system for your project is to describe the "what": what you want to build, and which files you want to build it from. And, in fact, simpler builds may need no more. In this chapter, you will see several examples of very simple build configurations using SCons, which will demonstrate how easy SCons makes it to build programs on different types of systems.

Here's the ubiquitous "Hello, World!" program in C:

#include <stdio.h>

int
main()
{
        printf("Hello, world!\n");
}
   

And here's how to build it using SCons. Save the code above into hello.c, and enter the following into a file named SConstruct:

Program('hello.c')
      

This minimal build file gives SCons three key pieces of information: what you want to build (a program); what you want to call that program (its base name will be hello), and the source file you want it built from (the hello.c file). Program is a Builder, an SCons function that you use to instruct SCons about the "what" of your build.

That's it. Now run the scons command to build the program. On a POSIX-compliant system like Linux or UNIX, you'll see something like:

% scons
scons: Reading SConscript files ...
scons: done reading SConscript files.
scons: Building targets ...
cc -o hello.o -c hello.c
cc -o hello hello.o
scons: done building targets.

On a Windows system with the Microsoft Visual C++ compiler, you'll see something like:

C:\>scons
scons: Reading SConscript files ...
scons: done reading SConscript files.
scons: Building targets ...
cl /Fohello.obj /c hello.c /nologo
link /nologo /OUT:hello.exe hello.obj
embedManifestExeCheck(target, source, env)
scons: done building targets.

Notice that SCons deduced quite a bit here: it figured out the name of the program to build, including operating system specific suffixes (hello or hello.exe), based off the basename of the source file; it knows an intermediate object file should be built (hello.o or hello.obj); and it knows how to build those things using the compiler that is appropriate on the system you're using. It was not necessary to instruct SCons about any of those details. This is an example of how SCons makes it easy to write portable software builds.

For the programming languages SCons already knows about, it will mostly just figure it out. Here's the "Hello, World!" example in Fortran:

program hello
  print *, 'Hello, World!'
end program hello
   
Program('hello', 'hello.f90')
   
$ scons
scons: Reading SConscript files ...
scons: done reading SConscript files.
scons: Building targets ...
gfortran -o hello.o -c hello.f90
gfortran -o hello hello.o
scons: done building targets.
   

The Program builder is only one of many builders (also called a builder method) that SCons provides to build different types of files. Another is the Object builder method, which tells SCons to build an object file from the specified source file:

Object('hello.c')
      

Now when you run the scons command to build the program, it will build just the hello.o object file on a POSIX system:

% scons
scons: Reading SConscript files ...
scons: done reading SConscript files.
scons: Building targets ...
cc -o hello.o -c hello.c
scons: done building targets.

And just the hello.obj object file on a Windows system (with the Microsoft Visual C++ compiler):

C:\>scons
scons: Reading SConscript files ...
scons: done reading SConscript files.
scons: Building targets ...
cl /Fohello.obj /c hello.c /nologo
scons: done building targets.

(Note that this guide will not continue to provide duplicate side-by-side POSIX and Windows output for all of the examples. Just keep in mind that, unless otherwise specified, any of the examples should work equally well on both types of systems.)

SCons also makes building with Java extremely easy. Unlike the Program and Object builder methods, however, the Java builder method requires that you specify the name of a destination directory in which you want the class files placed, followed by the source directory in which the .java files live:

Java('classes', 'src')
     

If the src directory contains a single hello.java file, then the output from running the scons command would look something like this (on a POSIX system):

% scons
scons: Reading SConscript files ...
scons: done reading SConscript files.
scons: Building targets ...
javac -d classes -sourcepath src src/hello.java
scons: done building targets.

Java builds will be covered in much more detail, including building a Java archive (.jar) and other types of files, in Chapter 24, Java Builds.

If you're used to build systems like Make you've already figured out that the SConstruct file is the SCons equivalent of a Makefile. That is, the SConstruct file is the input file that SCons reads to control the build.

One important way in which the SConstruct file is not exactly like a normal Python script, and is more like a Makefile, is that the order in which the SCons Builder functions are called in the SConstruct file does not affect the order in which SCons actually builds the programs and object files you want it to build. [1]. In other words, when you call the Program builder (or any other builder method), you're not telling SCons to build the program at that moment. Instead, you're telling SCons what you want accomplished, and it's up to SCons to figure out how to do that, and to take those steps if/when it's necessary. you'll learn more about how SCons decides when building or rebuilding a target is necessary in Chapter 6, Dependencies, below.

SCons reflects this distinction between calling a builder method like Program and actually building the program by printing the status messages that indicate when it's "just reading" the SConstruct file, and when it's actually building the target files. This is to make it clear when SCons is executing the Python statements that make up the SConstruct file, and when SCons is actually executing the commands or other actions to build the necessary files.

Let's clarify this with an example. Python has a print function that prints a string of characters to the screen. If you put print calls around the calls to the Program builder method:

print("Calling Program('hello.c')")
Program('hello.c')
print("Calling Program('goodbye.c')")
Program('goodbye.c')
print("Finished calling Program()")
       

Then when you execute SCons, you will see the output from calling the print function in between the messages about reading the SConscript files, indicating that is when the Python statements are being executed:

% scons
scons: Reading SConscript files ...
Calling Program('hello.c')
Calling Program('goodbye.c')
Finished calling Program()
scons: done reading SConscript files.
scons: Building targets ...
cc -o goodbye.o -c goodbye.c
cc -o goodbye goodbye.o
cc -o hello.o -c hello.c
cc -o hello hello.o
scons: done building targets.

Notice that SCons built the goodbye program first, even though the "reading SConscript" output shows that Program('hello.c') was called first in the SConstruct file.



[1] In programming parlance, the SConstruct file is declarative, meaning you tell SCons what you want done and let it figure out the order in which to do it, rather than strictly imperative, where you specify explicitly the order in which to do things.

Of course, most builds are more complicated than in the previous chapter. In this chapter, you will learn about builds that incorporate multiple source files, and then about building multiple targets that share some source files.

You've seen that when you call the Program builder method, it builds the resulting program with the same base name as the source file. That is, the following call to build an executable program from the hello.c source file will build an executable program named hello on POSIX systems, and an executable program named hello.exe on Windows systems:

Program('hello.c')
    

If you want to build a program with a different base name than the base of the source file name (or even the same name), you simply put the target file name to the left of the source file name:

Program('new_hello', 'hello.c')
       

SCons requires the target file name first, followed by the source file name, so that the order mimics that of an assignment statement in most programming languages, including Python: "target = source files". For an alternative way to supply this information, see Section 3.6, “Keyword Arguments”.

Now SCons will build an executable program named new_hello when run on a POSIX system:

% scons -Q
cc -o hello.o -c hello.c
cc -o new_hello hello.o

And SCons will build an executable program named new_hello.exe when run on a Windows system:

C:\>scons -Q
cl /Fohello.obj /c hello.c /nologo
link /nologo /OUT:new_hello.exe hello.obj
embedManifestExeCheck(target, source, env)

You can also use the Glob function to find all files matching a certain template, using the standard shell pattern matching characters * (to match everything), ? (to match a single character) and [abc] to match any of a, b or c. [!abc] is also supported, to match any character except a, b or c. This makes many multi-source-file builds quite easy:

Program('program', Glob('*.c'))
    

Glob has powerful capabilities - it matches even if the file does not currently exist, but SCons can determine that it would exist after a build. You will meet it again reading about variant directories (see Chapter 15, Separating Source and Build Trees: Variant Directories) and repositories (see Chapter 16, Building From Code Repositories).

One drawback to the use of a Python list for source files is that each file name must be enclosed in quotes (either single quotes or double quotes). This can get cumbersome and difficult to read when the list of file names is long. Fortunately, SCons and Python provide a number of ways to make sure that the SConstruct file stays easy to read.

To make long lists of file names easier to deal with, SCons provides a Split function that takes a quoted list of file names, with the names separated by spaces or other white-space characters, and turns it into a list of separate file names. Using the Split function turns the previous example into:

Program('program', Split('main.c file1.c file2.c'))
    

(If you're already familiar with Python, you'll have realized that this is similar to the split() method of Python string objects.. Unlike the split() method, however, the Split function does not require a string as input and will wrap up a single non-string object in a list, or return its argument untouched if it's already a list. This comes in handy as a way to make sure arbitrary values can be passed to SCons functions without having to check the type of the variable by hand.)

Putting the call to the Split function inside the Program call can also be a little unwieldy. A more readable alternative is to assign the output from the Split call to a variable name, and then use the variable when calling the Program function:

src_files = Split('main.c file1.c file2.c')
Program('program', src_files)
    

Lastly, the Split function doesn't care how much white space separates the file names in the quoted string. This allows you to create lists of file names that span multiple lines, which often makes for easier editing:

src_files = Split("""
    main.c
    file1.c
    file2.c
""")
Program('program', src_files)
    

(Note this example uses the Python "triple-quote" syntax, which allows a string to span multiple lines. The three quotes can be either single or double quotes as long as they match.)

It's common to re-use code by sharing source files between multiple programs. One way to do this is to create a library from the common source files, which can then be linked into resulting programs. (Creating libraries is discussed in Chapter 4, Building and Linking with Libraries, below.)

A more straightforward, but perhaps less convenient, way to share source files between multiple programs is simply to include the common files in the lists of source files for each program:

Program(Split('foo.c common1.c common2.c'))
Program('bar', Split('bar1.c bar2.c common1.c common2.c'))
       

SCons recognizes that the object files for the common1.c and common2.c source files each need to be built only once, even though the resulting object files are each linked in to both of the resulting executable programs:

% scons -Q
cc -o bar1.o -c bar1.c
cc -o bar2.o -c bar2.c
cc -o common1.o -c common1.c
cc -o common2.o -c common2.c
cc -o bar bar1.o bar2.o common1.o common2.o
cc -o foo.o -c foo.c
cc -o foo foo.o common1.o common2.o

If two or more programs share a lot of common source files, repeating the common files in the list for each program can be a maintenance problem when you need to change the list of common files. You can simplify this by creating a separate Python list to hold the common file names, and concatenating it with other lists using the Python + operator:

common = ['common1.c', 'common2.c']
foo_files = ['foo.c'] + common
bar_files = ['bar1.c', 'bar2.c'] + common
Program('foo', foo_files)
Program('bar', bar_files)
    

This is functionally equivalent to the previous example.

It's often useful to organize large software projects by collecting parts of the software into one or more libraries. SCons makes it easy to create libraries and to use them in the programs.

You build your own libraries by specifying Library instead of Program:

Library('foo', ['f1.c', 'f2.c', 'f3.c'])
      

SCons uses the appropriate library prefix and suffix for your system. So on POSIX or Linux systems, the above example would build as follows (although ranlib may not be called on all systems):

% scons -Q
cc -o f1.o -c f1.c
cc -o f2.o -c f2.c
cc -o f3.o -c f3.c
ar rc libfoo.a f1.o f2.o f3.o
ranlib libfoo.a

On a Windows system, a build of the above example would look like:

C:\>scons -Q
cl /Fof1.obj /c f1.c /nologo
cl /Fof2.obj /c f2.c /nologo
cl /Fof3.obj /c f3.c /nologo
lib /nologo /OUT:foo.lib f1.obj f2.obj f3.obj

The rules for the target name of the library are similar to those for programs: if you don't explicitly specify a target library name, SCons will deduce one from the name of the first source file specified, and SCons will add an appropriate file prefix and suffix if you leave them off.

The previous example shows building a library from a list of source files. You can, however, also give the Library call object files, and it will correctly realize they are object files. In fact, you can arbitrarily mix source code files and object files in the source list:

Library('foo', ['f1.c', 'f2.o', 'f3.c', 'f4.o'])
        

And SCons realizes that only the source code files must be compiled into object files before creating the final library:

% scons -Q
cc -o f1.o -c f1.c
cc -o f3.o -c f3.c
ar rc libfoo.a f1.o f2.o f3.o f4.o
ranlib libfoo.a

Of course, in this example, the object files must already exist for the build to succeed. See Chapter 5, Node Objects, below, for information about how you can build object files explicitly and include the built files in a library.

The Library function builds a traditional static library. If you want to be explicit about the type of library being built, you can use the synonym StaticLibrary function instead of Library:

StaticLibrary('foo', ['f1.c', 'f2.c', 'f3.c'])
        

There is no functional difference between the StaticLibrary and Library functions.

If you want to build a shared library (on POSIX systems) or a DLL file (on Windows systems), you use the SharedLibrary function:

SharedLibrary('foo', ['f1.c', 'f2.c', 'f3.c'])
        

The output on POSIX:

% scons -Q
cc -o f1.os -c f1.c
cc -o f2.os -c f2.c
cc -o f3.os -c f3.c
cc -o libfoo.so -shared f1.os f2.os f3.os

And the output on Windows:

C:\>scons -Q
cl /Fof1.obj /c f1.c /nologo
cl /Fof2.obj /c f2.c /nologo
cl /Fof3.obj /c f3.c /nologo
link /nologo /dll /out:foo.dll /implib:foo.lib f1.obj f2.obj f3.obj
RegServerFunc(target, source, env)
embedManifestDllCheck(target, source, env)

Notice again that SCons takes care of building the output file correctly, adding the -shared option for a POSIX compilation, and the /dll option on Windows.

Usually, you build a library because you want to link it with one or more programs. You link libraries with a program by specifying the libraries in the $LIBS construction variable, and by specifying the directory in which the library will be found in the $LIBPATH construction variable:

Library('foo', ['f1.c', 'f2.c', 'f3.c'])
Program('prog.c', LIBS=['foo', 'bar'], LIBPATH='.')
      

Notice, of course, that you don't need to specify a library prefix (like lib) or suffix (like .a or .lib). SCons uses the correct prefix or suffix for the current system.

On a POSIX or Linux system, a build of the above example would look like:

% scons -Q
cc -o f1.o -c f1.c
cc -o f2.o -c f2.c
cc -o f3.o -c f3.c
ar rc libfoo.a f1.o f2.o f3.o
ranlib libfoo.a
cc -o prog.o -c prog.c
cc -o prog prog.o -L. -lfoo -lbar

On a Windows system, a build of the above example would look like:

C:\>scons -Q
cl /Fof1.obj /c f1.c /nologo
cl /Fof2.obj /c f2.c /nologo
cl /Fof3.obj /c f3.c /nologo
lib /nologo /OUT:foo.lib f1.obj f2.obj f3.obj
cl /Foprog.obj /c prog.c /nologo
link /nologo /OUT:prog.exe /LIBPATH:. foo.lib bar.lib prog.obj
embedManifestExeCheck(target, source, env)

As usual, notice that SCons has taken care of constructing the correct command lines to link with the specified library on each system.

Note also that, if you only have a single library to link with, you can specify the library name in single string, instead of a Python list, so that:

Program('prog.c', LIBS='foo', LIBPATH='.')
    

is equivalent to:

Program('prog.c', LIBS=['foo'], LIBPATH='.')
    

This is similar to the way that SCons handles either a string or a list to specify a single source file.

By default, the linker will only look in certain system-defined directories for libraries. SCons knows how to look for libraries in directories that you specify with the $LIBPATH construction variable. $LIBPATH consists of a list of directory names, like so:

Program('prog.c', LIBS = 'm',
                  LIBPATH = ['/usr/lib', '/usr/local/lib'])
      

Using a Python list is preferred because it's portable across systems. Alternatively, you could put all of the directory names in a single string, separated by the system-specific path separator character: a colon on POSIX systems:

LIBPATH = '/usr/lib:/usr/local/lib'
    

or a semi-colon on Windows systems:

LIBPATH = 'C:\\lib;D:\\lib'
    

(Note that Python requires that the backslash separators in a Windows path name be escaped within strings.)

When the linker is executed, SCons will create appropriate flags so that the linker will look for libraries in the same directories as SCons. So on a POSIX or Linux system, a build of the above example would look like:

% scons -Q
cc -o prog.o -c prog.c
cc -o prog prog.o -L/usr/lib -L/usr/local/lib -lm

On a Windows system, a build of the above example would look like:

C:\>scons -Q
cl /Foprog.obj /c prog.c /nologo
link /nologo /OUT:prog.exe /LIBPATH:\usr\lib /LIBPATH:\usr\local\lib m.lib prog.obj
embedManifestExeCheck(target, source, env)

Note again that SCons has taken care of the system-specific details of creating the right command-line options.

Internally, SCons represents all of the files and directories it knows about as Nodes. These internal objects (not object files) can be used in a variety of ways to make your SConscript files portable and easy to read.

All builder methods return a list of Node objects that identify the target file or files that will be built. These returned Nodes can be passed as arguments to other builder methods.

For example, suppose that we want to build the two object files that make up a program with different options. This would mean calling the Object builder once for each object file, specifying the desired options:

Object('hello.c', CCFLAGS='-DHELLO')
Object('goodbye.c', CCFLAGS='-DGOODBYE')
    

One way to combine these object files into the resulting program would be to call the Program builder with the names of the object files listed as sources:

Object('hello.c', CCFLAGS='-DHELLO')
Object('goodbye.c', CCFLAGS='-DGOODBYE')
Program(['hello.o', 'goodbye.o'])
    

The problem with specifying the names as strings is that our SConstruct file is no longer portable across operating systems. It won't, for example, work on Windows because the object files there would be named hello.obj and goodbye.obj, not hello.o and goodbye.o.

A better solution is to assign the lists of targets returned by the calls to the Object builder to variables, which we can then concatenate in our call to the Program builder:

hello_list = Object('hello.c', CCFLAGS='-DHELLO')
goodbye_list = Object('goodbye.c', CCFLAGS='-DGOODBYE')
Program(hello_list + goodbye_list)
      

This makes our SConstruct file portable again, the build output on Linux looking like:

% scons -Q
cc -o goodbye.o -c -DGOODBYE goodbye.c
cc -o hello.o -c -DHELLO hello.c
cc -o hello hello.o goodbye.o

And on Windows:

C:\>scons -Q
cl /Fogoodbye.obj /c goodbye.c -DGOODBYE
cl /Fohello.obj /c hello.c -DHELLO
link /nologo /OUT:hello.exe hello.obj goodbye.obj
embedManifestExeCheck(target, source, env)

We'll see examples of using the list of nodes returned by builder methods throughout the rest of this guide.

So far we've seen how SCons handles one-time builds. But one of the main functions of a build tool like SCons is to rebuild only what is necessary when source files change--or, put another way, SCons should not waste time rebuilding things that don't need to be rebuilt. You can see this at work simply by re-invoking SCons after building our simple hello example:

% scons -Q
cc -o hello.o -c hello.c
cc -o hello hello.o
% scons -Q
scons: `.' is up to date.

The second time it is executed, SCons realizes that the hello program is up-to-date with respect to the current hello.c source file, and avoids rebuilding it. You can see this more clearly by naming the hello program explicitly on the command line:

% scons -Q hello
cc -o hello.o -c hello.c
cc -o hello hello.o
% scons -Q hello
scons: `hello' is up to date.

Note that SCons reports "...is up to date" only for target files named explicitly on the command line, to avoid cluttering the output.

Another aspect of avoiding unnecessary rebuilds is the fundamental build tool behavior of rebuilding things when an input file changes, so that the built software is up to date. By default, SCons keeps track of this through a content signature, or hash, of the contents of each file, although you can easily configure SCons to use the modification times (or time stamps) instead. You can even write your own Python function for deciding if an input file should trigger a rebuild.

By default, SCons uses a cryptographic hash of the file's contents, not the file's modification time, to decide whether a file has changed. This means that you may be surprised by the default SCons behavior if you are used to the Make convention of forcing a rebuild by updating the file's modification time (using the touch command, for example):

% scons -Q hello
cc -o hello.o -c hello.c
cc -o hello hello.o
% touch hello.c
% scons -Q hello
scons: `hello' is up to date.

Even though the file's modification time has changed, SCons realizes that the contents of the hello.c file have not changed, and therefore that the hello program need not be rebuilt. This avoids unnecessary rebuilds when, for example, someone rewrites the contents of a file without making a change. But if the contents of the file really do change, then SCons detects the change and rebuilds the program as required:

% scons -Q hello
cc -o hello.o -c hello.c
cc -o hello hello.o
%     [CHANGE THE CONTENTS OF hello.c]
% scons -Q hello
cc -o hello.o -c hello.c
cc -o hello hello.o

Note that you can, if you wish, specify the default behavior of using content signatures explicitly, using the Decider function as follows:

Program('hello.c')
Decider('content')
      

You can also use the string 'MD5' as a synonym for 'content' when calling the Decider function - this older name is deprecated since SCons now supports a choice of hash functions, not just the MD5 function.

If you prefer, you can configure SCons to use the modification time of a file, not the file contents, when deciding if a target needs to be rebuilt. SCons gives you two ways to use time stamps to decide if an input file has changed since the last time a target has been built.

The most familiar way to use time stamps is the way Make does: that is, have SCons decide that a target must be rebuilt if a source file's modification time is newer than the target file. To do this, call the Decider function as follows:

Object('hello.c')
Decider('timestamp-newer')
        

This makes SCons act like Make when a file's modification time is updated (using the touch command, for example):

% scons -Q hello.o
cc -o hello.o -c hello.c
% touch hello.c
% scons -Q hello.o
cc -o hello.o -c hello.c

And, in fact, because this behavior is the same as the behavior of Make, you can also use the string 'make' as a synonym for 'timestamp-newer' when calling the Decider function:

Object('hello.c')
Decider('make')
      

One drawback to using times stamps exactly like Make is that if an input file's modification time suddenly becomes older than a target file, the target file will not be rebuilt. This can happen if an old copy of a source file is restored from a backup archive, for example. The contents of the restored file will likely be different than they were the last time a dependent target was built, but the target won't be rebuilt because the modification time of the source file is not newer than the target.

Because SCons actually stores information about the source files' time stamps whenever a target is built, it can handle this situation by checking for an exact match of the source file time stamp, instead of just whether or not the source file is newer than the target file. To do this, specify the argument 'timestamp-match' when calling the Decider function:

Object('hello.c')
Decider('timestamp-match')
        

When configured this way, SCons will rebuild a target whenever a source file's modification time has changed. So if we use the touch -t option to change the modification time of hello.c to an old date (January 1, 1989), SCons will still rebuild the target file:

% scons -Q hello.o
cc -o hello.o -c hello.c
% touch -t 198901010000 hello.c
% scons -Q hello.o
cc -o hello.o -c hello.c

In general, the only reason to prefer timestamp-newer instead of timestamp-match, would be if you have some specific reason to require this Make-like behavior of not rebuilding a target when an otherwise-modified source file is older.

As a performance enhancement, SCons provides a way to use a file's content signature, but to read those contents only when the file's timestamp has changed. To do this, call the Decider function with 'content-timestamp' argument as follows:

Program('hello.c')
Decider('content-timestamp')
        

So configured, SCons will still behave like it does when using Decider('content'):

% scons -Q hello
cc -o hello.o -c hello.c
cc -o hello hello.o
% touch hello.c
% scons -Q hello
scons: `hello' is up to date.
% edit hello.c
    [CHANGE THE CONTENTS OF hello.c]
% scons -Q hello
cc -o hello.o -c hello.c
cc -o hello hello.o
      

However, the second call to SCons in the above output, when the build is up-to-date, will have been performed by simply looking at the modification time of the hello.c file, not by opening it and performing a signature calcuation on its contents. This can significantly speed up many up-to-date builds.

The only drawback to using Decider('content-timestamp') is that SCons will not rebuild a target file if a source file was modified within one second of the last time SCons built the file. While most developers are programming, this isn't a problem in practice, since it's unlikely that someone will have built and then thought quickly enough to make a substantive change to a source file within one second. Certain build scripts or continuous integration tools may, however, rely on the ability to apply changes to files automatically and then rebuild as quickly as possible, in which case use of Decider('content-timestamp') may not be appropriate.

The different string values that we've passed to the Decider function are essentially used by SCons to pick one of several specific internal functions that implement various ways of deciding if a dependency (usually a source file) has changed since a target file has been built. As it turns out, you can also supply your own function to decide if a dependency has changed.

For example, suppose we have an input file that contains a lot of data, in some specific regular format, that is used to rebuild a lot of different target files, but each target file really only depends on one particular section of the input file. We'd like to have each target file depend on only its section of the input file. However, since the input file may contain a lot of data, we want to open the input file only if its timestamp has changed. This could be done with a custom Decider function that might look something like this:

Program('hello.c')
def decide_if_changed(dependency, target, prev_ni, repo_node=None):
    if dependency.get_timestamp() != prev_ni.timestamp:
        dep = str(dependency)
        tgt = str(target)
        if specific_part_of_file_has_changed(dep, tgt):
            return True
    return False
Decider(decide_if_changed)
        

Note that in the function definition, the dependency (input file) is the first argument, and then the target. Both of these are passed to the functions as SCons Node objects, which we convert to strings using the Python str().

The third argument, prev_ni, is an object that holds the content signature and/or timestamp information that was recorded about the dependency the last time the target was built. A prev_ni object can hold different information, depending on the type of thing that the dependency argument represents. For normal files, the prev_ni object has the following attributes:

csig

The content signature: a cryptgraphic hash, or checksum, of the file contents of the dependency file the last time the target was built.

size

The size in bytes of the dependency file the last time the target was built.

timestamp

The modification time of the dependency file the last time the target was built.

These attributes may not be present at the time of the first run. Without any prior build, no targets have been created and no .sconsign DB file exists yet. So you should always check whether the prev_ni attribute in question is available (use the Python hasattr method or a try-except block).

The fourth argument repo_node is the Node to use if it is not None when comparing BuildInfo. This is typically only set when the target node only exists in a Repository

Note that ignoring some of the arguments in your custom Decider function is a perfectly normal thing to do, if they don't impact the way you want to decide if the dependency file has changed.

We finally present a small example for a csig-based decider function. Note how the signature information for the dependency file has to get initialized via get_csig during each function call (this is mandatory!).

env = Environment()


def config_file_decider(dependency, target, prev_ni, repo_node=None):
    import os.path

    # We always have to init the .csig value...
    dep_csig = dependency.get_csig()
    # .csig may not exist, because no target was built yet...
    if not prev_ni.hasattr("csig"):
        return True
    # Target file may not exist yet
    if not os.path.exists(str(target.abspath)):
        return True
    if dep_csig != prev_ni.csig:
        # Some change on source file => update installed one
        return True
    return False


def update_file():
    with open("test.txt", "a") as f:
        f.write("some line\n")


update_file()

# Activate our own decider function
env.Decider(config_file_decider)

env.Install("install", "test.txt")
      

The previous examples have all demonstrated calling the global Decider function to configure all dependency decisions that SCons makes. Sometimes, however, you want to be able to configure different decision-making for different targets. When that's necessary, you can use the env.Decider method to affect only the configuration decisions for targets built with a specific construction environment.

For example, if we arbitrarily want to build one program using content signatures and another using file modification times from the same source we might configure it this way:

env1 = Environment(CPPPATH = ['.'])
env2 = env1.Clone()
env2.Decider('timestamp-match')
env1.Program('prog-content', 'program1.c')
env2.Program('prog-timestamp', 'program2.c')
        

If both of the programs include the same inc.h file, then updating the modification time of inc.h (using the touch command) will cause only prog-timestamp to be rebuilt:

% scons -Q
cc -o program1.o -c -I. program1.c
cc -o prog-content program1.o
cc -o program2.o -c -I. program2.c
cc -o prog-timestamp program2.o
% touch inc.h
% scons -Q
cc -o program2.o -c -I. program2.c
cc -o prog-timestamp program2.o

Now suppose that our "Hello, World!" program actually has an #include line to include the hello.h file in the compilation:

#include <hello.h>
int
main()
{
    printf("Hello, %s!\n", string);
}
      

And, for completeness, the hello.h file looks like this:

#define string    "world"
      

In this case, we want SCons to recognize that, if the contents of the hello.h file change, the hello program must be recompiled. To do this, we need to modify the SConstruct file like so:

Program('hello.c', CPPPATH='.')
      

The $CPPPATH value tells SCons to look in the current directory ('.') for any files included by C source files (.c or .h files). With this assignment in the SConstruct file:

% scons -Q hello
cc -o hello.o -c -I. hello.c
cc -o hello hello.o
% scons -Q hello
scons: `hello' is up to date.
%     [CHANGE THE CONTENTS OF hello.h]
% scons -Q hello
cc -o hello.o -c -I. hello.c
cc -o hello hello.o

First, notice that SCons constructed the -I. argument from the '.' in the $CPPPATH variable so that the compilation would find the hello.h file in the local directory.

Second, realize that SCons knows that the hello program must be rebuilt because it scans the contents of the hello.c file for the #include lines that indicate another file is being included in the compilation. SCons records these as implicit dependencies of the target file, Consequently, when the hello.h file changes, SCons realizes that the hello.c file includes it, and rebuilds the resulting hello program that depends on both the hello.c and hello.h files.

Like the $LIBPATH variable, the $CPPPATH variable may be a list of directories, or a string separated by the system-specific path separation character (':' on POSIX/Linux, ';' on Windows). Either way, SCons creates the right command-line options so that the following example:

Program('hello.c', CPPPATH = ['include', '/home/project/inc'])
      

Will look like this on POSIX or Linux:

% scons -Q hello
cc -o hello.o -c -Iinclude -I/home/project/inc hello.c
cc -o hello hello.o

And like this on Windows:

C:\>scons -Q hello.exe
cl /Fohello.obj /c hello.c /nologo /Iinclude /I\home\project\inc
link /nologo /OUT:hello.exe hello.obj
embedManifestExeCheck(target, source, env)

Scanning each file for #include lines does take some extra processing time. When you're doing a full build of a large system, the scanning time is usually a very small percentage of the overall time spent on the build. You're most likely to notice the scanning time, however, when you rebuild all or part of a large system: SCons will likely take some extra time to "think about" what must be built before it issues the first build command (or decides that everything is up to date and nothing must be rebuilt).

In practice, having SCons scan files saves time relative to the amount of potential time lost to tracking down subtle problems introduced by incorrect dependencies. Nevertheless, the "waiting time" while SCons scans files can annoy individual developers waiting for their builds to finish. Consequently, SCons lets you cache the implicit dependencies that its scanners find, for use by later builds. You can do this by specifying the --implicit-cache option on the command line:

% scons -Q --implicit-cache hello
cc -o hello.o -c hello.c
cc -o hello hello.o
% scons -Q hello
scons: `hello' is up to date.

If you don't want to specify --implicit-cache on the command line each time, you can make it the default behavior for your build by setting the implicit_cache option in an SConscript file:

SetOption('implicit_cache', 1)
    

SCons does not cache implicit dependencies like this by default because the --implicit-cache causes SCons to simply use the implicit dependencies stored during the last run, without any checking for whether or not those dependencies are still correct. Specifically, this means --implicit-cache instructs SCons to not rebuild "correctly" in the following cases:

  • When --implicit-cache is used, SCons will ignore any changes that may have been made to search paths (like $CPPPATH or $LIBPATH,). This can lead to SCons not rebuilding a file if a change to $CPPPATH would normally cause a different, same-named file from a different directory to be used.

  • When --implicit-cache is used, SCons will not detect if a same-named file has been added to a directory that is earlier in the search path than the directory in which the file was found last time.

Sometimes a file depends on another file that is not detected by an SCons scanner. For this situation, SCons allows you to specific explicitly that one file depends on another file, and must be rebuilt whenever that file changes. This is specified using the Depends method:

hello = Program('hello.c')
Depends(hello, 'other_file')
    
% scons -Q hello
cc -c hello.c -o hello.o
cc -o hello hello.o
% scons -Q hello
scons: `hello' is up to date.
% edit other_file
    [CHANGE THE CONTENTS OF other_file]
% scons -Q hello
cc -c hello.c -o hello.o
cc -o hello hello.o
    

Note that the dependency (the second argument to Depends) may also be a list of Node objects (for example, as returned by a call to a Builder):

hello = Program('hello.c')
goodbye = Program('goodbye.c')
Depends(hello, goodbye)
    

in which case the dependency or dependencies will be built before the target(s):

% scons -Q hello
cc -c goodbye.c -o goodbye.o
cc -o goodbye goodbye.o
cc -c hello.c -o hello.o
cc -o hello hello.o
    

SCons has built-in scanners for a number of languages. Sometimes these scanners fail to extract certain implicit dependencies due to limitations of the scanner implementation.

The following example illustrates a case where the built-in C scanner is unable to extract the implicit dependency on a header file.

#define FOO_HEADER <foo.h>
#include FOO_HEADER

int main() {
    return FOO;
}
      
% scons -Q
cc -o hello.o -c -I. hello.c
cc -o hello hello.o
%    [CHANGE CONTENTS OF foo.h]
% scons -Q
scons: `.' is up to date.

Apparently, the scanner does not know about the header dependency. Not being a full-fledged C preprocessor, the scanner does not expand the macro.

In these cases, you may also use the compiler to extract the implicit dependencies. ParseDepends can parse the contents of the compiler output in the style of Make, and explicitly establish all of the listed dependencies.

The following example uses ParseDepends to process a compiler generated dependency file which is generated as a side effect during compilation of the object file:

obj = Object('hello.c', CCFLAGS='-MD -MF hello.d', CPPPATH='.')
SideEffect('hello.d', obj)
ParseDepends('hello.d')
Program('hello', obj)
      
% scons -Q
cc -o hello.o -c -MD -MF hello.d -I. hello.c
cc -o hello hello.o
%    [CHANGE CONTENTS OF foo.h]
% scons -Q
cc -o hello.o -c -MD -MF hello.d -I. hello.c

Parsing dependencies from a compiler-generated .d file has a chicken-and-egg problem, that causes unnecessary rebuilds:

% scons -Q
cc -o hello.o -c -MD -MF hello.d -I. hello.c
cc -o hello hello.o
% scons -Q --debug=explain
scons: rebuilding `hello.o' because `foo.h' is a new dependency
cc -o hello.o -c -MD -MF hello.d -I. hello.c
% scons -Q
scons: `.' is up to date.
    

In the first pass, the dependency file is generated while the object file is compiled. At that time, SCons does not know about the dependency on foo.h. In the second pass, the object file is regenerated because foo.h is detected as a new dependency.

ParseDepends immediately reads the specified file at invocation time and just returns if the file does not exist. A dependency file generated during the build process is not automatically parsed again. Hence, the compiler-extracted dependencies are not stored in the signature database during the same build pass. This limitation of ParseDepends leads to unnecessary recompilations. Therefore, ParseDepends should only be used if scanners are not available for the employed language or not powerful enough for the specific task.

Sometimes it makes sense to not rebuild a program, even if a dependency file changes. In this case, you would tell SCons specifically to ignore a dependency using the Ignore function as follows:

hello_obj=Object('hello.c')
hello = Program(hello_obj)
Ignore(hello_obj, 'hello.h')
      
% scons -Q hello
cc -c -o hello.o hello.c
cc -o hello hello.o
% scons -Q hello
scons: `hello' is up to date.
% edit hello.h
  [CHANGE THE CONTENTS OF hello.h]
% scons -Q hello
scons: `hello' is up to date.
    

Now, the above example is a little contrived, because it's hard to imagine a real-world situation where you wouldn't want to rebuild hello if the hello.h file changed. A more realistic example might be if the hello program is being built in a directory that is shared between multiple systems that have different copies of the stdio.h include file. In that case, SCons would notice the differences between the different systems' copies of stdio.h and would rebuild hello each time you change systems. You could avoid these rebuilds as follows:

hello = Program('hello.c', CPPPATH=['/usr/include'])
Ignore(hello, '/usr/include/stdio.h')
    

Ignore can also be used to prevent a generated file from being built by default. This is due to the fact that directories depend on their contents. So to ignore a generated file from the default build, you specify that the directory should ignore the generated file. Note that the file will still be built if the user specifically requests the target on scons command line, or if the file is a dependency of another file which is requested and/or is built by default.

hello_obj=Object('hello.c')
hello = Program(hello_obj)
Ignore('.',[hello,hello_obj])
      
% scons -Q
scons: `.' is up to date.
% scons -Q hello
cc -o hello.o -c hello.c
cc -o hello hello.o
% scons -Q hello
scons: `hello' is up to date.

Occasionally, it may be useful to specify that a certain file or directory must, if necessary, be built or created before some other target is built, but that changes to that file or directory do not require that the target itself be rebuilt. Such a relationship is called an order-only dependency because it only affects the order in which things must be built--the dependency before the target--but it is not a strict dependency relationship because the target should not change in response to changes in the dependent file.

For example, suppose that you want to create a file every time you run a build that identifies the time the build was performed, the version number, etc., and which is included in every program that you build. The version file's contents will change every build. If you specify a normal dependency relationship, then every program that depends on that file would be rebuilt every time you ran SCons. For example, we could use some Python code in a SConstruct file to create a new version.c file with a string containing the current date every time we run SCons, and then link a program with the resulting object file by listing version.c in the sources:

import time

version_c_text = """
char *date = "%s";
""" % time.ctime(time.time())
open('version.c', 'w').write(version_c_text)

hello = Program(['hello.c', 'version.c'])
      

If we list version.c as an actual source file, though, then the version.o file will get rebuilt every time we run SCons (because the SConstruct file itself changes the contents of version.c) and the hello executable will get re-linked every time (because the version.o file changes):

% scons -Q hello
cc -o hello.o -c hello.c
cc -o version.o -c version.c
cc -o hello hello.o version.o
% sleep 1
% scons -Q hello
cc -o version.o -c version.c
cc -o hello hello.o version.o
% sleep 1
% scons -Q hello
cc -o version.o -c version.c
cc -o hello hello.o version.o

(Note that for the above example to work, we sleep for one second in between each run, so that the SConstruct file will create a version.c file with a time string that's one second later than the previous run.)

One solution is to use the Requires function to specify that the version.o must be rebuilt before it is used by the link step, but that changes to version.o should not actually cause the hello executable to be re-linked:

import time

version_c_text = """
char *date = "%s";
""" % time.ctime(time.time())
open('version.c', 'w').write(version_c_text)

version_obj = Object('version.c')

hello = Program('hello.c',
                LINKFLAGS = str(version_obj[0]))

Requires(hello, version_obj)
      

Notice that because we can no longer list version.c as one of the sources for the hello program, we have to find some other way to get it into the link command line. For this example, we're cheating a bit and stuffing the object file name (extracted from version_obj list returned by the Object builder call) into the $LINKFLAGS variable, because $LINKFLAGS is already included in the $LINKCOM command line.

With these changes, we get the desired behavior of only re-linking the hello executable when the hello.c has changed, even though the version.o is rebuilt (because the SConstruct file still changes the version.c contents directly each run):

% scons -Q hello
cc -o version.o -c version.c
cc -o hello.o -c hello.c
cc -o hello version.o hello.o
% sleep 1
% scons -Q hello
cc -o version.o -c version.c
scons: `hello' is up to date.
% sleep 1
%     [CHANGE THE CONTENTS OF hello.c]
% scons -Q hello
cc -o version.o -c version.c
cc -o hello.o -c hello.c
cc -o hello version.o hello.o
% sleep 1
% scons -Q hello
cc -o version.o -c version.c
scons: `hello' is up to date.

How SCons handles dependencies can also be affected by the AlwaysBuild method. When a file is passed to the AlwaysBuild method, like so:

hello = Program('hello.c')
AlwaysBuild(hello)
      

Then the specified target file (hello in our example) will always be considered out-of-date and rebuilt whenever that target file is evaluated while walking the dependency graph:

% scons -Q
cc -o hello.o -c hello.c
cc -o hello hello.o
% scons -Q
cc -o hello hello.o

The AlwaysBuild function has a somewhat misleading name, because it does not actually mean the target file will be rebuilt every single time SCons is invoked. Instead, it means that the target will, in fact, be rebuilt whenever the target file is encountered while evaluating the targets specified on the command line (and their dependencies). So specifying some other target on the command line, a target that does not itself depend on the AlwaysBuild target, will still be rebuilt only if it's out-of-date with respect to its dependencies:

% scons -Q
cc -o hello.o -c hello.c
cc -o hello hello.o
% scons -Q hello.o
scons: `hello.o' is up to date.

An environment is a collection of values that can affect how a program executes. SCons distinguishes between three different types of environments that can affect the behavior of SCons itself (subject to the configuration in the SConscript files), as well as the compilers and other tools it executes:

External Environment

The External Environment is the set of variables in the user's environment at the time the user runs SCons. These variables are not automatically part of an SCons build but are available to be examined if needed. See Section 7.1, “Using Values From the External Environment”, below.

Construction Environment

A Construction Environment is a distinct object created within a SConscript file and which contains values that affect how SCons decides what action to use to build a target, and even to define which targets should be built from which sources. One of the most powerful features of SCons is the ability to create multiple construction environments, including the ability to clone a new, customized construction environment from an existing construction environment. See Section 7.2, “Construction Environments”, below.

Execution Environment

An Execution Environment is the values that SCons sets when executing an external command (such as a compiler or linker) to build one or more targets. Note that this is not the same as the external environment (see above). See Section 7.3, “Controlling the Execution Environment for Issued Commands”, below.

Unlike Make, SCons does not automatically copy or import values between different environments (with the exception of explicit clones of construction environments, which inherit the values from their parent). This is a deliberate design choice to make sure that builds are, by default, repeatable regardless of the values in the user's external environment. This avoids a whole class of problems with builds where a developer's local build works because a custom variable setting causes a different compiler or build option to be used, but the checked-in change breaks the official build because it uses different environment variable settings.

Note that the SConscript writer can easily arrange for variables to be copied or imported between environments, and this is often very useful (or even downright necessary) to make it easy for developers to customize the build in appropriate ways. The point is not that copying variables between different environments is evil and must always be avoided. Instead, it should be up to the implementer of the build system to make conscious choices about how and when to import a variable from one environment to another, making informed decisions about striking the right balance between making the build repeatable on the one hand and convenient to use on the other.

It is rare that all of the software in a large, complicated system needs to be built exactly the same way. For example, different source files may need different options enabled on the command line, or different executable programs need to be linked with different libraries. SCons accommodates these different build requirements by allowing you to create and configure multiple construction environments that control how the software is built. A construction environment is an object that has a number of associated construction variables, each with a name and a value, just like a dictionary. (A construction environment also has an attached set of Builder methods, about which we'll learn more later.)

You can fetch individual values, known as Construction Variables, using the same syntax used for accessing individual named items in a Python dictionary:

env = Environment()
print("CC is: %s" % env['CC'])
print("LATEX is: %s" % env.get('LATEX', None))
        

This example SConstruct file doesn't contain instructions for building any targets, but because it's still a valid SConstruct it will be evaluated and the Python print calls will output the values of $CC and $LATEX for us (remember using the .get() method for fetching means we get a default value back, rather than a failure, if the variable is not set):

% scons -Q
CC is: cc
LATEX is: None
scons: `.' is up to date.

A construction environment is actually an object with associated methods and attributes. If you want to have direct access to only the dictionary of construction variables you can fetch this using the env.Dictionary method (although it's rarely necessary to use this method):

env = Environment(FOO='foo', BAR='bar')
cvars = env.Dictionary()
for key in ['OBJSUFFIX', 'LIBSUFFIX', 'PROGSUFFIX']:
    print("key = %s, value = %s" % (key, cvars[key]))
         

This SConstruct file will print the specified dictionary items for us on POSIX systems as follows:

% scons -Q
key = OBJSUFFIX, value = .o
key = LIBSUFFIX, value = .a
key = PROGSUFFIX, value = 
scons: `.' is up to date.

And on Windows:

C:\>scons -Q
key = OBJSUFFIX, value = .obj
key = LIBSUFFIX, value = .lib
key = PROGSUFFIX, value = .exe
scons: `.' is up to date.

If you want to loop and print the values of all of the construction variables in a construction environment, the Python code to do that in sorted order might look something like:

env = Environment()
for item in sorted(env.Dictionary().items()):
    print("construction variable = '%s', value = '%s'" % item)
      

It should be noted that for the previous example, there is actually a construction environment method that does the same thing more simply, and tries to format the output nicely as well:

env = Environment()
print(env.Dump())
      

Another way to get information from a construction environment is to use the subst method on a string containing $ expansions of construction variable names. As a simple example, the example from the previous section that used env['CC'] to fetch the value of $CC could also be written as:

env = Environment()
print("CC is: %s" % env.subst('$CC'))
      

One advantage of using subst to expand strings is that construction variables in the result get re-expanded until there are no expansions left in the string. So a simple fetch of a value like $CCCOM:

env = Environment(CCFLAGS='-DFOO')
print("CCCOM is: %s" % env['CCCOM'])
      

Will print the unexpanded value of $CCCOM, showing us the construction variables that still need to be expanded:

% scons -Q
CCCOM is: $CC $CCFLAGS $CPPFLAGS $_CPPDEFFLAGS $_CPPINCFLAGS -c -o $TARGET $SOURCES
scons: `.' is up to date.
      

Calling the subst method on $CCOM, however:

env = Environment(CCFLAGS='-DFOO')
print("CCCOM is: %s" % env.subst('$CCCOM'))
      

Will recursively expand all of the construction variables prefixed with $ (dollar signs), showing us the final output:

% scons -Q
CCCOM is: gcc -DFOO -c -o
scons: `.' is up to date.
      

Note that because we're not expanding this in the context of building something there are no target or source files for $TARGET and $SOURCES to expand.

If a problem occurs when expanding a construction variable, by default it is expanded to '' (an empty string), and will not cause scons to fail.

env = Environment()
print("value is: %s"%env.subst( '->$MISSING<-' ))
        
% scons -Q
value is: -><-
scons: `.' is up to date.

This default behaviour can be changed using the AllowSubstExceptions function. When a problem occurs with a variable expansion it generates an exception, and the AllowSubstExceptions function controls which of these exceptions are actually fatal and which are allowed to occur safely. By default, NameError and IndexError are the two exceptions that are allowed to occur: so instead of causing scons to fail, these are caught, the variable expanded to '' and scons execution continues. To require that all construction variable names exist, and that indexes out of range are not allowed, call AllowSubstExceptions with no extra arguments.

AllowSubstExceptions()
env = Environment()
print("value is: %s"%env.subst( '->$MISSING<-' ))
        
% scons -Q

scons: *** NameError `name 'MISSING' is not defined' trying to evaluate `$MISSING'
File "/home/my/project/SConstruct", line 3, in <module>

This can also be used to allow other exceptions that might occur, most usefully with the ${...} construction variable syntax. For example, this would allow zero-division to occur in a variable expansion in addition to the default exceptions allowed

AllowSubstExceptions(IndexError, NameError, ZeroDivisionError)
env = Environment()
print("value is: %s"%env.subst( '->${1 / 0}<-' ))
        
% scons -Q
value is: -><-
scons: `.' is up to date.

If AllowSubstExceptions is called multiple times, each call completely overwrites the previous list of allowed exceptions.

All of the Builder functions that we've introduced so far, like Program and Library, use a construction environment that contains settings for the various compilers and other tools that SCons configures by default, or otherwise knows about and has discovered on your system. If not invoked as methods of a specific construction environment, they use the default construction environment The goal of the default construction environment is to make many configurations "just work" to build software using readily available tools with a minimum of configuration changes.

If needed, you can control the default construction environment by using the DefaultEnvironment function to initialize various settings by passing them as keyword arguments:

DefaultEnvironment(CC='/usr/local/bin/gcc')
      

When configured as above, all calls to the Program or Object Builder will build object files with the /usr/local/bin/gcc compiler.

The DefaultEnvironment function returns the initialized default construction environment object, which can then be manipulated like any other construction environment (note that the default environment works like a singleton - it can have only one instance - so the keyword arguments are processed only on the first call. On any subsequent call the existing object is returned). So the following would be equivalent to the previous example, setting the $CC variable to /usr/local/bin/gcc but as a separate step after the default construction environment has been initialized:

def_env = DefaultEnvironment()
def_env['CC'] = '/usr/local/bin/gcc'
      

One very common use of the DefaultEnvironment function is to speed up SCons initialization. As part of trying to make most default configurations "just work," SCons will actually search the local system for installed compilers and other utilities. This search can take time, especially on systems with slow or networked file systems. If you know which compiler(s) and/or other utilities you want to configure, you can control the search that SCons performs by specifying some specific tool modules with which to initialize the default construction environment:

def_env = DefaultEnvironment(tools=['gcc', 'gnulink'], CC='/usr/local/bin/gcc')
      

So the above example would tell SCons to explicitly configure the default environment to use its normal GNU Compiler and GNU Linker settings (without having to search for them, or any other utilities for that matter), and specifically to use the compiler found at /usr/local/bin/gcc.

The real advantage of construction environments is that you can create as many different ones as you need, each tailored to a different way to build some piece of software or other file. If, for example, we need to build one program with the -O2 flag and another with the -g (debug) flag, we would do this like so:

opt = Environment(CCFLAGS='-O2')
dbg = Environment(CCFLAGS='-g')

opt.Program('foo', 'foo.c')

dbg.Program('bar', 'bar.c')
        
% scons -Q
cc -o bar.o -c -g bar.c
cc -o bar bar.o
cc -o foo.o -c -O2 foo.c
cc -o foo foo.o

We can even use multiple construction environments to build multiple versions of a single program. If you do this by simply trying to use the Program builder with both environments, though, like this:

opt = Environment(CCFLAGS='-O2')
dbg = Environment(CCFLAGS='-g')

opt.Program('foo', 'foo.c')

dbg.Program('foo', 'foo.c')
        

Then SCons generates the following error:

% scons -Q

scons: *** Two environments with different actions were specified for the same target: foo.o
File "/home/my/project/SConstruct", line 6, in <module>

This is because the two Program calls have each implicitly told SCons to generate an object file named foo.o, one with a $CCFLAGS value of -O2 and one with a $CCFLAGS value of -g. SCons can't just decide that one of them should take precedence over the other, so it generates the error. To avoid this problem, we must explicitly specify that each environment compile foo.c to a separately-named object file using the Object builder, like so:

opt = Environment(CCFLAGS='-O2')
dbg = Environment(CCFLAGS='-g')

o = opt.Object('foo-opt', 'foo.c')
opt.Program(o)

d = dbg.Object('foo-dbg', 'foo.c')
dbg.Program(d)
        

Notice that each call to the Object builder returns a value, an internal SCons object that represents the object file that will be built. We then use that object as input to the Program builder. This avoids having to specify explicitly the object file name in multiple places, and makes for a compact, readable SConstruct file. Our SCons output then looks like:

% scons -Q
cc -o foo-dbg.o -c -g foo.c
cc -o foo-dbg foo-dbg.o
cc -o foo-opt.o -c -O2 foo.c
cc -o foo-opt foo-opt.o

Sometimes you want more than one construction environment to share the same values for one or more variables. Rather than always having to repeat all of the common variables when you create each construction environment, you can use the env.Clone method to create a copy of a construction environment.

Like the Environment call that creates a construction environment, the Clone method takes construction variable assignments, which will override the values in the copied construction environment. For example, suppose we want to use gcc to create three versions of a program, one optimized, one debug, and one with neither. We could do this by creating a "base" construction environment that sets $CC to gcc, and then creating two copies, one which sets $CCFLAGS for optimization and the other which sets $CCFLAGS for debugging:

env = Environment(CC='gcc')
opt = env.Clone(CCFLAGS='-O2')
dbg = env.Clone(CCFLAGS='-g')

env.Program('foo', 'foo.c')

o = opt.Object('foo-opt', 'foo.c')
opt.Program(o)

d = dbg.Object('foo-dbg', 'foo.c')
dbg.Program(d)
        

Then our output would look like:

% scons -Q
gcc -o foo.o -c foo.c
gcc -o foo foo.o
gcc -o foo-dbg.o -c -g foo.c
gcc -o foo-dbg foo-dbg.o
gcc -o foo-opt.o -c -O2 foo.c
gcc -o foo-opt foo-opt.o

You can replace existing construction variable values using the env.Replace method:

env = Environment(CCFLAGS='-DDEFINE1')
env.Replace(CCFLAGS='-DDEFINE2')
env.Program('foo.c')
        

The replacing value (-DDEFINE2 in the above example) completely replaces the value in the construction environment:

% scons -Q
cc -o foo.o -c -DDEFINE2 foo.c
cc -o foo foo.o

You can safely call Replace for construction variables that don't exist in the construction environment:

env = Environment()
env.Replace(NEW_VARIABLE='xyzzy')
print("NEW_VARIABLE = %s" % env['NEW_VARIABLE'])
        

In this case, the construction variable simply gets added to the construction environment:

% scons -Q
NEW_VARIABLE = xyzzy
scons: `.' is up to date.

Because the variables aren't expanded until the construction environment is actually used to build the targets, and because SCons function and method calls are order-independent, the last replacement "wins" and is used to build all targets, regardless of the order in which the calls to Replace() are interspersed with calls to builder methods:

env = Environment(CCFLAGS='-DDEFINE1')
print("CCFLAGS = %s" % env['CCFLAGS'])
env.Program('foo.c')

env.Replace(CCFLAGS='-DDEFINE2')
print("CCFLAGS = %s" % env['CCFLAGS'])
env.Program('bar.c')
        

The timing of when the replacement actually occurs relative to when the targets get built becomes apparent if we run scons without the -Q option:

% scons
scons: Reading SConscript files ...
CCFLAGS = -DDEFINE1
CCFLAGS = -DDEFINE2
scons: done reading SConscript files.
scons: Building targets ...
cc -o bar.o -c -DDEFINE2 bar.c
cc -o bar bar.o
cc -o foo.o -c -DDEFINE2 foo.c
cc -o foo foo.o
scons: done building targets.

Because the replacement occurs while the SConscript files are being read, the $CCFLAGS variable has already been set to -DDEFINE2 by the time the foo.o target is built, even though the call to the Replace method does not occur until later in the SConscript file.

Sometimes it's useful to be able to specify that a construction variable should be set to a value only if the construction environment does not already have that variable defined You can do this with the env.SetDefault method, which behaves similarly to the setdefault method of Python dictionary objects:

env.SetDefault(SPECIAL_FLAG='-extra-option')
      

This is especially useful when writing your own Tool modules to apply variables to construction environments.

You can append a value to an existing construction variable using the env.Append method:

env = Environment(CPPDEFINES=['MY_VALUE'])
env.Append(CPPDEFINES=['LAST'])
env.Program('foo.c')
        

Note $CPPDEFINES is the preferred way to set preprocessor defines, as SCons will generate the command line arguments using the correct prefix/suffix for the platform, leaving the usage portable. If you use $CCFLAGS and $SHCCFLAGS, you need to include them in their final form, which is less portable.

% scons -Q
cc -o foo.o -c -DMY_VALUE -DLAST foo.c
cc -o foo foo.o

If the construction variable doesn't already exist, the Append method will create it:

env = Environment()
env.Append(NEW_VARIABLE = 'added')
print("NEW_VARIABLE = %s"%env['NEW_VARIABLE'])
        

Which yields:

% scons -Q
NEW_VARIABLE = added
scons: `.' is up to date.

Note that the Append function tries to be "smart" about how the new value is appended to the old value. If both are strings, the previous and new strings are simply concatenated. Similarly, if both are lists, the lists are concatenated. If, however, one is a string and the other is a list, the string is added as a new element to the list.

Sometimes it's useful to add a new value only if the existing construction variable doesn't already contain the value. This can be done using the env.AppendUnique method:

env.AppendUnique(CCFLAGS=['-g'])
      

In the above example, the -g would be added only if the $CCFLAGS variable does not already contain a -g value.

You can prepend a value to the beginning of an existing construction variable using the env.Prepend method:

env = Environment(CPPDEFINES=['MY_VALUE'])
env.Prepend(CPPDEFINES=['FIRST'])
env.Program('foo.c')
        

SCons then generates the preprocessor define arguments from CPPDEFINES values with the correct prefix/suffix. For example on Linux or POSIX, the following arguments would be generated: -DFIRST and -DMY_VALUE

% scons -Q
cc -o foo.o -c -DFIRST -DMY_VALUE foo.c
cc -o foo foo.o

If the construction variable doesn't already exist, the Prepend method will create it:

env = Environment()
env.Prepend(NEW_VARIABLE='added')
print("NEW_VARIABLE = %s" % env['NEW_VARIABLE'])
        

Which yields:

% scons -Q
NEW_VARIABLE = added
scons: `.' is up to date.

Like the Append function, the Prepend function tries to be "smart" about how the new value is appended to the old value. If both are strings, the previous and new strings are simply concatenated. Similarly, if both are lists, the lists are concatenated. If, however, one is a string and the other is a list, the string is added as a new element to the list.

Rather than creating a cloned construction environment for specific tasks, you can override or add construction variables when calling a builder method by passing them as keyword arguments. The values of these overridden or added variables will only be in effect when building that target, and will not affect other parts of the build. For example, if you want to add additional libraries for just one program:

env.Program('hello', 'hello.c', LIBS=['gl', 'glut'])
      

or generate a shared library with a non-standard suffix:

env.SharedLibrary(
    target='word',
    source='word.cpp',
    SHLIBSUFFIX='.ocx',
    LIBSUFFIXES=['.ocx'],
)
      

When overriding this way, the Python keyword arguments in the builder call mean "set to this value". If you want your override to augment an existing value, you have to take some extra steps. Inside the builder call, it is possible to substitute in the existing value by using a string containing the variable name prefaced by a dollar sign ($).

env = Environment(CPPDEFINES="FOO")
env.Object(target="foo1.o", source="foo.c")
env.Object(target="foo2.o", source="foo.c", CPPDEFINES="BAR")
env.Object(target="foo3.o", source="foo.c", CPPDEFINES=["BAR", "$CPPDEFINES"])
        

Which yields:

% scons -Q
cc -o foo1.o -c -DFOO foo.c
cc -o foo2.o -c -DBAR foo.c
cc -o foo3.o -c -DBAR -DFOO foo.c

It is also possible to use the parse_flags keyword argument in an override to merge command-line style arguments into the appropriate construction variables. This works like the env.MergeFlags method, which will be fully described in the next chapter.

This example adds 'include' to $CPPPATH, 'EBUG' to $CPPDEFINES, and 'm' to $LIBS:

env = Environment()
env.Program('hello', 'hello.c', parse_flags='-Iinclude -DEBUG -lm')
        

So when executed:

% scons -Q
cc -o hello.o -c -DEBUG -Iinclude hello.c
cc -o hello hello.o -lm

Using temporary overrides this way is lighter weight than making a full construction environment, so it can help performance in large projects which have lots of special case values to set. However, keep in mind that this only works well when the targets are unique. Using builder overrides to try to build the same target with different sets of flags or other construction variables will lead to the scons: *** Two environments with different actions... error described in Section 7.2.6, “Multiple Construction Environments above. In this case you will actually want to create separate environments.

When SCons builds a target file, it does not execute the commands with the external environment that you used to execute SCons. Instead, it builds an execution environment from the values stored in the $ENV construction variable and uses that for executing commands.

The most important ramification of this behavior is that the PATH environment variable, which controls where the operating system will look for commands and utilities, will almost certainly not be the same as in the external environment from which you called SCons. This means that SCons might not necessarily find all of the tools that you can successfully execute from the command line.

The default value of the PATH environment variable on a POSIX system is /usr/local/bin:/opt/bin:/bin:/usr/bin:/snap/bin. The default value of the PATH environment variable on a Windows system comes from the Windows registry value for the command interpreter. If you want to execute any commands--compilers, linkers, etc.--that are not in these default locations, you need to set the PATH value in the $ENV dictionary in your construction environment.

The simplest way to do this is to initialize explicitly the value when you create the construction environment; this is one way to do that:

path = ['/usr/local/bin', '/bin', '/usr/bin']
env = Environment(ENV={'PATH': path})
    

Assigning a dictionary to the $ENV construction variable in this way completely resets the execution environment, so that the only variable that will be set when external commands are executed will be the PATH value. If you want to use the rest of the values in $ENV and only set the value of PATH, you can assign a value only to that variable:

env['ENV']['PATH'] = ['/usr/local/bin', '/bin', '/usr/bin']
    

Note that SCons does allow you to define the directories in the PATH in a string with paths separated by the pathname-separator character for your system (':' on POSIX systems, ';' on Windows).

env['ENV']['PATH'] = '/usr/local/bin:/bin:/usr/bin'
    

But doing so makes your SConscript file less portable, since it will be correct only for the system type that matches the separator. You can use the Python os.pathsep for for greater portability - don't worry too much if this Python syntax doesn't make sense since there are other ways available:

import os
env['ENV']['PATH'] = os.pathsep.join(['/usr/local/bin', '/bin', '/usr/bin'])
    

One of the most common requirements for manipulating a variable in the execution environment is to add one or more custom directories to a path search variable like PATH on Linux or POSIX systems, or %PATH% on Windows, so that a locally-installed compiler or other utility can be found when SCons tries to execute it to update a target. SCons provides env.PrependENVPath and env.AppendENVPath functions to make adding things to execution variables convenient. You call these functions by specifying the variable to which you want the value added, and then value itself. So to add some /usr/local directories to the $PATH and $LIB variables, you might:

env = Environment(ENV=os.environ.copy())
env.PrependENVPath('PATH', '/usr/local/bin')
env.AppendENVPath('LIB', '/usr/local/lib')
      

Note that the added values are strings, and if you want to add multiple directories to a variable like $PATH, you must include the path separator character in the string (: on Linux or POSIX, ; on Windows, or use os.pathsep for portability).

In some cases you may want to specify a different location to search for tools. The Environment function contains an option for this called toolpath This can be used to add additional search directories.

# Tool located within the toolpath directory option
env = Environment(
    tools=['SomeTool'],
    toolpath=['/opt/SomeToolPath', '/opt/SomeToolPath2']
)
env.SomeTool(targets, sources)

# The search locations in this example would include:
/opt/SomeToolPath/SomeTool.py
/opt/SomeToolPath/SomeTool/__init__.py
/opt/SomeToolPath2/SomeTool.py
/opt/SomeToolPath2/SomeTool/__init__.py
SCons/Tool/SomeTool.py
SCons/Tool/SomeTool/__init__.py
./site_scons/site_tools/SomeTool.py
./site_scons/site_tools/SomeTool/__init__.py
      

This chapter describes the MergeFlags, ParseFlags, and ParseConfig methods of a construction environment, as well as the parse_flags keyword argument to methods that construct environments.

SCons construction environments have a MergeFlags method that merges values from a passed-in argument into the construction environment. If the argument is a dictionary, MergeFlags treats each value in the dictionary as a list of options you would pass to a command (such as a compiler or linker). MergeFlags will not duplicate an option if it already exists in the construction variable. If the argument is a string, MergeFlags calls the ParseFlags method to burst it out into a dictionary first, then acts on the result.

MergeFlags tries to be intelligent about merging options, knowing that different construction variables may have different needs. When merging options to any variable whose name ends in PATH, MergeFlags keeps the leftmost occurrence of the option, because in typical lists of directory paths, the first occurrence "wins." When merging options to any other variable name, MergeFlags keeps the rightmost occurrence of the option, because in a list of typical command-line options, the last occurrence "wins."

env = Environment()
env.Append(CCFLAGS='-option -O3 -O1')
flags = {'CCFLAGS': '-whatever -O3'}
env.MergeFlags(flags)
print("CCFLAGS:", env['CCFLAGS'])
   
% scons -Q
CCFLAGS: ['-option', '-O1', '-whatever', '-O3']
scons: `.' is up to date.

Note that the default value for $CCFLAGS is an internal SCons object which automatically converts the options you specify as a string into a list.

env = Environment()
env.Append(CPPPATH=['/include', '/usr/local/include', '/usr/include'])
flags = {'CPPPATH': ['/usr/opt/include', '/usr/local/include']}
env.MergeFlags(flags)
print("CPPPATH:", env['CPPPATH'])
   
% scons -Q
CPPPATH: ['/include', '/usr/local/include', '/usr/include', '/usr/opt/include']
scons: `.' is up to date.

Note that the default value for $CPPPATH is a normal Python list, so you should give its values as a list in the dictionary you pass to the MergeFlags function.

If MergeFlags is passed anything other than a dictionary, it calls the ParseFlags method to convert it into a dictionary.

env = Environment()
env.Append(CCFLAGS='-option -O3 -O1')
env.Append(CPPPATH=['/include', '/usr/local/include', '/usr/include'])
env.MergeFlags('-whatever -I/usr/opt/include -O3 -I/usr/local/include')
print("CCFLAGS:", env['CCFLAGS'])
print("CPPPATH:", env['CPPPATH'])
   
% scons -Q
CCFLAGS: ['-option', '-O1', '-whatever', '-O3']
CPPPATH: ['/include', '/usr/local/include', '/usr/include', '/usr/opt/include']
scons: `.' is up to date.

In the combined example above, ParseFlags has sorted the options into their corresponding variables and returned a dictionary for MergeFlags to apply to the construction variables in the specified construction environment.

It is also possible to merge construction variable values from arguments given to the Environment call itself. If the parse_flags keyword argument is given, its value is distributed to construction variables in the new environment in the same way as described for the MergeFlags method. This also works when calling env.Clone, as well as in overrides to builder methods (see Section 7.2.14, “Overriding Construction Variable Settings”).

env = Environment(parse_flags="-I/opt/include -L/opt/lib -lfoo")
for k in ('CPPPATH', 'LIBPATH', 'LIBS'):
    print("%s:" % k, env.get(k))
env.Program("f1.c")
   
% scons -Q
CPPPATH: ['/opt/include']
LIBPATH: ['/opt/lib']
LIBS: ['foo']
cc -o f1.o -c -I/opt/include f1.c
cc -o f1 f1.o -L/opt/lib -lfoo

SCons has a bewildering array of construction variables for different types of options when building programs. Sometimes you may not know exactly which variable should be used for a particular option.

SCons construction environments have a ParseFlags method that takes a set of typical command-line options and distributes them into the appropriate construction variables Historically, it was created to support the ParseConfig method, so it focuses on options used by the GNU Compiler Collection (GCC) for the C and C++ toolchains.

ParseFlags returns a dictionary containing the options distributed into their respective construction variables. Normally, this dictionary would then be passed to MergeFlags to merge the options into a construction environment, but the dictionary can be edited if desired to provide additional functionality. (Note that if the flags are not going to be edited, calling MergeFlags with the options directly will avoid an additional step.)

env = Environment()
d = env.ParseFlags("-I/opt/include -L/opt/lib -lfoo")
for k, v in sorted(d.items()):
    if v:
        print(k, v)
env.MergeFlags(d)
env.Program("f1.c")
   
% scons -Q
CPPPATH ['/opt/include']
LIBPATH ['/opt/lib']
LIBS ['foo']
cc -o f1.o -c -I/opt/include f1.c
cc -o f1 f1.o -L/opt/lib -lfoo

Note that if the options are limited to generic types like those above, they will be correctly translated for other platform types:

C:\>scons -Q
CPPPATH ['/opt/include']
LIBPATH ['/opt/lib']
LIBS ['foo']
cl /Fof1.obj /c f1.c /nologo /I\opt\include
link /nologo /OUT:f1.exe /LIBPATH:\opt\lib foo.lib f1.obj
embedManifestExeCheck(target, source, env)

Since the assumption is that the flags are used for the GCC toolchain, unrecognized flags are placed in $CCFLAGS so they will be used for both C and C++ compiles:

env = Environment()
d = env.ParseFlags("-whatever")
for k, v in sorted(d.items()):
    if v:
        print(k, v)
env.MergeFlags(d)
env.Program("f1.c")
   
% scons -Q
CCFLAGS -whatever
cc -o f1.o -c -whatever f1.c
cc -o f1 f1.o

ParseFlags will also accept a (recursive) list of strings as input; the list is flattened before the strings are processed:

env = Environment()
d = env.ParseFlags(["-I/opt/include", ["-L/opt/lib", "-lfoo"]])
for k, v in sorted(d.items()):
    if v:
        print(k, v)
env.MergeFlags(d)
env.Program("f1.c")
   
% scons -Q
CPPPATH ['/opt/include']
LIBPATH ['/opt/lib']
LIBS ['foo']
cc -o f1.o -c -I/opt/include f1.c
cc -o f1 f1.o -L/opt/lib -lfoo

If a string begins with a an exclamation mark (!), the string is passed to the shell for execution. The output of the command is then parsed:

env = Environment()
d = env.ParseFlags(["!echo -I/opt/include", "!echo -L/opt/lib", "-lfoo"])
for k, v in sorted(d.items()):
    if v:
        print(k, v)
env.MergeFlags(d)
env.Program("f1.c")
   
% scons -Q
CPPPATH ['/opt/include']
LIBPATH ['/opt/lib']
LIBS ['foo']
cc -o f1.o -c -I/opt/include f1.c
cc -o f1 f1.o -L/opt/lib -lfoo

ParseFlags is regularly updated for new options; consult the man page for details about those currently recognized.

Configuring the right options to build programs to work with libraries--especially shared libraries--that are available on POSIX systems can be complex. To help this situation, various utilies with names that end in config return the command-line options for the GNU Compiler Collection (GCC) that are needed to build and link against those libraries; for example, the command-line options to use a library named lib could be found by calling a utility named lib-config.

A more recent convention is that these options are available through the generic pkg-config program, providing a common framework, error handling, and the like, so that all the package creator has to do is provide the set of strings for his particular package.

SCons construction variables have a ParseConfig method that asks the host system to execute a command and then configures the appropriate construction variables based on the output of that command. This lets you run a program like pkg-config or a more specific utility to help set up your build.

env = Environment()
env['CPPPATH'] = ['/lib/compat']
env.ParseConfig("pkg-config x11 --cflags --libs")
print("CPPPATH:", env['CPPPATH'])
   

SCons will execute the specified command string, parse the resultant flags, and add the flags to the appropriate environment variables.

% scons -Q
CPPPATH: ['/lib/compat', '/usr/X11/include']
scons: `.' is up to date.
 

In the example above, SCons has added the include directory to $CPPPATH (Depending upon what other flags are emitted by the pkg-config command, other variables may have been extended as well.)

Note that the options are merged with existing options using the MergeFlags method, so that each option only occurs once in the construction variable.

env = Environment()
env.ParseConfig("pkg-config x11 --cflags --libs")
env.ParseConfig("pkg-config x11 --cflags --libs")
print("CPPPATH:", "CPPPATH:", env['CPPPATH'])
   
% scons -Q
CPPPATH: ['/usr/X11/include']
scons: `.' is up to date.
 

A key aspect of creating a usable build configuration is providing useful output from the build so its users can readily understand what the build is doing and get information about how to control the build. SCons provides several ways of controlling output from the build configuration to help make the build more useful and understandable.

It's often very useful to be able to give users some help that describes the specific targets, build options, etc., that can be used for your build. SCons provides the Help function to allow you to specify this help text:

Help("""
Type: 'scons program' to build the production program,
      'scons debug' to build the debug version.
""")
       

Optionally, you can specify the append flag:

Help("""
Type: 'scons program' to build the production program,
      'scons debug' to build the debug version.
""", append=True)
       

(Note the above use of the Python triple-quote syntax, which comes in very handy for specifying multi-line strings like help text.)

When the SConstruct or SConscript files contain a call to the Help function, the specified help text will be displayed in response to the SCons -h option:

% scons -h
scons: Reading SConscript files ...
scons: done reading SConscript files.

Type: 'scons program' to build the production program,
      'scons debug' to build the debug version.

Use scons -H for help about SCons built-in command-line options.

The SConscript files may contain multiple calls to the Help function, in which case the specified text(s) will be concatenated when displayed. This allows you to define fragments of help text together with the corresponding feature, even if spread across multiple SConscript files. In this situation, the order in which the SConscript files are called will determine the order in which the Help functions are called, which will determine the order in which the various bits of text will get concatenated.

Calling Help("text") overwrites the help text that otherwise would be collected from any command-line options defined in AddOption calls. To preserve the AddOption help text, add the append=True keyword argument when calling Help. This also preserves the option help for the scons command itself. To preserve only the AddOption help, also add the local_only=True keyword argument. (This only matters the first time you call Append, on any subsequent calls the text you passed is added to the existing help text).

Another use would be to make the help text conditional on some variable. For example, suppose you only want to display a line about building a Windows-only version of a program when actually run on Windows. The following SConstruct file:

env = Environment()

Help("\nType: 'scons program' to build the production program.\n")

if env['PLATFORM'] == 'win32':
    Help("\nType: 'scons windebug' to build the Windows debug version.\n")
       

Will display the complete help text on Windows:

C:\>scons -h
scons: Reading SConscript files ...
scons: done reading SConscript files.

Type: 'scons program' to build the production program.

Type: 'scons windebug' to build the Windows debug version.

Use scons -H for help about SCons built-in command-line options.

But only show the relevant option on a Linux or UNIX system:

% scons -h
scons: Reading SConscript files ...
scons: done reading SConscript files.

Type: 'scons program' to build the production program.

Use scons -H for help about SCons built-in command-line options.

If there is no Help text in the SConstruct or SConscript files, SCons will revert to displaying its standard list that describes the SCons command-line options. This list is also always displayed whenever the -H option is used.

Sometimes the commands executed to compile object files or link programs (or build other targets) can get very long, long enough to make it difficult for users to distinguish error messages or other important build output from the commands themselves. All of the default $*COM variables that specify the command lines used to build various types of target files have a corresponding $*COMSTR variable that can be set to an alternative string that will be displayed when the target is built.

For example, suppose you want to have SCons display a "Compiling" message whenever it's compiling an object file, and a "Linking" when it's linking an executable. You could write a SConstruct file that looks like:

env = Environment(CCCOMSTR = "Compiling $TARGET",
                  LINKCOMSTR = "Linking $TARGET")
env.Program('foo.c')
       

Which would then yield the output:

% scons -Q
Compiling foo.o
Linking foo
    

SCons performs complete variable substitution on $*COMSTR variables, so they have access to all of the standard variables like $TARGET $SOURCES, etc., as well as any construction variables that happen to be configured in the construction environment used to build a specific target.

Of course, sometimes it's still important to be able to see the exact command that SCons will execute to build a target. For example, you may simply need to verify that SCons is configured to supply the right options to the compiler, or a developer may want to cut-and-paste a compile command to add a few options for a custom test.

One common way to give users control over whether or not SCons should print the actual command line or a short, configured summary is to add support for a VERBOSE command-line variable to your SConstruct file. A simple configuration for this might look like:

env = Environment()
if ARGUMENTS.get('VERBOSE') != '1':
    env['CCCOMSTR'] = "Compiling $TARGET"
    env['LINKCOMSTR'] = "Linking $TARGET"
env.Program('foo.c')
       

By only setting the appropriate $*COMSTR variables if the user specifies VERBOSE=1 on the command line, the user has control over how SCons displays these particular command lines:

% scons -Q
Compiling foo.o
Linking foo
% scons -Q -c
Removed foo.o
Removed foo
% scons -Q VERBOSE=1
cc -o foo.o -c foo.c
cc -o foo foo.o
    

A gentle reminder here: many of the commands for building come in pairs, depending on whether the intent is to build an object for use in a shared library or not. The command strings mirror this, so it may be necessary to set, for example, both CCCOMSTR and SHCCCOMSTR to get the desired results.

Another aspect of providing good build output is to give the user feedback about what SCons is doing even when nothing is being built at the moment. This can be especially true for large builds when most of the targets are already up-to-date. Because SCons can take a long time making absolutely sure that every target is, in fact, up-to-date with respect to a lot of dependency files, it can be easy for users to mistakenly conclude that SCons is hung or that there is some other problem with the build.

One way to deal with this perception is to configure SCons to print something to let the user know what it's "thinking about." The Progress function allows you to specify a string that will be printed for every file that SCons is "considering" while it is traversing the dependency graph to decide what targets are or are not up-to-date.

Progress('Evaluating $TARGET\n')
Program('f1.c')
Program('f2.c')
      

Note that the Progress function does not arrange for a newline to be printed automatically at the end of the string (as does the Python print function), and we must specify the \n that we want printed at the end of the configured string. This configuration, then, will have SCons print that it is Evaluating each file that it encounters in turn as it traverses the dependency graph:

% scons -Q
Evaluating SConstruct
Evaluating f1.c
Evaluating f1.o
cc -o f1.o -c f1.c
Evaluating f1
cc -o f1 f1.o
Evaluating f2.c
Evaluating f2.o
cc -o f2.o -c f2.c
Evaluating f2
cc -o f2 f2.o
Evaluating .

Of course, normally you don't want to add all of these additional lines to your build output, as that can make it difficult for the user to find errors or other important messages. A more useful way to display this progress might be to have the file names printed directly to the user's screen, not to the same standard output stream where build output is printed, and to use a carriage return character (\r) so that each file name gets re-printed on the same line. Such a configuration would look like:

Progress('$TARGET\r',
         file=open('/dev/tty', 'w'),
         overwrite=True)
Program('f1.c')
Program('f2.c')
    

Note that we also specified the overwrite=True argument to the Progress function, which causes SCons to "wipe out" the previous string with space characters before printing the next Progress string. Without the overwrite=True argument, a shorter file name would not overwrite all of the charactes in a longer file name that precedes it, making it difficult to tell what the actual file name is on the output. Also note that we opened up the /dev/tty file for direct access (on POSIX) to the user's screen. On Windows, the equivalent would be to open the con: file name.

Also, it's important to know that although you can use $TARGET to substitute the name of the node in the string, the Progress function does not perform general variable substitution (because there's not necessarily a construction environment involved in evaluating a node like a source file, for example).

You can also specify a list of strings to the Progress function, in which case SCons will display each string in turn. This can be used to implement a "spinner" by having SCons cycle through a sequence of strings:

Progress(['-\r', '\\\r', '|\r', '/\r'], interval=5)
Program('f1.c')
Program('f2.c')
    

Note that here we have also used the interval= keyword argument to have SCons only print a new "spinner" string once every five evaluated nodes. Using an interval= count, even with strings that use $TARGET like our examples above, can be a good way to lessen the work that SCons expends printing Progress strings, while still giving the user feedback that indicates SCons is still working on evaluating the build.

Lastly, you can have direct control over how to print each evaluated node by passing a Python function (or other Python callable) to the Progress function. Your function will be called for each evaluated node, allowing you to implement more sophisticated logic like adding a counter:

screen = open('/dev/tty', 'w')
count = 0
def progress_function(node)
    count += 1
    screen.write('Node %4d: %s\r' % (count, node))

Progress(progress_function)
      

Of course, if you choose, you could completely ignore the node argument to the function, and just print a count, or anything else you wish.

(Note that there's an obvious follow-on question here: how would you find the total number of nodes that will be evaluated so you can tell the user how close the build is to finishing? Unfortunately, in the general case, there isn't a good way to do that, short of having SCons evaluate its dependency graph twice, first to count the total and the second time to actually build the targets. This would be necessary because you can't know in advance which target(s) the user actually requested to be built. The entire build may consist of thousands of Nodes, for example, but maybe the user specifically requested that only a single object file be built.)

SCons, like most build tools, returns zero status to the shell on success and nonzero status on failure. Sometimes it's useful to give more information about the build status at the end of the run, for instance to print an informative message, send an email, or page the poor slob who broke the build.

SCons provides a GetBuildFailures method that you can use in a python atexit function to get a list of objects describing the actions that failed while attempting to build targets. There can be more than one if you're using -j. Here's a simple example:

import atexit

def print_build_failures():
    from SCons.Script import GetBuildFailures
    for bf in GetBuildFailures():
        print("%s failed: %s" % (bf.node, bf.errstr))
atexit.register(print_build_failures)
      

The atexit.register call registers print_build_failures as an atexit callback, to be called before SCons exits. When that function is called, it calls GetBuildFailures to fetch the list of failed objects. See the man page for the detailed contents of the returned objects; some of the more useful attributes are .node, .errstr, .filename, and .command. The filename is not necessarily the same file as the node; the node is the target that was being built when the error occurred, while the filenameis the file or dir that actually caused the error. Note: only call GetBuildFailures at the end of the build; calling it at any other time is undefined.

Here is a more complete example showing how to turn each element of GetBuildFailures into a string:

# Make the build fail if we pass fail=1 on the command line
if ARGUMENTS.get('fail', 0):
    Command('target', 'source', ['/bin/false'])

def bf_to_str(bf):
    """Convert an element of GetBuildFailures() to a string
    in a useful way."""
    import SCons.Errors
    if bf is None: # unknown targets product None in list
        return '(unknown tgt)'
    elif isinstance(bf, SCons.Errors.StopError):
        return str(bf)
    elif bf.node:
        return str(bf.node) + ': ' + bf.errstr
    elif bf.filename:
        return bf.filename + ': ' + bf.errstr
    return 'unknown failure: ' + bf.errstr
import atexit

def build_status():
    """Convert the build status to a 2-tuple, (status, msg)."""
    from SCons.Script import GetBuildFailures
    bf = GetBuildFailures()
    if bf:
        # bf is normally a list of build failures; if an element is None,
        # it's because of a target that scons doesn't know anything about.
        status = 'failed'
        failures_message = "\n".join(["Failed building %s" % bf_to_str(x)
                           for x in bf if x is not None])
    else:
        # if bf is None, the build completed successfully.
        status = 'ok'
        failures_message = ''
    return (status, failures_message)

def display_build_status():
    """Display the build status.  Called by atexit.
    Here you could do all kinds of complicated things."""
    status, failures_message = build_status()
    if status == 'failed':
       print("FAILED!!!!")  # could display alert, ring bell, etc.
    elif status == 'ok':
       print("Build succeeded.")
    print(failures_message)

atexit.register(display_build_status)
      

When this runs, you'll see the appropriate output:

% scons -Q
scons: `.' is up to date.
Build succeeded.
% scons -Q fail=1
scons: *** [target] Source `source' not found, needed by target `target'.
FAILED!!!!
Failed building target: Source `source' not found, needed by target `target'.

SCons provides a number of ways for you as the writer of the SConscript files to give you (and your users) the ability to control the build execution. The arguments that can be specified on the command line are broken down into three types:

Options

Command-line options always begin with one or two - (hyphen) characters. SCons provides ways for you to examine and set options values from within your SConscript files, as well as the ability to define your own custom options. See Section 10.1, “Command-Line Options”, below.

Variables

Any command-line argument containing an = (equal sign) is considered a variable setting with the form variable=value. SCons provides direct access to all of the command-line variable settings, the ability to apply command-line variable settings to construction environments, and functions for configuring specific types of variables (Boolean values, path names, etc.) with automatic validation of the specified values. See Section 10.2, “Command-Line variable=value Build Variables”, below.

Targets

Any command-line argument that is not an option or a variable setting (does not begin with a hyphen and does not contain an equal sign) is considered a target that the you are telling SCons to build. SCons provides access to the list of specified targets, as well as ways to set the default list of targets from within the SConscript files. See Section 10.3, “Command-Line Targets”, below.

SCons has many command-line options that control its behavior. An SCons command-line option always begins with one or two hyphen (-) characters.

You may find yourself using the same command-line options every time you run SCons. For example, you might find it saves time to specify -j 2 to have SCons run up to two build commands in parallel. To avoid having to type -j 2 by hand every time, you can set the external environment variable SCONSFLAGS to a string containing -j 2, as well as any other command-line options that you want SCons to always use. SCONSFLAGS is an exception to the usual rule that SCons itself avoids looking at environment variables from the shell you are running.

If, for example, you are using a POSIX shell such as bash or zsh and you always want SCons to use the -Q option, you can set the SCONSFLAGS environment as follows:

% scons
scons: Reading SConscript files ...
scons: done reading SConscript files.
scons: Building targets ...
    ... [build output] ...
scons: done building targets.
% export SCONSFLAGS="-Q"
% scons
    ... [build output] ...

For csh-style shells on POSIX systems you can set the SCONSFLAGS environment variable as follows:

$ setenv SCONSFLAGS "-Q"
      

For the Windows command shell (cmd) you can set the SCONSFLAGS environment variable as follows:

C:\Users\foo> set SCONSFLAGS="-Q"
      

To set SCONSFLAGS more permanently you can add the setting to the shell's startup file on POSIX systems, and on Windows you can use the System Properties control panel applet to select Environment Variables and set it there.

SCons provides the GetOption function to get the values set by the various command-line options.

One use case for GetOption is to check whether or not the -h or --help option has been specified. Normally, SCons does not print its help text until after it has read all of the SConscript files, because it's possible that help text has been added by some subsidiary SConscript file deep in the source tree hierarchy. Of course, reading all of the SConscript files takes extra time. If you know that your configuration does not define any additional help text in subsidiary SConscript files, you can speed up displaying the command-line help by using the GetOption function to load the subsidiary SConscript files only if the -h or --help option has not been specified like this:

if not GetOption('help'):
    SConscript('src/SConscript', export='env')
      

In general, the string that you pass to the GetOption function to fetch the value of a command-line option setting is the same as the "most common" long option name (beginning with two hyphen characters), although there are some exceptions. The list of SCons command-line options and the GetOption strings for fetching them, are available in the Section 10.1.4, “Strings for Getting or Setting Values of SCons Command-Line Options” section, below.

GetOption can be used to retrieve the values of options defined by calls to AddOption. A GetOption call must appear after the AddOption call for that option. If the AddOption call supplied a dest keyword argument, a string with that name is what to pass as the argument to GetOption, otherwise it is a (possibly modified) version of the first long option name - see AddOption.

You can also set the values of SCons command-line options from within the SConscript files by using the SetOption function. The strings that you use to set the values of SCons command-line options are available in the Section 10.1.4, “Strings for Getting or Setting Values of SCons Command-Line Options” section, below.

One use of the SetOption function is to specify a value for the -j or --jobs option, so that you get the improved performance of a parallel build without having to specify the option by hand. A complicating factor is that a good value for the -j option is somewhat system-dependent. One rough guideline is that the more processors your system has, the higher you want to set the -j value, in order to take advantage of the number of CPUs.

For example, suppose the administrators of your development systems have standardized on setting a NUM_CPU environment variable to the number of processors on each system. A little bit of Python code to access the environment variable and the SetOption function provides the right level of flexibility:

import os

num_cpu = int(os.environ.get('NUM_CPU', 2))
SetOption('num_jobs', num_cpu)
print("running with -j %s" % GetOption('num_jobs'))
        

The above snippet of code sets the value of the --jobs option to the value specified in the NUM_CPU environment variable. (This is one of the exception cases where the string is spelled differently from the from command-line option. The string for fetching or setting the --jobs value is num_jobs for historical reasons.) The code in this example prints the num_jobs value for illustrative purposes. It uses a default value of 2 to provide some minimal parallelism even on single-processor systems:

% scons -Q
running with -j 2
scons: `.' is up to date.

But if the NUM_CPU environment variable is set, then use that for the default number of jobs:

% export NUM_CPU="4"
% scons -Q
running with -j 4
scons: `.' is up to date.

But any explicit -j or --jobs value you specify on the command line is used first, regardless of whether or not the NUM_CPU environment variable is set:

% scons -Q -j 7
running with -j 7
scons: `.' is up to date.
% export NUM_CPU="4"
% scons -Q -j 3
running with -j 3
scons: `.' is up to date.

The strings that you can pass to the GetOption and SetOption functions usually correspond to the first long-form option name (that is, name beginning with two hyphen characters: --), after replacing any remaining hyphen characters with underscores.

SetOption is not currently supported for options added with AddOption.

The full list of strings and the variables they correspond to is as follows:

String for GetOption and SetOption Command-Line Option(s)
cache_debug --cache-debug
cache_disable --cache-disable
cache_force --cache-force
cache_show --cache-show
clean -c, --clean, --remove
config --config
directory -C, --directory
diskcheck --diskcheck
duplicate --duplicate
file -f, --file, --makefile , --sconstruct
help -h, --help
ignore_errors --ignore-errors
implicit_cache --implicit-cache
implicit_deps_changed --implicit-deps-changed
implicit_deps_unchanged --implicit-deps-unchanged
interactive --interact, --interactive
keep_going -k, --keep-going
max_drift --max-drift
no_exec -n, --no-exec, --just-print, --dry-run, --recon
no_site_dir --no-site-dir
num_jobs -j, --jobs
profile_file --profile
question -q, --question
random --random
repository -Y, --repository, --srcdir
silent -s, --silent, --quiet
site_dir --site-dir
stack_size --stack-size
taskmastertrace_file --taskmastertrace
warn --warn --warning

SCons also allows you to define your own command-line options with the AddOption function. The AddOption function takes the same arguments as the add_option method from the standard Python library module optparse. [2]

Once you add a custom command-line option with the AddOption function, the value of the option (if any) is immediately available using the standard GetOption function. The argument to GetOption must be the name of the variable which holds the option. If the dest keyword argument to AddOption is specified, the value is the variable name. given. If not given, it is the name (without the leading hyphens) of the first long option name given to AddOption after replacing any remaining hyphen characters with underscores, since hyphens are not legal in Python identifier names.

SetOption is not currently supported for options added with AddOption.

One useful example of using this functionality is to provide a --prefix to help describe where to install files:

AddOption(
    '--prefix',
    dest='prefix',
    type='string',
    nargs=1,
    action='store',
    metavar='DIR',
    help='installation prefix',
)

env = Environment(PREFIX=GetOption('prefix'))

installed_foo = env.Install('$PREFIX/usr/bin', 'foo.in')
Default(installed_foo)
        

The above code uses the GetOption function to set the $PREFIX construction variable to a value you specify with a command-line option of --prefix. Because $PREFIX expands to a null string if it's not initialized, running SCons without the option of --prefix installs the file in the /usr/bin/ directory:

% scons -Q -n
Install file: "foo.in" as "/usr/bin/foo.in"

But specifying --prefix=/tmp/install on the command line causes the file to be installed in the /tmp/install/usr/bin/ directory:

% scons -Q -n --prefix=/tmp/install
Install file: "foo.in" as "/tmp/install/usr/bin/foo.in"

Note

Option-arguments separated from long options by whitespace, rather than by an =, cannot be correctly resolved by SCons. While --input=ARG is clearly opt followed by arg, for --input ARG it is not possible to tell without instructions whether ARG is an argument belonging to the input option or a positional argument. SCons treats positional arguments as either command-line build options or command-line targets which are made available for use in an SConscript (see the immediately following sections for details). Thus, they must be collected before SConscript processing takes place. Since AddOption calls, which provide the processing instructions to resolve any ambiguity, happen in an SConscript, SCons does not know in time for options added this way, and unexpected things happen, such as option-arguments assigned as targets and/or exceptions due to missing option-arguments.

As a result, this usage style should be avoided when invoking scons. For single-argument options, use the --input=ARG form on the command line. For multiple-argument options (nargs greater than one), set nargs to one in AddOption calls and either: combine the option-arguments into one word with a separator, and parse the result in your own code (see the built-in --debug option, which allows specifying multiple arguments as a single comma-separated word, for an example of such usage); or allow the option to be specified multiple times by setting action='append'. Both methods can be supported at the same time.

You may want to control various aspects of your build by allowing variable=value values to be specified on the command line. For example, suppose you want to be able to build a debug version of a program by running SCons as follows:

% scons -Q debug=1
    

SCons provides an ARGUMENTS dictionary that stores all of the variable=value assignments from the command line. This allows you to modify aspects of your build in response to specifications on the command line. (Note that unless you want to require a variable always be specified you probably want to use the Python dictionary get method, which allows you to designate a default value to be used if there is no specification on the command line.)

The following code sets the $CCFLAGS construction variable in response to the debug flag being set in the ARGUMENTS dictionary:

env = Environment()
debug = ARGUMENTS.get('debug', 0)
if int(debug):
    env.Append(CCFLAGS='-g')
env.Program('prog.c')
       

This results in the -g compiler option being used when debug=1 is used on the command line:

% scons -Q debug=0
cc -o prog.o -c prog.c
cc -o prog prog.o
% scons -Q debug=0
scons: `.' is up to date.
% scons -Q debug=1
cc -o prog.o -c -g prog.c
cc -o prog prog.o
% scons -Q debug=1
scons: `.' is up to date.

SCons keeps track of the precise command line used to build each object file, and as a result can determine that the object and executable files need rebuilding when the value of the debug argument has changed.

The ARGUMENTS dictionary has two minor drawbacks. First, because it is a dictionary, it can only store one value for each specified keyword, and thus only "remembers" the last setting for each keyword on the command line. This makes the ARGUMENTS dictionary less than ideal if you want to allow specifying multiple values on the command line for a given keyword. Second, it does not preserve the order in which the variable settings were specified, which is a problem if you want the configuration to behave differently in response to the order in which the build variable settings were specified on the command line.

To accomodate these requirements, SCons provides an ARGLIST variable that gives you direct access to variable=value settings on the command line, in the exact order they were specified, and without removing any duplicate settings. Each element in the ARGLIST variable is itself a two-element list containing the keyword and the value of the setting, and you must loop through, or otherwise select from, the elements of ARGLIST to process the specific settings you want in whatever way is appropriate for your configuration. For example, the following code lets you add to the CPPDEFINES construction variable by specifying multiple define= settings on the command line:

cppdefines = []
for key, value in ARGLIST:
    if key == 'define':
        cppdefines.append(value)
env = Environment(CPPDEFINES=cppdefines)
env.Object('prog.c')
       

Yields the following output:

% scons -Q define=FOO
cc -o prog.o -c -DFOO prog.c
% scons -Q define=FOO define=BAR
cc -o prog.o -c -DFOO -DBAR prog.c

Note that the ARGLIST and ARGUMENTS variables do not interfere with each other, but rather provide slightly different views into how you specified variable=value settings on the command line. You can use both variables in the same SCons configuration. In general, the ARGUMENTS dictionary is more convenient to use, (since you can just fetch variable settings through Python dictionary access), and the ARGLIST list is more flexible (since you can examine the specific order in which the command-line variable settings were given).

Being able to use a command-line build variable like debug=1 is handy, but it can be a chore to write specific Python code to recognize each such variable, check for errors and provide appropriate messages, and apply the values to a construction variable. To help with this, SCons provides a Variables class to define such build variables easily, and a mechanism to apply the build variables to a construction environment. This allows you to control how the build variables affect construction environments.

For example, suppose that you want to set a RELEASE construction variable on the command line whenever the time comes to build a program for release, and that the value of this variable should be added to the command line with the appropriate define to pass the value to the C compiler. Here's how you might do that by setting the appropriate value in a dictionary for the $CPPDEFINES construction variable:

vars = Variables(None, ARGUMENTS)
vars.Add('RELEASE', default=0)
env = Environment(variables=vars, CPPDEFINES={'RELEASE_BUILD': '${RELEASE}'})
env.Program(['foo.c', 'bar.c'])
        

This SConstruct file first creates a Variables object which uses the values from the command-line options dictionary ARGUMENTS (the vars=Variables(None, ARGUMENTS) call). It then uses the object's Add method to indicate that the RELEASE variable can be set on the command line, and that if not set the default value is 0. The newly created Variables object is passed to the Environment call used to create the construction environment using a variables keyword argument. This then allows you to set the RELEASE build variable on the command line and have the variable show up in the command line used to build each object from a C source file:

% scons -Q RELEASE=1
cc -o bar.o -c -DRELEASE_BUILD=1 bar.c
cc -o foo.o -c -DRELEASE_BUILD=1 foo.c
cc -o foo foo.o bar.o

Historical note: In old SCons (prior to 0.98.1), these build variables were known as "command-line build options." At that time, class was named Options and the predefined functions to construct options were named BoolOption, EnumOption, ListOption, PathOption, PackageOption and AddOptions (contrast with the current names in Section 10.2.4, “Pre-Defined Build Variable Functions”, below). You may encounter these names in older SConscript files, wiki pages, blog entries, StackExchange articles, etc. These old names no longer work, but a mental substitution of Variable for Option allows the concepts to transfer to current usage models.

SCons provides a number of convenience functions that provide ready-made behaviors for various types of command-line build variables. These functions all return a tuple which is ready to be passed to the Add or AddVariables method call. You are of course free to define your own behaviors as well.

It is often handy to be able to specify a variable that controls a simple Boolean variable with a true or false value. It would be even more handy to accomodate different preferences for how to represent true or false values. The BoolVariable function makes it easy to accomodate these common representations of true or false.

The BoolVariable function takes three arguments: the name of the build variable, the default value of the build variable, and the help string for the variable. It then returns appropriate information for passing to the Add method of a Variables object, like so:

vars = Variables('custom.py')
vars.Add(BoolVariable('RELEASE', help='Set to build for release', default=False))
env = Environment(variables=vars, CPPDEFINES={'RELEASE_BUILD': '${RELEASE}'})
env.Program('foo.c')
          

With this build variable in place, the RELEASE variable can now be enabled by setting it to the value yes or t:

% scons -Q RELEASE=yes foo.o
cc -o foo.o -c -DRELEASE_BUILD=True foo.c
% scons -Q RELEASE=t foo.o
cc -o foo.o -c -DRELEASE_BUILD=True foo.c

Other values that equate to true include y, 1, on and all.

Conversely, RELEASE may now be given a false value by setting it to no or f:

% scons -Q RELEASE=no foo.o
cc -o foo.o -c -DRELEASE_BUILD=False foo.c
% scons -Q RELEASE=f foo.o
cc -o foo.o -c -DRELEASE_BUILD=False foo.c

Other values that equate to false include n, 0, off and none.

Lastly, if you try to specify any other value, SCons supplies an appropriate error message:

% scons -Q RELEASE=bad_value foo.o

scons: *** Error converting option: RELEASE
Invalid value for boolean option: bad_value
File "/home/my/project/SConstruct", line 3, in <module>

Suppose that you want to allow setting a COLOR variable that selects a background color to be displayed by an application, but that you want to restrict the choices to a specific set of allowed colors. You can set this up quite easily using the EnumVariable function, which takes a list of allowed_values in addition to the variable name, default value, and help text arguments:

vars = Variables('custom.py')
vars.Add(
    EnumVariable(
        'COLOR',
        help='Set background color',
        default='red',
        allowed_values=('red', 'green', 'blue'),
    )
)
env = Environment(variables=vars, CPPDEFINES={'COLOR': '"${COLOR}"'})
env.Program('foo.c')
Help(vars.GenerateHelpText(env))
          

You can now explicitly set the COLOR build variable to any of the specified allowed values:

% scons -Q COLOR=red foo.o
cc -o foo.o -c -DCOLOR="red" foo.c
% scons -Q COLOR=blue foo.o
cc -o foo.o -c -DCOLOR="blue" foo.c
% scons -Q COLOR=green foo.o
cc -o foo.o -c -DCOLOR="green" foo.c

But, importantly, an attempt to set COLOR to a value that's not in the list generates an error message:

% scons -Q COLOR=magenta foo.o

scons: *** Invalid value for option COLOR: magenta.  Valid values are: ('red', 'green', 'blue')
File "/home/my/project/SConstruct", line 10, in <module>

This example can also serve to further illustrate help generation: the help message here picks up not only the help text, but augments it with information gathered from allowed_values and default:

% scons -Q -h

COLOR: Set background color (red|green|blue)
    default: red
    actual: red

Use scons -H for help about SCons built-in command-line options.

The EnumVariable function also provides a way to map alternate names to allowed values. Suppose, for example, you want to allow the word navy to be used as a synonym for blue. You do this by adding a map dictionary that maps its key values to the desired allowed value:

vars = Variables('custom.py')
vars.Add(
    EnumVariable(
        'COLOR',
        help='Set background color',
        default='red',
        allowed_values=('red', 'green', 'blue'),
        map={'navy': 'blue'},
    )
)
env = Environment(variables=vars, CPPDEFINES={'COLOR': '"${COLOR}"'})
env.Program('foo.c')
          

Now you can supply navy on the command line, and SCons translates that into blue when it comes time to use the COLOR variable to build a target:

% scons -Q COLOR=navy foo.o
cc -o foo.o -c -DCOLOR="blue" foo.c

By default, when using the EnumVariable function, the allowed values are case-sensitive:

% scons -Q COLOR=Red foo.o

scons: *** Invalid value for option COLOR: Red.  Valid values are: ('red', 'green', 'blue')
File "/home/my/project/SConstruct", line 10, in <module>
% scons -Q COLOR=BLUE foo.o

scons: *** Invalid value for option COLOR: BLUE.  Valid values are: ('red', 'green', 'blue')
File "/home/my/project/SConstruct", line 10, in <module>
% scons -Q COLOR=nAvY foo.o

scons: *** Invalid value for option COLOR: nAvY.  Valid values are: ('red', 'green', 'blue')
File "/home/my/project/SConstruct", line 10, in <module>

The EnumVariable function can take an additional ignorecase keyword argument that, when set to 1, tells SCons to allow case differences when the values are specified:

vars = Variables('custom.py')
vars.Add(
    EnumVariable(
        'COLOR',
        help='Set background color',
        default='red',
        allowed_values=('red', 'green', 'blue'),
        map={'navy': 'blue'},
        ignorecase=1,
    )
)
env = Environment(variables=vars, CPPDEFINES={'COLOR': '"${COLOR}"'})
env.Program('foo.c')
          

Which yields the output:

% scons -Q COLOR=Red foo.o
cc -o foo.o -c -DCOLOR="Red" foo.c
% scons -Q COLOR=BLUE foo.o
cc -o foo.o -c -DCOLOR="BLUE" foo.c
% scons -Q COLOR=nAvY foo.o
cc -o foo.o -c -DCOLOR="blue" foo.c
% scons -Q COLOR=green foo.o
cc -o foo.o -c -DCOLOR="green" foo.c

Notice that an ignorecase value of 1 preserves the case-spelling supplied, only ignoring the case for matching. If you want SCons to translate the names into lower-case, regardless of the case used by the user, specify an ignorecase value of 2:

vars = Variables('custom.py')
vars.Add(
    EnumVariable(
        'COLOR',
        help='Set background color',
        default='red',
        allowed_values=('red', 'green', 'blue'),
        map={'navy': 'blue'},
        ignorecase=2,
    )
)
env = Environment(variables=vars, CPPDEFINES={'COLOR': '"${COLOR}"'})
env.Program('foo.c')
          

Now SCons uses values of red, green or blue regardless of how those values are spelled on the command line:

% scons -Q COLOR=Red foo.o
cc -o foo.o -c -DCOLOR="red" foo.c
% scons -Q COLOR=nAvY foo.o
cc -o foo.o -c -DCOLOR="blue" foo.c
% scons -Q COLOR=GREEN foo.o
cc -o foo.o -c -DCOLOR="green" foo.c

Another way in which you might want to control a build variable is to specify a list of allowed values, of which one or more can be chosen (where EnumVariable allows exactly one value to be chosen). SCons provides this through the ListVariable function. If, for example, you want to be able to set a COLORS variable to one or more of the allowed values:

vars = Variables('custom.py')
vars.Add(
    ListVariable(
        'COLORS', help='List of colors', default=0, names=['red', 'green', 'blue']
    )
)
env = Environment(variables=vars, CPPDEFINES={'COLORS': '"${COLORS}"'})
env.Program('foo.c')
          

You can now specify a comma-separated list of allowed values, which get translated into a space-separated list for passing to the build commands:

% scons -Q COLORS=red,blue foo.o
cc -o foo.o -c -DCOLORS="red -Dblue" foo.c
% scons -Q COLORS=blue,green,red foo.o
cc -o foo.o -c -DCOLORS="blue -Dgreen -Dred" foo.c

In addition, the ListVariable function lets you specify explicit keywords of all or none to select all of the allowed values, or none of them, respectively:

% scons -Q COLORS=all foo.o
cc -o foo.o -c -DCOLORS="red -Dgreen -Dblue" foo.c
% scons -Q COLORS=none foo.o
cc -o foo.o -c -DCOLORS="" foo.c

And, of course, an illegal value still generates an error message:

% scons -Q COLORS=magenta foo.o

scons: *** Error converting option: COLORS
Invalid value(s) for option: magenta
File "/home/my/project/SConstruct", line 7, in <module>

You can use this last characteristic as a way to enforce at least one of your valid options being chosen by specifying the valid values with the names parameter and then giving a value not in that list as the default parameter - that way if no value is given on the command line, the default is chosen, SCons errors out as this is invalid. The example is, in fact, set up that way by using 0 as the default:

% scons -Q foo.o

scons: *** Error converting option: COLORS
Invalid value(s) for option: 0
File "/home/my/project/SConstruct", line 7, in <module>

This technique works for EnumVariable as well.

SCons provides a PathVariable function to make it easy to create a build variable to control an expected path name. If, for example, you need to define a preprocessor macro that controls the location of a configuration file:

vars = Variables('custom.py')
vars.Add(
    PathVariable(
        'CONFIG', help='Path to configuration file', default='/etc/my_config'
    )
)
env = Environment(variables=vars, CPPDEFINES={'CONFIG_FILE': '"$CONFIG"'})
env.Program('foo.c')
          

This allows you to override the CONFIG build variable on the command line as necessary:

% scons -Q foo.o
cc -o foo.o -c -DCONFIG_FILE="/etc/my_config" foo.c
% scons -Q CONFIG=/usr/local/etc/other_config foo.o
scons: `foo.o' is up to date.

By default, PathVariable checks to make sure that the specified path exists and generates an error if it doesn't:

% scons -Q CONFIG=/does/not/exist foo.o

scons: *** Path for option CONFIG does not exist: /does/not/exist
File "/home/my/project/SConstruct", line 7, in <module>

PathVariable provides a number of methods that you can use to change this behavior. If you want to ensure that any specified paths are, in fact, files and not directories, use the PathVariable.PathIsFile method as the validation function:

vars = Variables('custom.py')
vars.Add(
    PathVariable(
        'CONFIG',
        help='Path to configuration file',
        default='/etc/my_config',
        validator=PathVariable.PathIsFile,
    )
)
env = Environment(variables=vars, CPPDEFINES={'CONFIG_FILE': '"$CONFIG"'})
env.Program('foo.c')
          

Conversely, to ensure that any specified paths are directories and not files, use the PathVariable.PathIsDir method as the validation function:

vars = Variables('custom.py')
vars.Add(
    PathVariable(
        'DBDIR',
        help='Path to database directory',
        default='/var/my_dbdir',
        validator=PathVariable.PathIsDir,
    )
)
env = Environment(variables=vars, CPPDEFINES={'DBDIR': '"$DBDIR"'})
env.Program('foo.c')
          

If you want to make sure that any specified paths are directories, and you would like the directory created if it doesn't already exist, use the PathVariable.PathIsDirCreate method as the validation function:

vars = Variables('custom.py')
vars.Add(
    PathVariable(
        'DBDIR',
        help='Path to database directory',
        default='/var/my_dbdir',
        validator=PathVariable.PathIsDirCreate,
    )
)
env = Environment(variables=vars, CPPDEFINES={'DBDIR': '"$DBDIR"'})
env.Program('foo.c')
          

Lastly, if you don't care whether the path exists, is a file, or a directory, use the PathVariable.PathAccept method to accept any path you supply:

vars = Variables('custom.py')
vars.Add(
    PathVariable(
        'OUTPUT',
        help='Path to output file or directory',
        default=None,
        validator=PathVariable.PathAccept,
    )
)
env = Environment(variables=vars, CPPDEFINES={'OUTPUT': '"$OUTPUT"'})
env.Program('foo.c')
          

Humans, of course, occasionally misspell variable names in their command-line settings. SCons does not generate an error or warning for any unknown variables specified on the command line, because it can not reliably tell whether a given "misspelled" variable is really unknown and a potential problem or not. After all, you might be processing arguments directly using ARGUMENTS or ARGLIST with some Python code in your SConscript file.

If, however, you are using a Variables object to define a specific set of command-line build variables that you expect to be able to set, you may want to provide an error message or warning of your own if a variable setting is specified that is not among the defined list of variable names known to the Variables object. You can do this by calling the UnknownVariables method of the Variables object to get the settings Variables did not recognize:

vars = Variables(None)
vars.Add('RELEASE', help='Set to 1 to build for release', default=0)
env = Environment(variables=vars, CPPDEFINES={'RELEASE_BUILD': '${RELEASE}'})
unknown = vars.UnknownVariables()
if unknown:
    print("Unknown variables: %s" % " ".join(unknown.keys()))
    Exit(1)
env.Program('foo.c')
        

The UnknownVariables method returns a dictionary containing the keywords and values of any variables specified on the command line that are not among the variables known to the Variables object (from having been specified using the Variables object's Add method). The example above, checks whether the dictionary returned by UnknownVariables is non-empty, and if so prints the Python list containing the names of the unknown variables and then calls the Exit function to terminate SCons:

% scons -Q NOT_KNOWN=foo
Unknown variables: NOT_KNOWN

Of course, you can process the items in the dictionary returned by the UnknownVariables function in any way appropriate to your build configuration, including just printing a warning message but not exiting, logging an error somewhere, etc.

Note that you must delay the call of UnknownVariables until after you have applied the Variables object to a construction environment with the variables= keyword argument of an Environment call: the variables in the object are not fully processed until this has happened.

You can control which targets SCons builds by default - that is, when there are no targets specified on the command line. As mentioned previously, SCons normally builds every target in or below the current directory unless you explicitly specify one or more targets on the command line. Sometimes, however, you may want to specify that only certain programs, or programs in certain directories, should be built by default. You do this with the Default function:

env = Environment()
hello = env.Program('hello.c')
env.Program('goodbye.c')
Default(hello)
         

This SConstruct file knows how to build two programs, hello and goodbye, but only builds the hello program by default:

% scons -Q
cc -o hello.o -c hello.c
cc -o hello hello.o
% scons -Q
scons: `hello' is up to date.
% scons -Q goodbye
cc -o goodbye.o -c goodbye.c
cc -o goodbye goodbye.o

Note that, even when you use the Default function in your SConstruct file, you can still explicitly specify the current directory (.) on the command line to tell SCons to build everything in (or below) the current directory:

% scons -Q .
cc -o goodbye.o -c goodbye.c
cc -o goodbye goodbye.o
cc -o hello.o -c hello.c
cc -o hello hello.o

You can also call the Default function more than once, in which case each call adds to the list of targets to be built by default:

env = Environment()
prog1 = env.Program('prog1.c')
Default(prog1)
prog2 = env.Program('prog2.c')
prog3 = env.Program('prog3.c')
Default(prog3)
         

Or you can specify more than one target in a single call to the Default function:

env = Environment()
prog1 = env.Program('prog1.c')
prog2 = env.Program('prog2.c')
prog3 = env.Program('prog3.c')
Default(prog1, prog3)
      

Either of these last two examples build only the prog1 and prog3 programs by default:

% scons -Q
cc -o prog1.o -c prog1.c
cc -o prog1 prog1.o
cc -o prog3.o -c prog3.c
cc -o prog3 prog3.o
% scons -Q .
cc -o prog2.o -c prog2.c
cc -o prog2 prog2.o

You can list a directory as an argument to Default:

env = Environment()
env.Program(['prog1/main.c', 'prog1/foo.c'])
env.Program(['prog2/main.c', 'prog2/bar.c'])
Default('prog1')
         

In which case only the target(s) in that directory are built by default:

% scons -Q
cc -o prog1/foo.o -c prog1/foo.c
cc -o prog1/main.o -c prog1/main.c
cc -o prog1/main prog1/main.o prog1/foo.o
% scons -Q
scons: `prog1' is up to date.
% scons -Q .
cc -o prog2/bar.o -c prog2/bar.c
cc -o prog2/main.o -c prog2/main.c
cc -o prog2/main prog2/main.o prog2/bar.o

Lastly, if for some reason you don't want any targets built by default, you can use the Python None variable:

env = Environment()
prog1 = env.Program('prog1.c')
prog2 = env.Program('prog2.c')
Default(None)
         

Which would produce build output like:

% scons -Q
scons: *** No targets specified and no Default() targets found.  Stop.
Found nothing to build
% scons -Q .
cc -o prog1.o -c prog1.c
cc -o prog1 prog1.o
cc -o prog2.o -c prog2.c
cc -o prog2 prog2.o

SCons provides a DEFAULT_TARGETS variable that lets you get at the current list of default targets specified by calls to the Default function or method. The DEFAULT_TARGETS variable has two important differences from the COMMAND_LINE_TARGETS variable. First, the DEFAULT_TARGETS variable is a list of internal SCons nodes, so you need to convert the list elements to strings if you want to print them or look for a specific target name. You can do this easily by calling the str on the elements in a list comprehension:

prog1 = Program('prog1.c')
Default(prog1)
print("DEFAULT_TARGETS is %s" % [str(t) for t in DEFAULT_TARGETS])
           

(Keep in mind that all of the manipulation of the DEFAULT_TARGETS list takes place during the first phase when SCons is reading up the SConscript files, which is obvious if you leave off the -Q flag when you run SCons:)

% scons
scons: Reading SConscript files ...
DEFAULT_TARGETS is ['prog1']
scons: done reading SConscript files.
scons: Building targets ...
cc -o prog1.o -c prog1.c
cc -o prog1 prog1.o
scons: done building targets.

Second, the contents of the DEFAULT_TARGETS list changes in response to calls to the Default function, as you can see from the following SConstruct file:

prog1 = Program('prog1.c')
Default(prog1)
print("DEFAULT_TARGETS is now %s" % [str(t) for t in DEFAULT_TARGETS])
prog2 = Program('prog2.c')
Default(prog2)
print("DEFAULT_TARGETS is now %s" % [str(t) for t in DEFAULT_TARGETS])
           

Which yields the output:

% scons
scons: Reading SConscript files ...
DEFAULT_TARGETS is now ['prog1']
DEFAULT_TARGETS is now ['prog1', 'prog2']
scons: done reading SConscript files.
scons: Building targets ...
cc -o prog1.o -c prog1.c
cc -o prog1 prog1.o
cc -o prog2.o -c prog2.c
cc -o prog2 prog2.o
scons: done building targets.

In practice, this simply means that you need to pay attention to the order in which you call the Default function and refer to the DEFAULT_TARGETS list, to make sure that you don't examine the list before you have added the default targets you expect to find in it.

You have already seen the COMMAND_LINE_TARGETS variable, which contains a list of targets specified on the command line, and the DEFAULT_TARGETS variable, which contains a list of targets specified via calls to the Default method or function. Sometimes, however, you want a list of whatever targets SCons tries to build, regardless of whether the targets came from the command line or a Default call. You could code this up by hand, as follows:

if COMMAND_LINE_TARGETS:
    targets = COMMAND_LINE_TARGETS
else:
    targets = DEFAULT_TARGETS
      

SCons, however, provides a convenient BUILD_TARGETS variable that eliminates the need for this by-hand manipulation. Essentially, the BUILD_TARGETS variable contains a list of the command-line targets, if any were specified, and if no command-line targets were specified, it contains a list of the targets specified via the Default method or function.

Because BUILD_TARGETS may contain a list of SCons nodes, you must convert the list elements to strings if you want to print them or look for a specific target name, just like the DEFAULT_TARGETS list:

prog1 = Program('prog1.c')
Program('prog2.c')
Default(prog1)
print("BUILD_TARGETS is %s" % [str(t) for t in BUILD_TARGETS])
        

Notice how the value of BUILD_TARGETS changes depending on whether a target is specified on the command line - BUILD_TARGETS takes from DEFAULT_TARGETS only if there are no COMMAND_LINE_TARGETS:

% scons -Q
BUILD_TARGETS is ['prog1']
cc -o prog1.o -c prog1.c
cc -o prog1 prog1.o
% scons -Q prog2
BUILD_TARGETS is ['prog2']
cc -o prog2.o -c prog2.c
cc -o prog2 prog2.o
% scons -Q -c .
BUILD_TARGETS is ['.']
Removed prog1.o
Removed prog1
Removed prog2.o
Removed prog2


[2] The AddOption function is, in fact, implemented using a subclass of optparse.OptionParser.

Once a program is built, it is often appropriate to install it in another directory for public use. You use the Install method to arrange for a program, or any other file, to be copied into a destination directory:

env = Environment()
hello = env.Program('hello.c')
env.Install('/usr/bin', hello)
     

Note, however, that installing a file is still considered a type of file "build." This is important when you remember that the default behavior of SCons is to build files in or below the current directory. If, as in the example above, you are installing files in a directory outside of the top-level SConstruct file's directory tree, you must specify that directory (or a higher directory, such as /) for it to install anything there:

% scons -Q
cc -o hello.o -c hello.c
cc -o hello hello.o
% scons -Q /usr/bin
Install file: "hello" as "/usr/bin/hello"

It can, however, be cumbersome to remember (and type) the specific destination directory in which the program (or other file) should be installed. A call to Default can be used to add the directory to the list of default targets, removing the need to type it, but sometimes you don't want to install on every build. This is an area where the Alias function comes in handy, allowing you, for example, to create a pseudo-target named install that can expand to the specified destination directory:

env = Environment()
hello = env.Program('hello.c')
env.Install('/usr/bin', hello)
env.Alias('install', '/usr/bin')
    

This then yields the more natural ability to install the program in its destination as a separate invocation, as follows:

% scons -Q
cc -o hello.o -c hello.c
cc -o hello hello.o
% scons -Q install
Install file: "hello" as "/usr/bin/hello"

If a shared library is created with the $SHLIBVERSION variable set, scons will create symbolic links as needed based on that variable. To properly install such a library including the symbolic links, use the InstallVersionedLib function.

For example, on a Linux system, this instruction:

foo =  env.SharedLibrary(target="foo", source="foo.c", SHLIBVERSION="1.2.3")
    

Will produce a shared library libfoo.so.1.2.3 and symbolic links libfoo.so and libfoo.so.1 which point to libfoo.so.1.2.3. You can use the Node returned by the SharedLibrary builder in order to install the library and its symbolic links in one go without having to list them individually:

env.InstallVersionedLib(target="lib", source=foo)
    

On systems which expect a shared library to be installed both with a name that indicates the version, for run-time resolution, and as a plain name, for link-time resolution, the InstallVersionedLib function can be used. Symbolic links appropriate to the type of system will be generated based on symlinks of the source library.

SCons provides a number of platform-independent functions, called factories, that perform common file system manipulations like copying, moving or deleting files and directories, or making directories. These functions are factories because they don't perform the action at the time they're called, they each return an Action object that can be executed at the appropriate time.

Suppose you want to arrange to make a copy of a file, and don't have a suitable pre-existing builder. [3] One way would be to use the Copy action factory in conjunction with the Command builder:

Command("file.out", "file.in", Copy("$TARGET", "$SOURCE"))
      

Notice that the action returned by the Copy factory will expand the $TARGET and $SOURCE strings at the time file.out is built, and that the order of the arguments is the same as that of a builder itself--that is, target first, followed by source:

% scons -Q
Copy("file.out", "file.in")

You can, of course, name a file explicitly instead of using $TARGET or $SOURCE:

Command("file.out", [], Copy("$TARGET", "file.in"))
      

Which executes as:

% scons -Q
Copy("file.out", "file.in")

The usefulness of the Copy factory becomes more apparent when you use it in a list of actions passed to the Command builder. For example, suppose you needed to run a file through a utility that only modifies files in-place, and can't "pipe" input to output. One solution is to copy the source file to a temporary file name, run the utility, and then copy the modified temporary file to the target, which the Copy factory makes extremely easy:

Command(
    "file.out",
    "file.in",
    action=[
        Copy("tempfile", "$SOURCE"),
        "modify tempfile",
        Copy("$TARGET", "tempfile"),
    ],
)
      

The output then looks like:

% scons -Q
Copy("tempfile", "file.in")
modify tempfile
Copy("file.out", "tempfile")

The Copy factory has a third optional argument which controls how symlinks are copied.

# Symbolic link shallow copied as a new symbolic link:
Command("LinkIn", "LinkOut", Copy("$TARGET", "$SOURCE", symlinks=True))

# Symbolic link target copied as a file or directory:
Command("LinkIn", "FileOrDirectoryOut", Copy("$TARGET", "$SOURCE", symlinks=False))
      

If you need to delete a file, then the Delete factory can be used in much the same way as the Copy factory. For example, if we want to make sure that the temporary file in our last example doesn't exist before we copy to it, we could add Delete to the beginning of the command list:

Command(
    "file.out",
    "file.in",
    action=[
        Delete("tempfile"),
        Copy("tempfile", "$SOURCE"),
        "modify tempfile",
        Copy("$TARGET", "tempfile"),
    ],
)
      

Which then executes as follows:

% scons -Q
Delete("tempfile")
Copy("tempfile", "file.in")
modify tempfile
Copy("file.out", "tempfile")

Of course, like all of these Action factories, the Delete factory also expands $TARGET and $SOURCE variables appropriately. For example:

Command(
    "file.out",
    "file.in",
    action=[
        Delete("$TARGET"),
        Copy("$TARGET", "$SOURCE"),
    ],
)
      

Executes as:

% scons -Q
Delete("file.out")
Copy("file.out", "file.in")

Note, however, that you typically don't need to call the Delete factory explicitly in this way; by default, SCons deletes its target(s) for you before executing any action.

One word of caution about using the Delete factory: it has the same variable expansions available as any other factory, including the $SOURCE variable. Specifying Delete("$SOURCE") is not something you usually want to do!

We've been showing you how to use Action factories in the Command function. You can also execute an Action returned by a factory (or actually, any Action) at the time the SConscript file is read by using the Execute function. For example, if we need to make sure that a directory exists before we build any targets,

Execute(Mkdir('/tmp/my_temp_directory'))
      

Notice that this will create the directory while the SConscript file is being read:

% scons
scons: Reading SConscript files ...
Mkdir("/tmp/my_temp_directory")
scons: done reading SConscript files.
scons: Building targets ...
scons: `.' is up to date.
scons: done building targets.

If you're familiar with Python, you may wonder why you would want to use this instead of just calling the native Python os.mkdir() function. The advantage here is that the Mkdir action will behave appropriately if the user specifies the SCons -n or -q options--that is, it will print the action but not actually make the directory when -n is specified, or make the directory but not print the action when -q is specified.

The Execute function returns the exit status or return value of the underlying action being executed. It will also print an error message if the action fails and returns a non-zero value. SCons will not, however, actually stop the build if the action fails. If you want the build to stop in response to a failure in an action called by Execute, you must do so by explicitly checking the return value and calling the Exit function (or a Python equivalent):

if Execute(Mkdir('/tmp/my_temp_directory')):
    # A problem occurred while making the temp directory.
    Exit(1)
    


[3] Unfortunately, in the early days of SCons design, we used the name Copy for the function that returns a copy of the environment, otherwise that would be the logical choice for a Builder that copies a file or directory tree to a target location.

There are two occasions when SCons will, by default, remove target files. The first is when SCons determines that an target file needs to be rebuilt and removes the existing version of the target before executing The second is when SCons is invoked with the -c option to "clean" a tree of its built targets. These behaviours can be suppressed with the Precious and NoClean functions, respectively.

The source code for large software projects rarely stays in a single directory, but is nearly always divided into a hierarchy of directories. Organizing a large software build using SCons involves creating a hierarchy of build scripts which are connected together using the SConscript function.

Subsidiary SConscript files make it easy to create a build hierarchy because all of the file and directory names in a subsidiary SConscript files are interpreted relative to the directory in which that SConscript file lives. Typically, this allows the SConscript file containing the instructions to build a target file to live in the same directory as the source files from which the target will be built, making it easy to update how the software is built whenever files are added or deleted (or other changes are made). It also tends to keep scripts more readable as they don't need to be filled with complex paths.

For example, suppose we want to build two programs prog1 and prog2 in two separate directories with the same names as the programs. One typical way to do this would be with a top-level SConstruct file like this:

SConscript(['prog1/SConscript', 'prog2/SConscript'])
      

And subsidiary SConscript files that look like this:

env = Environment()
env.Program('prog1', ['main.c', 'foo1.c', 'foo2.c'])
      

And this:

env = Environment()
env.Program('prog2', ['main.c', 'bar1.c', 'bar2.c'])
      

Then, when we run SCons in the top-level directory, our build looks like:

% scons -Q
cc -o prog1/foo1.o -c prog1/foo1.c
cc -o prog1/foo2.o -c prog1/foo2.c
cc -o prog1/main.o -c prog1/main.c
cc -o prog1/prog1 prog1/main.o prog1/foo1.o prog1/foo2.o
cc -o prog2/bar1.o -c prog2/bar1.c
cc -o prog2/bar2.o -c prog2/bar2.c
cc -o prog2/main.o -c prog2/main.c
cc -o prog2/prog2 prog2/main.o prog2/bar1.o prog2/bar2.o

Notice the following: First, you can have files with the same names in multiple directories, like main.c in the above example. Second, when building, SCons stays in the top-level directory (where the SConstruct file lives) and issues commands that use the path names from the top-level directory to the target and source files within the hierarchy. This works because SCons reads all the SConscript files in one pass, interpreting each in its local context, building up a tree of information, before starting to execute the needed builds in a second pass. This is quite different than some other build tools which implement a heirarcical build by recursing.

If you need to use a file from another directory, it's sometimes more convenient to specify the path to a file in another directory from the top-level SConstruct directory, even when you're using that file in a subsidiary SConscript file in a subdirectory. You can tell SCons to interpret a path name as relative to the top-level SConstruct directory, not the local directory of the SConscript file, by prepending a # (hash mark) in front of the path name:

env = Environment()
env.Program('prog', ['main.c', '#lib/foo1.c', 'foo2.c'])
       

In this example, the lib directory is directly underneath the top-level SConstruct directory. If the above SConscript file is in a subdirectory named src/prog, the output would look like:

% scons -Q
cc -o lib/foo1.o -c lib/foo1.c
cc -o src/prog/foo2.o -c src/prog/foo2.c
cc -o src/prog/main.o -c src/prog/main.c
cc -o src/prog/prog src/prog/main.o lib/foo1.o src/prog/foo2.o

(Notice that the lib/foo1.o object file is built in the same directory as its source file. See Chapter 15, Separating Source and Build Trees: Variant Directories, below, for information about how to build the object file in a different subdirectory.)

A couple of notes on top-relative paths:

  1. SCons doesn't care whether you add a slash after the #. Some people consider '#/lib/foo1.c' more readable than '#lib/foo1.c', but they're functionally equivalent.

  2. The top-relative syntax is only evaluated by SCons, the Python language itself does not understand about it. This becomes immediately obvious if you like to use print for debugging, or write a Python function that wants to evaluate a path. You can force SCons to evaluate a top-relative path and produce a string that can be used by Python code by creating a Node object from it:

path = "#/include"

print("path =", path)
print("force-interpreted path =", Entry(path))
        

Which shows:

% scons -Q
path = #/include
force-interpreted path = include
scons: `.' is up to date.

In the previous example, each of the subsidiary SConscript files created its own construction environment by calling Environment separately. This obviously works fine, but if each program must be built with the same construction variables, it's cumbersome and error-prone to initialize separate construction environments in the same way over and over in each subsidiary SConscript file.

SCons supports the ability to export variables from an SConscript file so they can be imported by other SConscript files, thus allowing you to share common initialized values throughout your build hierarchy.

There are two ways to export a variable from an SConscript file. The first way is to call the Export function. Export is pretty flexible - in the simplest form, you pass it a string that represents the name of the variable, and Export stores that with its value:

env = Environment()
Export('env')
      

You may export more than one variable name at a time:

env = Environment()
debug = ARGUMENTS['debug']
Export('env', 'debug')
      

Because a Python identifier cannot contain spaces, Export assumes a string containing spaces is is a shortcut for multiple variable names to export and splits it up for you:

env = Environment()
debug = ARGUMENTS['debug']
Export('env debug')
      

You can also pass Export a dictionary of values. This form allows the opportunity to export a variable from the current scope under a different name - in this example, the value of foo is exported under the name "bar":

env = Environment()
foo = "FOO"
args = {"env": env, "bar": foo}
Export(args)
      

Export will also accept arguments in keyword style. This form adds the ability to create exported variables that have not actually been set locally in the SConscript file. When used this way, the key is the intended variable name, not a string representation as with the other forms:

Export(MODE="DEBUG", TARGET="arm")
      

The styles can be mixed, though Python function calling syntax requires all non-keyword arguments to precede any keyword arguments in the call.

The Export function adds the variables to a global location from which other SConscript files can import. Calls to Export are cumulative. When you call Export you are actually updating a Python dictionary, so it is fine to export a variable you have already exported, but when doing so, the previous value is lost.

The other way to export is you can specify a list of variables as a second argument to the SConscript function call:

SConscript('src/SConscript', 'env')
      

Or (preferably, for readability) using the exports keyword argument:

SConscript('src/SConscript', exports='env')
      

These calls export the specified variables to only the listed SConscript file(s). You may specify more than one SConscript file in a list:

SConscript(['src1/SConscript', 'src2/SConscript'], exports='env')
      

This is functionally equivalent to calling the SConscript function multiple times with the same exports argument, one per SConscript file.

Once a variable has been exported from a calling SConscript file, it may be used in other SConscript files by calling the Import function:

Import('env')
env.Program('prog', ['prog.c'])
      

The Import call makes the previously defined env variable available to the SConscript file. Assuming env is a construction environment, after import it can be used to build programs, libraries, etc. The use case of passing around a construction environment is extremely common in larger scons builds.

Like the Export function, the Import function can be called with multiple variable names:

Import('env', 'debug')
env = env.Clone(DEBUG=debug)
env.Program('prog', ['prog.c'])
      

In this example, we pull in the common construction environment env, and use the value of the debug variable to make a modified copy by passing that to a Clone call.

The Import function will (like Export) split a string containing white-space into separate variable names:

Import('env debug')
env = env.Clone(DEBUG=debug)
env.Program('prog', ['prog.c'])
      

Import prefers a local definition to a global one, so that if there is a global export of foo, and the calling SConscript has exported foo to this SConscript, the import will find the foo exported to this SConscript.

Lastly, as a special case, you may import all of the variables that have been exported by supplying an asterisk to the Import function:

Import('*')
env = env.Clone(DEBUG=debug)
env.Program('prog', ['prog.c'])
      

If you're dealing with a lot of SConscript files, this can be a lot simpler than keeping arbitrary lists of imported variables up to date in each file.

Sometimes, you would like to be able to use information from a subsidiary SConscript file in some way. For example, suppose that you want to create one library from object files built by several subsidiary SConscript files. You can do this by using the Return function to return values from the subsidiary SConscript files to the calling file. Like Import and Export, Return takes a string representation of the variable name, not the variable name itself.

If, for example, we have two subdirectories foo and bar that should each contribute an object file to a library, what we'd like to be able to do is collect the object files from the subsidiary SConscript calls like this:

env = Environment()
Export('env')
objs = []
for subdir in ['foo', 'bar']:
    o = SConscript('%s/SConscript' % subdir)
    objs.append(o)
env.Library('prog', objs)
        

We can do this by using the Return function in the foo/SConscript file like this:

Import('env')
obj = env.Object('foo.c')
Return('obj')
        

(The corresponding bar/SConscript file should be pretty obvious.) Then when we run SCons, the object files from the subsidiary subdirectories are all correctly archived in the desired library:

% scons -Q
cc -o bar/bar.o -c bar/bar.c
cc -o foo/foo.o -c foo/foo.c
ar rc libprog.a foo/foo.o bar/bar.o
ranlib libprog.a

It is often useful to keep built files completely separate from the source files. Two main benefits are the ability to have different configurations simultaneously without build conflicts, and being version-control friendly.

Consider if you have a project to build an embedded software system for a variety of different controller hardware. The system is able to share a lot of code, so it makes sense to use a common source tree, but certain build options in the source code and header files differ. For a regular in-place build, the build outputs go in the same place as the source code. If you build Controller A first, followed by Controller B, on the Controller B build everything that uses different build options has to be rebuilt since those objects will be different (the build lines, including preprocessor defines, are part of SCons's out-of-date calculation for this reason). If you go back and build for Controller A again, things have to be rebuilt again for the same reason. However, if you can separate the locations of the output files, so each controller has its own location for build outputs, this problem can be avoided.

Having a separated build tree also helps you keep your source tree clean - there is less chance of accidentally checking in build products to version control that were not intended to be checked in. You can add a separated build directory to your version control system's list of items not to track. You can even remove the whole build tree with a single command without risking removing any of the source code.

The key to making this separation work is the ability to do out-of-tree builds: building under a separate root than the sources being built. You set up out of tree builds by establishing what SCons calls a variant directory, a place where you can build a single variant of your software (of course you can define more than one of these if you need to). Since SCons tracks targets by their path, it is able to distinguish build products like build/A/network.obj of the Controller A build from build/B/network.obj of the Controller B build, thus avoiding conflicts.

SCons provides two ways to establish variant directories, one through the SConscript function that we have already seen, and the second through a more flexible VariantDir function.

The variant directory mechanism does support doing multiple builds in one invocation of SCons, but the remainder of this chapter will focus on setting up a single build. You can combine these techniques with ones from the previous chapter and elsewhere in this Guide to set up more complex scenarios.

Note

The VariantDir function used to be called BuildDir, a name which was changed because it turned out to be confusing: the SCons functionality differs from a familiar model of a "build directory" implemented by certain other build systems like GNU Autotools. You might still find references to the old name on the Internet in postings about SCons, but it no longer works.

The most straightforward way to establish a variant directory tree relies on the fact that the usual way to set up a build hierarchy is to have an SConscript file in the source directory. If you pass a variant_dir argument to the SConscript function call:

SConscript('src/SConscript', variant_dir='build')
      

SCons will then build all of the files in the build directory:

% ls src
SConscript  hello.c
% scons -Q
cc -o build/hello.o -c build/hello.c
cc -o build/hello build/hello.o
% ls src
SConscript  hello.c
% ls build
SConscript  hello  hello.c  hello.o

No files were built in src: the object file build/hello.o and the executable file build/hello were built in the build directory, as expected. But notice that even though our hello.c file actually lives in the src directory, SCons has compiled a build/hello.c file to create the object file, and that file is now seen in build.

You can ask SCons to show the dependency tree to illustrate a bit more:

% scons -Q --tree=prune
cc -o build/hello.o -c build/hello.c
cc -o build/hello build/hello.o
+-.
  +-SConstruct
  +-build
  | +-build/SConscript
  | +-build/hello
  | | +-build/hello.o
  | |   +-build/hello.c
  | +-build/hello.c
  | +-[build/hello.o]
  +-src
    +-src/SConscript
    +-src/hello.c

What's happened is that SCons has duplicated the hello.c file from the src directory to the build directory, and built the program from there (it also duplicated SConscript). The next section explains why SCons does this.

The nice thing about the SConscript approach is it is almost invisible to you: this build looks just like an ordinary in-place build except for the extra variant_dir argument in the SConscript call. SCons handles all the path adjustments for the out of tree build directory while it processes that SConscript file.

When you set up a variant directory SCons conceptually behaves as if you requested a build in that directory. As noted in the previous chapter, all builds actually happen from the top level directory, but as an aid to understanding how SCons operates, think of it as build in place in the variant directory, not build in source but send build artifacts to the variant directory. It turns out in place builds are easier to get right than out of tree builds - so by default SCons simulates an in place build by making the variant directory look just like the source directory. The most straightforward way to do that is by making copies of the files needed for the build.

The most direct reason to duplicate source files in variant directories is simply that some tools (mostly older versions) are written to only build their output files in the same directory as the source files - such tools often don't have any option to specify the output file, and the tool just uses a predefined output file name, or uses a derived variant of the source file name, dropping the result in the same directory. In this case, the choices are either to build the output file in the source directory and move it to the variant directory, or to duplicate the source files in the variant directory.

Additionally, relative references between files can cause problems which are resolved by just duplicating the hierarchy of source files into the variant directory. You can see this at work in use of the C preprocessor #include mechanism with double quotes, not angle brackets:

#include "file.h"
    

The de facto standard behavior for most C compilers in this case is to first look in the same directory as the source file that contains the #include line, then to look in the directories in the preprocessor search path. Add to this that the SCons implementation of support for code repositories (described below) means not all of the files will be found in the same directory hierarchy, and the simplest way to make sure that the right include file is found is to duplicate the source files into the variant directory, which provides a correct build regardless of the original location(s) of the source files.

Although source-file duplication guarantees a correct build even in these edge cases, it can usually be safely disabled. The next section describes how you can disable the duplication of source files in the variant directory.

In most cases and with most tool sets, SCons can use sources directly from the source directory without duplicating them into the variant directory before building, and everything will work just fine. You can disable the default SCons duplication behavior by specifying duplicate=False when you call the SConscript function:

SConscript('src/SConscript', variant_dir='build', duplicate=False)
    

When this flag is specified, the results of a build look more like the mental model people may have from other build systems - that is, the output files end up in the variant directory while the source files do not.

% ls src
SConscript
hello.c
% scons -Q
cc -c src/hello.c -o build/hello.o
cc -o build/hello build/hello.o
% ls build
hello
hello.o
    

If disabling duplication causes any problems, just return to the more cautious approach by letting SCons go back to duplicating files.

You can also use the VariantDir function to establish that target files should be built in a separate directory tree from the source files:

VariantDir('build', 'src')
env = Environment()
env.Program('build/hello.c')
      

When using this form, you have to tell SCons that sources and targets are in the variant directory, and those references will trigger the remapping, necessary file copying, etc. for an already established variant directory. Here is the same example in a more spelled out form to show this more clearly:

VariantDir('build', 'src')
env = Environment()
env.Program(target='build/hello', source=['build/hello.c'])
    

When using the VariantDir function directly, SCons still duplicates the source files in the variant directory by default:

% ls src
hello.c
% scons -Q
cc -o build/hello.o -c build/hello.c
cc -o build/hello build/hello.o
% ls build
hello  hello.c  hello.o

You can specify the same duplicate=False argument that you can specify for an SConscript call:

VariantDir('build', 'src', duplicate=False)
env = Environment()
env.Program('build/hello.c')
      

In which case SCons will disable duplication of the source files:

% ls src
hello.c
% scons -Q
cc -o build/hello.o -c src/hello.c
cc -o build/hello build/hello.o
% ls build
hello  hello.o

Even when using the VariantDir function, it is more natural to use it with a subsidiary SConscript file, because then you don't have to adjust your individual build instructions to use the variant directory path. For example, if the src/SConscript looks like this:

env = Environment()
env.Program('hello.c')
      

Then our SConstruct file could look like:

VariantDir('build', 'src')
SConscript('build/SConscript')
      

Yielding the following output:

% ls src
SConscript  hello.c
% scons -Q
cc -o build/hello.o -c build/hello.c
cc -o build/hello build/hello.o
% ls build
SConscript  hello  hello.c  hello.o

This is completely equivalent to the use of SConscript with the variant_dir argument from earlier in this chapter, but did require callng the SConscript using the already established variant directory path to trigger that behavior. If you call SConscript('src/SConscript') you would get a normal in-place build in src.

The Glob file name pattern matching function works just as usual when using VariantDir. For example, if the src/SConscript looks like this:

env = Environment()
env.Program('hello', Glob('*.c'))
      

Then with the same SConstruct file as in the previous section, and source files f1.c and f2.c in src, we would see the following output:

% ls src
SConscript  f1.c  f2.c  f2.h
% scons -Q
cc -o build/f1.o -c build/f1.c
cc -o build/f2.o -c build/f2.c
cc -o build/hello build/f1.o build/f2.o
% ls build
SConscript  f1.c  f1.o  f2.c  f2.h  f2.o  hello

The Glob function returns Nodes in the build/ tree, as you'd expect.

The variant_dir keyword argument of the SConscript function provides everything we need to show how easy it is to create variant builds using SCons. Suppose, for example, that we want to build a program for both Windows and Linux platforms, but that we want to build it in directory on a network share with separate side-by-side build directories for the Windows and Linux versions of the program. We have to do a little bit of work to construct paths, to make sure unwanted location dependencies don't creep in. The top-relative path reference can be useful here. To avoid writing conditional code based on platform, we can build the variant_dir path dynamically:

platform = ARGUMENTS.get('OS', Platform())

include = "#export/$PLATFORM/include"
lib = "#export/$PLATFORM/lib"
bin = "#export/$PLATFORM/bin"

env = Environment(
    PLATFORM=platform,
    BINDIR=bin,
    INCDIR=include,
    LIBDIR=lib,
    CPPPATH=[include],
    LIBPATH=[lib],
    LIBS='world',
)

Export('env')

env.SConscript('src/SConscript', variant_dir='build/$PLATFORM')
    

This SConstruct file, when run on a Linux system, yields:

% scons -Q OS=linux
Install file: "build/linux/world/world.h" as "export/linux/include/world.h"
cc -o build/linux/hello/hello.o -c -Iexport/linux/include build/linux/hello/hello.c
cc -o build/linux/world/world.o -c -Iexport/linux/include build/linux/world/world.c
ar rc build/linux/world/libworld.a build/linux/world/world.o
ranlib build/linux/world/libworld.a
Install file: "build/linux/world/libworld.a" as "export/linux/lib/libworld.a"
cc -o build/linux/hello/hello build/linux/hello/hello.o -Lexport/linux/lib -lworld
Install file: "build/linux/hello/hello" as "export/linux/bin/hello"

The same SConstruct file on Windows would build:

C:\>scons -Q OS=windows
Install file: "build/windows/world/world.h" as "export/windows/include/world.h"
cl /Fobuild\windows\hello\hello.obj /c build\windows\hello\hello.c /nologo /Iexport\windows\include
cl /Fobuild\windows\world\world.obj /c build\windows\world\world.c /nologo /Iexport\windows\include
lib /nologo /OUT:build\windows\world\world.lib build\windows\world\world.obj
Install file: "build/windows/world/world.lib" as "export/windows/lib/world.lib"
link /nologo /OUT:build\windows\hello\hello.exe /LIBPATH:export\windows\lib world.lib build\windows\hello\hello.obj
embedManifestExeCheck(target, source, env)
Install file: "build/windows/hello/hello.exe" as "export/windows/bin/hello.exe"

In order to build several variants at once when using the variant_dir argument to SConscript, you can call the function repeatedely - this example does so in a loop. Note that the SConscript trick of passing a list of script files, or a list of source directories, does not work with variant_dir, SCons allows only a single SConscript to be given if variant_dir is used.

env = Environment(OS=ARGUMENTS.get('OS'))
for os in ['newell', 'post']:
    SConscript('src/SConscript', variant_dir='build/' + os)
    

Often, a software project will have one or more central repositories, directory trees that contain source code, or derived files, or both. You can eliminate additional unnecessary rebuilds of files by having SCons use files from one or more code repositories to build files in your local build tree.

We've already seen that SCons will scan the contents of a source file for #include file names and realize that targets built from that source file also depend on the #include file(s). For each directory in the $CPPPATH list, SCons will actually search the corresponding directories in any repository trees and establish the correct dependencies on any #include files that it finds in repository directory.

Unless the C compiler also knows about these directories in the repository trees, though, it will be unable to find the #include files. If, for example, the hello.c file in our previous example includes the hello.h in its current directory, and the hello.h only exists in the repository:

% scons -Q
cc -o hello.o -c hello.c
hello.c:1: hello.h: No such file or directory
    

In order to inform the C compiler about the repositories, SCons will add appropriate -I flags to the compilation commands for each directory in the $CPPPATH list. So if we add the current directory to the construction environment $CPPPATH like so:

env = Environment(CPPPATH = ['.'])
env.Program('hello.c')
Repository('/usr/repository1')
      

Then re-executing SCons yields:

% scons -Q
cc -o hello.o -c -I. -I/usr/repository1 hello.c
cc -o hello hello.o

The order of the -I options replicates, for the C preprocessor, the same repository-directory search path that SCons uses for its own dependency analysis. If there are multiple repositories and multiple $CPPPATH directories, SCons will add the repository directories to the beginning of each $CPPPATH directory, rapidly multiplying the number of -I flags. If, for example, the $CPPPATH contains three directories (and shorter repository path names!):

env = Environment(CPPPATH = ['dir1', 'dir2', 'dir3'])
env.Program('hello.c')
Repository('/r1', '/r2')
      

Then we'll end up with nine -I options on the command line, three (for each of the $CPPPATH directories) times three (for the local directory plus the two repositories):

% scons -Q
cc -o hello.o -c -Idir1 -I/r1/dir1 -I/r2/dir1 -Idir2 -I/r1/dir2 -I/r2/dir2 -Idir3 -I/r1/dir3 -I/r2/dir3 hello.c
cc -o hello hello.o

SCons relies on the C compiler's -I options to control the order in which the preprocessor will search the repository directories for #include files. This causes a problem, however, with how the C preprocessor handles #include lines with the file name included in double-quotes.

As we've seen, SCons will compile the hello.c file from the repository if it doesn't exist in the local directory. If, however, the hello.c file in the repository contains a #include line with the file name in double quotes:

#include "hello.h"
int
main(int argc, char *argv[])
{
    printf(HELLO_MESSAGE);
    return (0);
}
      

Then the C preprocessor will always use a hello.h file from the repository directory first, even if there is a hello.h file in the local directory, despite the fact that the command line specifies -I as the first option:

% scons -Q
cc -o hello.o -c -I. -I/usr/repository1 /usr/repository1/hello.c
cc -o hello hello.o

This behavior of the C preprocessor--always search for a #include file in double-quotes first in the same directory as the source file, and only then search the -I--can not, in general, be changed. In other words, it's a limitation that must be lived with if you want to use code repositories in this way. There are three ways you can possibly work around this C preprocessor behavior:

  1. Some modern versions of C compilers do have an option to disable or control this behavior. If so, add that option to $CFLAGS (or $CXXFLAGS or both) in your construction environment(s). Make sure the option is used for all construction environments that use C preprocessing!

  2. Change all occurrences of #include "file.h" to #include <file.h>. Use of #include with angle brackets does not have the same behavior--the -I directories are searched first for #include files--which gives SCons direct control over the list of directories the C preprocessor will search.

  3. Require that everyone working with compilation from repositories check out and work on entire directories of files, not individual files. (If you use local wrapper scripts around your source code control system's command, you could add logic to enforce this restriction there.

If a repository contains not only source files, but also derived files (such as object files, libraries, or executables), SCons will perform its normal MD5 signature calculation to decide if a derived file in a repository is up-to-date, or the derived file must be rebuilt in the local build directory. For the SCons signature calculation to work correctly, a repository tree must contain the .sconsign files that SCons uses to keep track of signature information.

Usually, this would be done by a build integrator who would run SCons in the repository to create all of its derived files and .sconsign files, or who would run SCons in a separate build directory and copy the resulting tree to the desired repository:

% cd /usr/repository1
% scons -Q
cc -o file1.o -c file1.c
cc -o file2.o -c file2.c
cc -o hello.o -c hello.c
cc -o hello hello.o file1.o file2.o

(Note that this is safe even if the SConstruct file lists /usr/repository1 as a repository, because SCons will remove the current build directory from its repository list for that invocation.)

Now, with the repository populated, we only need to create the one local source file we're interested in working with at the moment, and use the -Y option to tell SCons to fetch any other files it needs from the repository:

% cd $HOME/build
% edit hello.c
% scons -Q -Y /usr/repository1
cc -c -o hello.o hello.c
cc -o hello hello.o /usr/repository1/file1.o /usr/repository1/file2.o
    

Notice that SCons realizes that it does not need to rebuild local copies file1.o and file2.o files, but instead uses the already-compiled files from the repository.

Although SCons provides many useful methods for building common software products (programs, libraries, documents, etc.), you frequently want to be able to build some other type of file not supported directly by SCons. Fortunately, SCons makes it very easy to define your own Builder objects for any custom file types you want to build. (In fact, the SCons interfaces for creating Builder objects are flexible enough and easy enough to use that all of the the SCons built-in Builder objects are created using the mechanisms described in this section.)

A Builder object isn't useful until it's attached to a construction environment so that we can call it to arrange for files to be built. This is done through the $BUILDERS construction variable in an environment. The $BUILDERS variable is a Python dictionary that maps the names by which you want to call various Builder objects to the objects themselves. For example, if we want to call the Builder we just defined by the name Foo, our SConstruct file might look like:

bld = Builder(action='foobuild < $SOURCE > $TARGET')
env = Environment(BUILDERS={'Foo': bld})
    

With the Builder attached to our construction environment with the name Foo, we can now actually call it like so:

env.Foo('file.foo', 'file.input')
    

Then when we run SCons it looks like:

% scons -Q
foobuild < file.input > file.foo

Note, however, that the default $BUILDERS variable in a construction environment comes with a default set of Builder objects already defined: Program, Library, etc. And when we explicitly set the $BUILDERS variable when we create the construction environment, the default Builders are no longer part of the environment:

bld = Builder(action='foobuild < $SOURCE > $TARGET')
env = Environment(BUILDERS={'Foo': bld})
env.Foo('file.foo', 'file.input')
env.Program('hello.c')
       
% scons -Q
AttributeError: 'SConsEnvironment' object has no attribute 'Program':
  File "/home/my/project/SConstruct", line 7:
    env.Program('hello.c')

To be able to use both our own defined Builder objects and the default Builder objects in the same construction environment, you can either add to the $BUILDERS variable using the Append function:

env = Environment()
bld = Builder(action='foobuild < $SOURCE > $TARGET')
env.Append(BUILDERS={'Foo': bld})
env.Foo('file.foo', 'file.input')
env.Program('hello.c')
    

Or you can explicitly set the appropriately-named key in the $BUILDERS dictionary:

env = Environment()
bld = Builder(action='foobuild < $SOURCE > $TARGET')
env['BUILDERS']['Foo'] = bld
env.Foo('file.foo', 'file.input')
env.Program('hello.c')
    

Either way, the same construction environment can then use both the newly-defined Foo Builder and the default Program Builder:

% scons -Q
foobuild < file.input > file.foo
cc -o hello.o -c hello.c
cc -o hello hello.o

In SCons, you don't have to call an external command to build a file. You can, instead, define a Python function that a Builder object can invoke to build your target file (or files). Such a builder function definition looks like:

def build_function(target, source, env):
    # Code to build "target" from "source"
    return None
    

The arguments of a builder function are:

target

A list of Node objects representing the target or targets to be built by this function. The file names of these target(s) may be extracted using the Python str function.

source

A list of Node objects representing the sources to be used by this function to build the targets. The file names of these source(s) may be extracted using the Python str function.

env

The construction environment used for building the target(s). The function may use any of the environment's construction variables in any way to affect how it builds the targets.

The function will be constructed as a SCons FunctionAction and must return a 0 or None value if the target(s) are built successfully. The function may raise an exception or return any non-zero value to indicate that the build is unsuccessful. For more information on Actions see the Action Objects section of the man page.

Once you've defined the Python function that will build your target file, defining a Builder object for it is as simple as specifying the name of the function, instead of an external command, as the Builder's action argument:

def build_function(target, source, env):
    # Code to build "target" from "source"
    return None

bld = Builder(
    action=build_function,
    suffix='.foo',
    src_suffix='.input',
)
env = Environment(BUILDERS={'Foo': bld})
env.Foo('file')
       

And notice that the output changes slightly, reflecting the fact that a Python function, not an external command, is now called to build the target file:

% scons -Q
build_function(["file.foo"], ["file.input"])

SCons Builder objects can create an action "on the fly" by using a function called a Generator. (Note: this is not the same thing as a Python generator function described in PEP 255) This provides a great deal of flexibility to construct just the right list of commands to build your target. A generator looks like:

def generate_actions(source, target, env, for_signature):
    return 'foobuild < %s > %s' % (target[0], source[0])
    

The arguments of a generator are:

source

A list of Node objects representing the sources to be built by the command or other action generated by this function. The file names of these source(s) may be extracted using the Python str function.

target

A list of Node objects representing the target or targets to be built by the command or other action generated by this function. The file names of these target(s) may be extracted using the Python str function.

env

The construction environment used for building the target(s). The generator may use any of the environment's construction variables in any way to determine what command or other action to return.

for_signature

A flag that specifies whether the generator is being called to contribute to a build signature, as opposed to actually executing the command.

The generator must return a command string or other action that will be used to build the specified target(s) from the specified source(s).

Once you've defined a generator, you create a Builder to use it by specifying the generator keyword argument instead of action.

def generate_actions(source, target, env, for_signature):
    return 'foobuild < %s > %s' % (source[0], target[0])

bld = Builder(
    generator=generate_actions,
    suffix='.foo',
    src_suffix='.input',
)
env = Environment(BUILDERS={'Foo': bld})
env.Foo('file')
    
% scons -Q
foobuild < file.input > file.foo

Note that it's illegal to specify both an action and a generator for a Builder.

SCons supports the ability for a Builder to modify the lists of target(s) from the specified source(s). You do this by defining an emitter function that takes as its arguments the list of the targets passed to the builder, the list of the sources passed to the builder, and the construction environment. The emitter function should return the modified lists of targets that should be built and sources from which the targets will be built.

For example, suppose you want to define a Builder that always calls a foobuild program, and you want to automatically add a new target file named new_target and a new source file named new_source whenever it's called. The SConstruct file might look like this:

def modify_targets(target, source, env):
    target.append('new_target')
    source.append('new_source')
    return target, source

bld = Builder(
    action='foobuild $TARGETS - $SOURCES',
    suffix='.foo',
    src_suffix='.input',
    emitter=modify_targets,
)
env = Environment(BUILDERS={'Foo': bld})
env.Foo('file')
    

And would yield the following output:

% scons -Q
foobuild file.foo new_target - file.input new_source

One very flexible thing that you can do is use a construction variable to specify different emitter functions for different construction environments. To do this, specify a string containing a construction variable expansion as the emitter when you call the Builder function, and set that construction variable to the desired emitter function in different construction environments:

bld = Builder(
    action='./my_command $SOURCES > $TARGET',
    suffix='.foo',
    src_suffix='.input',
    emitter='$MY_EMITTER',
)

def modify1(target, source, env):
    return target, source + ['modify1.in']

def modify2(target, source, env):
    return target, source + ['modify2.in']

env1 = Environment(BUILDERS={'Foo': bld}, MY_EMITTER=modify1)
env2 = Environment(BUILDERS={'Foo': bld}, MY_EMITTER=modify2)
env1.Foo('file1')
env2.Foo('file2')
        

In this example, the modify1.in and modify2.in files get added to the source lists of the different commands:

% scons -Q
./my_command file1.input modify1.in > file1.foo
./my_command file2.input modify2.in > file2.foo

Defining an emitter to work with a custom Builder is a powerful concept, but sometimes all you really want is to be able to use an existing builder but change its concept of what targets are created. In this case, trying to recreate the logic of an existing Builder to supply a special emitter can be a lot of work. The typical case for this is when you want to use a compiler flag that causes additional files to be generated. For example the GNU linker accepts an option -Map which outputs a link map to the file specified by the option's argument. If this option is just supplied to the build, SCons will not consider the link map file a tracked target, which has various undesirable efffects.

To help with this, SCons provides construction variables which correspond to a few standard builders: $PROGEMITTER for Program; $LIBEMITTER for Library; $SHLIBEMITTER for SharedLibrary and $LDMODULEEMITTER for LoadableModule;. Adding an emitter to one of these will cause it to be invoked in addition to any existing emitter for the corresponding builder.

This example adds map creation as a linker flag, and modifies the standard Program emitter to know that map generation is a side-effect:

env = Environment()
map_filename = "${TARGET.name}.map"

def map_emitter(target, source, env):
    target.append(map_filename)
    return target, source

env.Append(LINKFLAGS="-Wl,-Map={},--cref".format(map_filename))
env.Append(PROGEMITTER=map_emitter)
env.Program('hello.c')
      

If you run this example, adding an option to tell SCons to dump some information about the dependencies it knows, it shows the map file option in use, and that SCons indeed knows about the map file, it's not just a silent side effect of the compiler:

% scons -Q --tree=prune
cc -o hello.o -c hello.c
cc -o hello -Wl,-Map=hello.map,--cref hello.o
+-.
  +-SConstruct
  +-hello
  | +-hello.o
  |   +-hello.c
  +-hello.c
  +-hello.map
  | +-[hello.o]
  +-[hello.o]

The site_scons directories give you a place to put Python modules and packages that you can import into your SConscript files (at the top level) add-on tools that can integrate into SCons (in a site_tools subdirectory), and a site_scons/site_init.py file that gets read before any SConstruct or SConscript file, allowing you to change SCons's default behavior.

Each system type (Windows, Mac, Linux, etc.) searches a canonical set of directories for site_scons; see the man page for details. The top-level SConstruct's site_scons dir (that is, the one in the project) is always searched last, and its dir is placed first in the tool path so it overrides all others.

If you get a tool from somewhere (the SCons wiki or a third party, for instance) and you'd like to use it in your project, a site_scons dir is the simplest place to put it. Tools come in two flavors; either a Python function that operates on an Environment or a Python module or package containing two functions, exists() and generate().

A single-function Tool can just be included in your site_scons/site_init.py file where it will be parsed and made available for use. For instance, you could have a site_scons/site_init.py file like this:

def TOOL_ADD_HEADER(env):
    """A Tool to add a header from $HEADER to the source file"""
    add_header = Builder(
        action=['echo "$HEADER" > $TARGET', 'cat $SOURCE >> $TARGET']
    )
    env.Append(BUILDERS={'AddHeader': add_header})
    env['HEADER'] = ''  # set default value
      

and a SConstruct like this:

# Use TOOL_ADD_HEADER from site_scons/site_init.py
env=Environment(tools=['default', TOOL_ADD_HEADER], HEADER="=====")
env.AddHeader('tgt', 'src')
    

The TOOL_ADD_HEADER tool method will be called to add the AddHeader tool to the environment.

A more full-fledged tool with exists() and generate() methods can be installed either as a module in the file site_scons/site_tools/toolname.py or as a package in the directory site_scons/site_tools/toolname. In the case of using a package, the exists() and generate() are in the file site_scons/site_tools/toolname/__init__.py. (In all the above case toolname is replaced by the name of the tool.) Since site_scons/site_tools is automatically added to the head of the tool search path, any tool found there will be available to all environments. Furthermore, a tool found there will override a built-in tool of the same name, so if you need to change the behavior of a built-in tool, site_scons gives you the hook you need.

Many people have a collection of utility Python functions they'd like to include in their SConscript files: just put them in site_scons/my_utils.py or any valid Python module name of your choice. For instance you can do something like this in site_scons/my_utils.py to add build_id and MakeWorkDir functions:

from SCons.Script import *  # for Execute and Mkdir

def build_id():
    """Return a build ID (stub version)"""
    return "100"

def MakeWorkDir(workdir):
    """Create the specified dir immediately"""
    Execute(Mkdir(workdir))
      

And then in your SConscript or any sub-SConscript anywhere in your build, you can import my_utils and use it:

import my_utils
print("build_id=" + my_utils.build_id())
my_utils.MakeWorkDir('/tmp/work')
    

You can put this collection in its own module in a site_scons and import it as in the example, or you can include it in site_scons/site_init.py, which is automatically imported (unless you disable site directories). Note that in order to refer to objects in the SCons namespace such as Environment or Mkdir or Execute in any file other than a SConstruct or SConscript you always need to do

from SCons.Script import *
    

This is true of modules in site_scons such as site_scons/site_init.py as well.

You can use any of the user- or machine-wide site directories such as ~/.scons/site_scons instead of ./site_scons, or use the --site-dir option to point to your own directory. site_init.py and site_tools will be located under that directory. To avoid using a site_scons directory at all, even if it exists, use the --no-site-dir option.

Creating a Builder and attaching it to a construction environment allows for a lot of flexibility when you want to re-use actions to build multiple files of the same type. This can, however, be cumbersome if you only need to execute one specific command to build a single file (or group of files). For these situations, SCons supports a Command builder that arranges for a specific action to be executed to build a specific file or files. This looks a lot like the other builders (like Program, Object, etc.), but takes as an additional argument the command to be executed to build the file:

env = Environment()
env.Command('foo.out', 'foo.in', "sed 's/x/y/' < $SOURCE > $TARGET")
     

When executed, SCons runs the specified command, substituting $SOURCE and $TARGET as expected:

% scons -Q
sed 's/x/y/' < foo.in > foo.out

This is often more convenient than creating a Builder object and adding it to the $BUILDERS variable of a construction environment.

Note that the action you specify to the Command Builder can be any legal SCons Action, such as a Python function:

env = Environment()

def build(target, source, env):
    # Whatever it takes to build
    return None

env.Command('foo.out', 'foo.in', build)
     

Which executes as follows:

% scons -Q
build(["foo.out"], ["foo.in"])

Note that $SOURCE and $TARGET are expanded in the source and target as well, so you can write:

env.Command('${SOURCE.basename}.out', 'foo.in', build)
     

which does the same thing as the previous example, but allows you to avoid repeating yourself.

It may be helpful to use the action keyword to specify the action, is this makes things more clear to the reader:

env.Command('${SOURCE.basename}.out', 'foo.in', action=build)
     

The method described in Section 9.2, “Controlling How SCons Prints Build Commands: the $*COMSTR Variables” for controlling build output works well when used with pre-defined builders which have pre-defined *COMSTR variables for that purpose, but that is not the case when calling Command, where SCons has no specific knowledge of the action ahead of time. If the action argument to Command is not already an Action object, it will construct one for you with suitable defaults, which include a message based on the type of action. However, you can also construct the Action object yourself to pass to Command, which gives you much more control. Here's an evolution of the example from above showing this approach:

env = Environment()

def build(target, source, env):
    # Whatever it takes to build
    return None

act = Action(build, cmdstr="Building ${TARGET}")
env.Command('foo.out', 'foo.in', action=act)
     

Which executes as follows:

% scons -Q
Building foo.out

The AddMethod function is used to add a method to an environment. It is typically used to add a "pseudo-builder," a function that looks like a Builder but wraps up calls to multiple other Builders or otherwise processes its arguments before calling one or more Builders.

In the following example, we want to install the program into the standard /usr/bin directory hierarchy, but also copy it into a local install/bin directory from which a package might be built:

def install_in_bin_dirs(env, source):
    """Install source in both bin dirs"""
    i1 = env.Install("$BIN", source)
    i2 = env.Install("$LOCALBIN", source)
    return [i1[0], i2[0]]  # Return a list, like a normal builder

env = Environment(BIN='/usr/bin', LOCALBIN='#install/bin')
env.AddMethod(install_in_bin_dirs, "InstallInBinDirs")
env.InstallInBinDirs(Program('hello.c'))  # installs hello in both bin dirs
     

This produces the following:

% scons -Q /
cc -o hello.o -c hello.c
cc -o hello hello.o
Install file: "hello" as "/usr/bin/hello"
Install file: "hello" as "install/bin/hello"

A pseudo-builder is useful because it gives you more flexibility parsing arguments than you can get with a standard Builder. The next example shows a pseudo-builder with a named argument that modifies the filename, and a separate optional argument for a resource file (rather than having the builder figure it out by file extension). This example also demonstrates using the global AddMethod function to add a method to the global Environment class, so it will be available in all subsequently created environments.

def BuildTestProg(env, testfile, resourcefile="", testdir="tests"):
    """Build the test program.

    Prepends "test_" to src and target and puts the target into testdir.
    If the build is running on Windows, also make use of a resource file,
    if supplied.
    """
    srcfile = f"test_{testfile}.c"
    target = f"{testdir}/test_{testfile}"
    if env['PLATFORM'] == 'win32' and resourcefile:
        resfile = env.RES(resourcefile)
        p = env.Program(target, [srcfile, resfile])
    else:
        p = env.Program(target, srcfile)
    return p

AddMethod(Environment, BuildTestProg)

env = Environment()
env.BuildTestProg('stuff', resourcefile='res.rc')
     

This produces the following on Linux:

% scons -Q
cc -o test_stuff.o -c test_stuff.c
cc -o tests/test_stuff test_stuff.o

And the following on Windows:

C:\>scons -Q
rc /nologo /fores.res res.rc
cl /Fotest_stuff.obj /c test_stuff.c /nologo
link /nologo /OUT:tests\test_stuff.exe test_stuff.obj res.res
embedManifestExeCheck(target, source, env)

Using AddMethod is better than just adding an instance method to a construction environment because it gets called as a proper method, and because AddMethod provides for copying the method to any clones of the construction environment instance.

SCons has built-in Scanners that know how to look in C/C++, Fortran, D, IDL, LaTeX, Python and SWIG source files for information about other files that targets built from those files depend on. For example, if you have a file format which uses #include to specify files which should be included into the source file when it is processed, you can use an existing scanner already included in SCons. You can use the same mechanisms that SCons uses to create its built-in Scanners to write Scanners of your own for file types that SCons does not know how to scan "out of the box."

Suppose, for example, that we want to create a simple Scanner for .k files. A .k file contains some text that will be processed, and can include other files on lines that begin with include followed by a file name:

include filename.k
    

Scanning a file will be handled by a Python function that you must supply. Here is a function that will use the Python re module to scan for the include lines in our example:

import re

include_re = re.compile(r'^include\s+(\S+)$', re.M)

def kfile_scan(node, env, path, arg=None):
    contents = node.get_text_contents()
    return env.File(include_re.findall(contents))
    

It is important to note that you have to return a list of File nodes from the scanner function, simple strings for the file names won't do. As in the examples we are showing here, you can use the File function of your current construction environment in order to create nodes on the fly from a sequence of file names with relative paths.

The scanner function must accept the four specified arguments and return a list of implicit dependencies. Presumably, these would be dependencies found from examining the contents of the file, although the function can perform any manipulation at all to generate the list of dependencies.

node

An SCons node object representing the file being scanned. The path name to the file can be used by converting the node to a string using the str function, or an internal SCons get_text_contents object method can be used to fetch the contents.

env

The construction environment in effect for this scan. The scanner function may choose to use construction variables from this environment to affect its behavior.

path

A list of directories that form the search path for included files for this Scanner. This is how SCons handles the $CPPPATH and $LIBPATH variables.

arg

An optional argument that can be passed to this scanner function when it is called from a scanner instance. The argument is only supplied if it was given when the scanner instance is created (see the manpage section "Scanner Objects"). This can be useful, for example, to distinguish which scanner type called us, if the function might be bound to several scanner objects. Since the argument is only supplied in the function call if it was defined for that scanner, the function needs to be prepared to possibly be called in different ways if multiple scanners are expected to use this function - giving the parameter a default value as shown above is a good way to do this. If the function to scanner relationship will be 1:1, just make sure they match.

A scanner object is created using the Scanner function, which typically takes an skeys argument to associate a file suffix with this Scanner. The scanner object must then be associated with the $SCANNERS construction variable in the current construction environment, typically by using the Append method:

kscan = Scanner(function=kfile_scan, skeys=['.k'])
env.Append(SCANNERS=kscan)
    

Let's put this all together. Our new file type, with the .k suffix, will be processed by a command named kprocess, which lives in non-standard location /usr/local/bin, so we add that path to the execution environment so SCons can find it. Here's what it looks like:

import re

include_re = re.compile(r'^include\s+(\S+)$', re.M)

def kfile_scan(node, env, path):
    contents = node.get_text_contents()
    includes = include_re.findall(contents)
    return env.File(includes)

kscan = Scanner(function=kfile_scan, skeys=['.k'])
env = Environment()
env.AppendENVPath('PATH', '/usr/local/bin')
env.Append(SCANNERS=kscan)

env.Command('foo', 'foo.k', 'kprocess < $SOURCES > $TARGET')
      

Assume a foo.k file like this:

some initial text
include other_file
some other text
      

Now if we run scons we can see that the scanner works - it identified the dependency other_file via the detected include line, although we get an error message because we forgot to create that file!

% scons -Q
scons: *** [foo] Implicit dependency `other_file' not found, needed by target `foo'.

If the build tool in question will use a path variable to search for included files or other dependencies, then the Scanner will need to take that path variable into account as well - the same way $CPPPATH is used for files processed by the C Preprocessor (used for C, C++, Fortran and others). Path variables may be lists of nodes or semicolon-separated strings (SCons uses a semicolon here irrespective of the pathlist separator used by the native operating system), and may contain construction variables to be expanded. A Scanner can take a path_function to process such a path variable; the function produces a tuple of paths that is passed to the scanner function as its path parameter.

To make this easy, SCons provides the premade FindPathDirs function which returns a callable to expand a given path variable (given as an SCons construction variable name) to a tuple of paths at the time the Scanner is called. Deferring evaluation until that point allows, for instance, the path to contain $TARGET references which differ for each file scanned.

Using FindPathDirs is easy. Continuing the above example, using $KPATH as the construction variable to hold the paths (analogous to $CPPPATH), we just modify the call to the Scanner factory function to include a path_function keyword argument:

kscan = Scanner(
    function=kfile_scan,
    skeys=['.k'],
    path_function=FindPathDirs('KPATH'),
)
      

FindPathDirs is called when the Scanner is created, and the callable object it returns is stored as an attribute in the scanner. When the scanner is invoked, it calls that object, which processes the $KPATH from the current construction environment, doing necessary expansions and, if necessary, adds related repository and variant directories, producing a (possibly empty) tuple of paths that is passed on to the scanner function. The scanner function is then responsible for using that list of paths to locate the include files identified by the scan. The next section will show an example of that.

As a side note, the returned method stores the path in an efficient way so lookups are fast even when variable substitutions may be needed. This is important since many files get scanned in a typical build.

One approach for introducing a Scanner into the build is in conjunction with a Builder. There are two relvant optional parameters we can use when creating a Builder: source_scanner and target_scanner. source_scanner is used for scanning source files, and target_scanner is used for scanning the target once it is generated.

import os, re

include_re = re.compile(r"^include\s+(\S+)$", re.M)

def kfile_scan(node, env, path, arg=None):
    includes = include_re.findall(node.get_text_contents())
    print(f"DEBUG: scan of {str(node)!r} found {includes}")
    deps = []
    for inc in includes:
        for dir in path:
            file = str(dir) + os.sep + inc
            if os.path.exists(file):
                deps.append(file)
                break
    print(f"DEBUG: scanned dependencies found: {deps}")
    return env.File(deps)

kscan = Scanner(
    function=kfile_scan,
    skeys=[".k"],
    path_function=FindPathDirs("KPATH"),
)

def build_function(target, source, env):
    # Code to build "target" from "source"
    return None

bld = Builder(
    action=build_function,
    suffix=".k",
    source_scanner=kscan,
    src_suffix=".input",
)

env = Environment(BUILDERS={"KFile": bld}, KPATH="inc")
env.KFile("file")
      

Running this example would only show that the stub build_function is getting called, so some debug prints were added to the scaner function, just to show the scanner is being invoked.

% scons -Q
DEBUG: scan of 'file.input' found ['other_file']
DEBUG: scanned dependencies found: ['inc/other_file']
build_function(["file.k"], ["file.input"])

The path-search implementation in kfile_scan works, but is quite simple-minded - a production scanner will probably do something more sophisticated.

An emitter function can modify the list of sources or targets passed to the action function when the Builder is triggered.

A scanner function will not affect the list of sources or targets seen by the Builder during the build action. The scanner function will, however, affect if the Builder should rebuild (if any of the files sourced by the Scanner have changed for example).

SCons has integrated support for build configuration similar in style to GNU Autoconf, but designed to be transparently multi-platform. The configuration system can help figure out if external build requirements such as system libraries or header files are available on the build system. This section describes how to use this SCons feature. (See also the SCons man page for additional information).

The basic framework for multi-platform build configuration in SCons is to create a configure context inside a construction environment by calling the Configure function, perform the desired checks for libraries, functions, header files, etc., and then call the configure context's Finish method to finish off the configuration:

env = Environment()
conf = Configure(env)
# Checks for libraries, header files, etc. go here!
env = conf.Finish()
    

The Finish call is required; if a new context is created while a context is active, even in a different construction environment, scons will complain and exit.

SCons provides a number of pre-defined basic checks, as well as a mechanism for adding your own custom checks.

There are a few possible strategies for failing configure checks. Some checks may be for features without which you cannot proceed. The simple approach here is just to exit SCons at that point - a number of the examples in this chapter are coded that way. If there are multiple hard requirements, however, it may be friendlier to the user to set a flag in case of any fails of hard requirements and accumulate a record of them, so that on the completion of the configure context they can all be listed prior to failing the build - as it can be frustrating to have to iterate through the setup, fixing one new requirement each iteration. Other checks may be for features which you can do without, and here the strategy will usually be to set a construction variable which the rest of the build can examine for its absence/presence, or to set particular compiler flags, library lists, etc. as appropriate for the circumstances, so you can proceed with the build appropriately based on available features.

Note that SCons uses its own dependency mechanism to determine when a check needs to be run--that is, SCons does not run the checks every time it is invoked, but caches the values returned by previous checks and uses the cached values unless something has changed. This saves a tremendous amount of developer time while working on cross-platform build issues.

The next sections describe the basic checks that SCons supports, as well as how to add your own custom checks.

A custom check is a Python function that checks for a certain condition to exist on the running system, usually using methods that SCons supplies to take care of the details of checking whether a compilation succeeds, a link succeeds, a program is runnable, etc. A simple custom check for the existence of a specific library might look as follows:

mylib_test_source_file = """
#include <mylib.h>
int main(int argc, char **argv)
{
    MyLibrary mylib(argc, argv);
    return 0;
}
"""

def CheckMyLibrary(context):
    context.Message('Checking for MyLibrary...')
    result = context.TryLink(mylib_test_source_file, '.c')
    context.Result(result)
    return result
    

The Message and Result methods should typically begin and end a custom check to let the user know what's going on: the Message call prints the specified message (with no trailing newline) and the Result call prints yes if the check succeeds and no if it doesn't. The TryLink method actually tests for whether the specified program text will successfully link.

(Note that a custom check can modify its check based on any arguments you choose to pass it, or by using or modifying the configure context environment in the context.env attribute.)

This custom check function is then attached to the configure context by passing a dictionary to the Configure call that maps a name of the check to the underlying function:

env = Environment()
conf = Configure(env, custom_tests={'CheckMyLibrary': CheckMyLibrary})
    

You'll typically want to make the check and the function name the same, as we've done here, to avoid potential confusion.

We can then put these pieces together and actually call the CheckMyLibrary check as follows:

mylib_test_source_file = """
#include <mylib.h>
int main(int argc, char **argv)
{
    MyLibrary mylib(argc, argv);
    return 0;
}
"""

def CheckMyLibrary(context):
    context.Message('Checking for MyLibrary... ')
    result = context.TryLink(mylib_test_source_file, '.c')
    context.Result(result)
    return result

env = Environment()
conf = Configure(env, custom_tests={'CheckMyLibrary': CheckMyLibrary})
if not conf.CheckMyLibrary():
    print('MyLibrary is not installed!')
    Exit(1)
env = conf.Finish()

# We would then add actual calls like Program() to build
# something using the "env" construction environment.
    

If MyLibrary is not installed on the system, the output will look like:

% scons
scons: Reading SConscript file ...
Checking for MyLibrary... no
MyLibrary is not installed!
    

If MyLibrary is installed, the output will look like:

% scons
scons: Reading SConscript file ...
Checking for MyLibrary... yes
scons: done reading SConscript
scons: Building targets ...
    .
    .
    .
    

On multi-developer software projects, you can sometimes speed up every developer's builds a lot by allowing them to share a cache of the derived files that they build. After all, it is relatively rare that any in-progress change affects more than a few derived files, most will be unchanged. Using a cache can also help an individual developer: for example if you wish to start work on a new feature in a clean tree, those build artifacts which could be reused can be retrieved from the cache to populate the tree and save a lot of initial build time. SCons makes this easy and reliable.

To enable caching of derived files, use the CacheDir function in any SConscript file:

CacheDir('/usr/local/build_cache')
       

The cache directory you specify must have read and write access for all developers who will be accessing the cached files (if --cache-readonly is used, only read access is required). It should also be in some central location that all builds will be able to access. In environments where developers are using separate systems (like individual workstations) for builds, this directory would typically be on a shared or NFS-mounted file system. While SCons will create the specified cache directory as needed, in this multi user scenario it is usually best to create it ahead of time so the access rights can be set up correctly.

Here's what happens: When a build has a CacheDir specified, every time a file is built, it is stored in that cache directory indexed by its build signature. On subsequent builds, before an action is invoked to build a file, the build signature is computed and SCons checks the derived-file cache directory to see if a file with the exact same build signature already exists. [4] If so, the derived file will not be built locally, but will be copied into the local build directory from the derived-file cache directory, like this:

% scons -Q
cc -o hello.o -c hello.c
cc -o hello hello.o
% scons -Q -c
Removed hello.o
Removed hello
% scons -Q
Retrieved `hello.o' from cache
Retrieved `hello' from cache

Note that the CacheDir feature requires that the build signature be calculated, even if you configure SCons to use timestamps to decide if files are up to date (see the Chapter 6, Dependencies chapter for information about the Decider function), since the build signature is used to determine if a target file exists in the cache. Consequently, using CacheDir may reduce or negate any performance improvements from using timestamps for up-to-date decisions.

You may want to disable caching for certain specific files in your configuration. For example, if you only want to put executable files in a central cache, but not the intermediate object files, you can use the NoCache function to specify that the object files should not be cached:

env = Environment()
obj = env.Object('hello.c')
env.Program('hello.c')
CacheDir('cache')
NoCache('hello.o')
       

Then when you run scons after cleaning the built targets, it will recompile the object file locally (since it doesn't exist in the derived-file cache directory), but still realize that the derived-file cache directory contains an up-to-date executable program that can be retrieved instead of re-linking:

% scons -Q
cc -o hello.o -c hello.c
cc -o hello hello.o
% scons -Q -c
Removed hello.o
Removed hello
% scons -Q
cc -o hello.o -c hello.c
Retrieved `hello' from cache
    

If you allow multiple builds to update the derived-file cache directory simultaneously, two builds that occur at the same time can sometimes start "racing" with one another to build the same files in the same order. If, for example, you are linking multiple files into an executable program:

Program('prog', ['f1.c', 'f2.c', 'f3.c', 'f4.c', 'f5.c'])
       

SCons will normally build the input object files on which the program depends in their normal, sorted order:

% scons -Q
cc -o f3.o -c f3.c
cc -o f5.o -c f5.c
cc -o f4.o -c f4.c
cc -o f1.o -c f1.c
cc -o f2.o -c f2.c
cc -o prog f1.o f2.o f3.o f4.o f5.o

But if two such builds take place simultaneously, they may each look in the cache at nearly the same time and both decide that f1.o must be rebuilt and pushed into the derived-file cache directory, then both decide that f2.o must be rebuilt (and pushed into the derived-file cache directory), then both decide that f3.o must be rebuilt... This won't cause any actual build problems--both builds will succeed, generate correct output files, and populate the cache--but it does represent wasted effort.

To alleviate such contention for the cache, you can use the --random command-line option to tell SCons to build dependencies in a random order:

  % scons -Q --random
  cc -o f3.o -c f3.c
  cc -o f1.o -c f1.c
  cc -o f5.o -c f5.c
  cc -o f2.o -c f2.c
  cc -o f4.o -c f4.c
  cc -o prog f1.o f2.o f3.o f4.o f5.o
    

Multiple builds using the --random option will usually build their dependencies in different, random orders, which minimizes the chances for a lot of contention for same-named files in the derived-file cache directory. Multiple simultaneous builds might still race to try to build the same target file on occasion, but long sequences of inefficient contention should be rare.

Note, of course, the --random option will cause the output that SCons prints to be inconsistent from invocation to invocation, which may be an issue when trying to compare output from different build runs.

If you want to make sure dependencies will be built in a random order without having to specify the --random on very command line, you can use the SetOption function to set the random option within any SConscript file:

SetOption('random', 1)
Program('prog', ['f1.c', 'f2.c', 'f3.c', 'f4.c', 'f5.c'])
       

You can customize the behavior of derived-file caching to add your own features, for example to support compressed and/or encrypted cache files, modify cache file permissions to better support shared caches, gather additional statistics and data, etc.

To define custom cache behavior, subclass the SCons.CacheDir.CacheDir class, specializing those methods you want to change. You can pass this custom class as the custom_class parameter when calling CacheDir for global reach, or when calling env.CacheDir for a specific environment. You can also set the construction variable $CACHEDIR_CLASS to the custom class - this needs to happen before configuring the cache in that environment. SCons will internally invoke and use your custom class when performing cache operations. The below example shows a simple use case of overriding the copy_from_cache method to record the total number of bytes pulled from the cache.

import os
import SCons.CacheDir

class CustomCacheDir(SCons.CacheDir.CacheDir):
    total_retrieved = 0

    @classmethod
    def copy_from_cache(cls, env, src, dst):
        # record total bytes pulled from cache
        cls.total_retrieved += os.stat(src).st_size
        return super().copy_from_cache(env, src, dst)

env = Environment()
env.CacheDir('scons-cache', custom_class=CustomCacheDir)
# ...
      


[4] A few inside details: SCons tracks two main kinds of cryptographic hashes: a content signature, which is a hash of the contents of a file participating in the build (dependencies as well as targets); and a build signature, which is a hash of the elements needed to build a target, such as the command line, the contents of the sources, and possibly information about tools used in the build. The hash function produces a unique signature from its inputs, no other set of inputs can produce that same signature. The build signature from building a target is used as the filename of the target file in the derived-file cache - that way lookups are efficient, just compute a build signature and see if a file exists with that as the name.

The use of the build signature provides protection from concflicts: if two developers have different setups, so they would produce built objects that are not identical, then because the difference in tools will show up in the build signature, which is used as the name of the cache entry, they will end up being stored as separate entries.

We've already seen how you can use the Alias function to create a target named install:

env = Environment()
hello = env.Program('hello.c')
env.Install('/usr/bin', hello)
env.Alias('install', '/usr/bin')
     

You can then use this alias on the command line to tell SCons more naturally that you want to install files:

% scons -Q install
cc -o hello.o -c hello.c
cc -o hello hello.o
Install file: "hello" as "/usr/bin/hello"

Like other Builder methods, though, the Alias method returns an object representing the alias being built. You can then use this object as input to anothother Builder. This is especially useful if you use such an object as input to another call to the Alias Builder, allowing you to create a hierarchy of nested aliases:

env = Environment()
p = env.Program('foo.c')
l = env.Library('bar.c')
env.Install('/usr/bin', p)
env.Install('/usr/lib', l)
ib = env.Alias('install-bin', '/usr/bin')
il = env.Alias('install-lib', '/usr/lib')
env.Alias('install', [ib, il])
     

This example defines separate install, install-bin, and install-lib aliases, allowing you finer control over what gets installed:

% scons -Q install-bin
cc -o foo.o -c foo.c
cc -o foo foo.o
Install file: "foo" as "/usr/bin/foo"
% scons -Q install-lib
cc -o bar.o -c bar.c
ar rc libbar.a bar.o
ranlib libbar.a
Install file: "libbar.a" as "/usr/lib/libbar.a"
% scons -Q -c /
Removed foo.o
Removed foo
Removed /usr/bin/foo
Removed bar.o
Removed libbar.a
Removed /usr/lib/libbar.a
% scons -Q install
cc -o foo.o -c foo.c
cc -o foo foo.o
Install file: "foo" as "/usr/bin/foo"
cc -o bar.o -c bar.c
ar rc libbar.a bar.o
ranlib libbar.a
Install file: "libbar.a" as "/usr/lib/libbar.a"

So far, we've been using examples of building C and C++ programs to demonstrate the features of SCons. SCons also supports building Java programs, but Java builds are handled slightly differently, which reflects the ways in which the Java compiler and tools build programs differently than other languages' tool chains.

The basic activity when programming in Java, of course, is to take one or more .java files containing Java source code and to call the Java compiler to turn them into one or more .class files. In SCons, you do this by giving the Java Builder a target directory in which to put the .class files, and a source directory that contains the .java files:

Java('classes', 'src')
      

If the src directory contains three .java source files, then running SCons might look like this:

% scons -Q
javac -d classes -sourcepath src src/Example1.java src/Example2.java src/Example3.java

SCons will actually search the src directory tree for all of the .java files. The Java compiler will then create the necessary class files in the classes subdirectory, based on the class names found in the .java files.

In addition to searching the source directory for .java files, SCons actually runs the .java files through a stripped-down Java parser that figures out what classes are defined. In other words, SCons knows, without you having to tell it, what .class files will be produced by the javac call. So our one-liner example from the preceding section:

Java('classes', 'src')
      

Will not only tell you reliably that the .class files in the classes subdirectory are up-to-date:

% scons -Q
javac -d classes -sourcepath src src/Example1.java src/Example2.java src/Example3.java
% scons -Q classes
scons: `classes' is up to date.

But it will also remove all of the generated .class files, even for inner classes, without you having to specify them manually. For example, if our Example1.java and Example3.java files both define additional classes, and the class defined in Example2.java has an inner class, running scons -c will clean up all of those .class files as well:

% scons -Q
javac -d classes -sourcepath src src/Example1.java src/Example2.java src/Example3.java
% scons -Q -c classes
Removed classes/Example1.class
Removed classes/AdditionalClass1.class
Removed classes/Example2$Inner2.class
Removed classes/Example2.class
Removed classes/Example3.class
Removed classes/AdditionalClass3.class

To ensure correct handling of .class dependencies in all cases, you need to tell SCons which Java version is being used. This is needed because Java 1.5 changed the .class file names for nested anonymous inner classes. Use the JAVAVERSION construction variable to specify the version in use. With Java 1.6, the one-liner example can then be defined like this:

Java('classes', 'src', JAVAVERSION='1.6')
    

See JAVAVERSION in the man page for more information.

After building the class files, it's common to collect them into a Java archive (.jar) file, which you do by calling the Jar Builder. If you want to just collect all of the class files within a subdirectory, you can just specify that subdirectory as the Jar source:

Java(target='classes', source='src')
Jar(target='test.jar', source='classes')
      

SCons will then pass that directory to the jar command, which will collect all of the underlying .class files:

% scons -Q
javac -d classes -sourcepath src src/Example1.java src/Example2.java src/Example3.java
jar cf test.jar classes

If you want to keep all of the .class files for multiple programs in one location, and only archive some of them in each .jar file, you can pass the Jar builder a list of files as its source. It's extremely simple to create multiple .jar files this way, using the lists of target class files created by calls to the Java builder as sources to the various Jar calls:

prog1_class_files = Java(target='classes', source='prog1')
prog2_class_files = Java(target='classes', source='prog2')
Jar(target='prog1.jar', source=prog1_class_files)
Jar(target='prog2.jar', source=prog2_class_files)
      

This will then create prog1.jar and prog2.jar next to the subdirectories that contain their .java files:

% scons -Q
javac -d classes -sourcepath prog1 prog1/Example1.java prog1/Example2.java
javac -d classes -sourcepath prog2 prog2/Example3.java prog2/Example4.java
jar cf prog1.jar -C classes Example1.class -C classes Example2.class
jar cf prog2.jar -C classes Example3.class -C classes Example4.class

You can generate C header and source files for implementing native methods, by using the JavaH Builder. There are several ways of using the JavaH Builder. One typical invocation might look like:

classes = Java(target='classes', source='src/pkg/sub')
JavaH(target='native', source=classes)
      

The source is a list of class files generated by the call to the Java Builder, and the target is the output directory in which we want the C header files placed. The target gets converted into the -d when SCons runs javah:

% scons -Q
javac -d classes -sourcepath src/pkg/sub src/pkg/sub/Example1.java src/pkg/sub/Example2.java src/pkg/sub/Example3.java
javah -d native -classpath classes pkg.sub.Example1 pkg.sub.Example2 pkg.sub.Example3

In this case, the call to javah will generate the header files native/pkg_sub_Example1.h, native/pkg_sub_Example2.h and native/pkg_sub_Example3.h. Notice that SCons remembered that the class files were generated with a target directory of classes, and that it then specified that target directory as the -classpath option to the call to javah.

Although it's more convenient to use the list of class files returned by the Java Builder as the source of a call to the JavaH Builder, you can specify the list of class files by hand, if you prefer. If you do, you need to set the $JAVACLASSDIR construction variable when calling JavaH:

Java(target='classes', source='src/pkg/sub')
class_file_list = [
    'classes/pkg/sub/Example1.class',
    'classes/pkg/sub/Example2.class',
    'classes/pkg/sub/Example3.class',
]
JavaH(target='native', source=class_file_list, JAVACLASSDIR='classes')
      

The $JAVACLASSDIR value then gets converted into the -classpath when SCons runs javah:

% scons -Q
javac -d classes -sourcepath src/pkg/sub src/pkg/sub/Example1.java src/pkg/sub/Example2.java src/pkg/sub/Example3.java
javah -d native -classpath classes pkg.sub.Example1 pkg.sub.Example2 pkg.sub.Example3

Lastly, if you don't want a separate header file generated for each source file, you can specify an explicit File Node as the target of the JavaH Builder:

classes = Java(target='classes', source='src/pkg/sub')
JavaH(target=File('native.h'), source=classes)
      

Because SCons assumes by default that the target of the JavaH builder is a directory, you need to use the File function to make sure that SCons doesn't create a directory named native.h. When a file is used, though, SCons correctly converts the file name into the javah -o option:

% scons -Q
javac -d classes -sourcepath src/pkg/sub src/pkg/sub/Example1.java src/pkg/sub/Example2.java src/pkg/sub/Example3.java
javah -o native.h -classpath classes pkg.sub.Example1 pkg.sub.Example2 pkg.sub.Example3

Note that the the javah command was removed from the JDK as of JDK 10, and the approved method (available since JDK 8) is to use javac to generate native headers at the same time as the Java source code is compiled.. As such the JavaH builder is of limited utility in later Java versions.

You can generate Remote Method Invocation stubs by using the RMIC Builder. The source is a list of directories, typically returned by a call to the Java Builder, and the target is an output directory where the _Stub.class and _Skel.class files will be placed:

classes = Java(target='classes', source='src/pkg/sub')
RMIC(target='outdir', source=classes)
      

As it did with the JavaH Builder, SCons remembers the class directory and passes it as the -classpath option to rmic:

% scons -Q
javac -d classes -sourcepath src/pkg/sub src/pkg/sub/Example1.java src/pkg/sub/Example2.java
rmic -d outdir -classpath classes pkg.sub.Example1 pkg.sub.Example2

This example would generate the files outdir/pkg/sub/Example1_Skel.class, outdir/pkg/sub/Example1_Stub.class, outdir/pkg/sub/Example2_Skel.class and outdir/pkg/sub/Example2_Stub.class.

The gettext toolset supports internationalization and localization of SCons-based projects. Builders provided by gettext automatize generation and updates of translation files. You can manage translations and translation templates similarly to how it's done with autotools.

Let's start with a very simple project, the "Hello world" program for example

/* hello.c */
#include <stdio.h>
int main(int argc, char* argv[])
{
  printf("Hello world\n");
  return 0;
}
    

Prepare a SConstruct to compile the program as usual.

# SConstruct
env = Environment()
hello = Program(["hello.c"])
    

Now we'll convert the project to a multi-lingual one. If you don't already have GNU gettext utilities installed, install them from your preffered package repository, or download from http://ftp.gnu.org/gnu/gettext/. For the purpose of this example, you should have following three locales installed on your system: en_US, de_DE and pl_PL. On debian, for example, you may enable certain locales through dpkg-reconfigure locales.

First prepare the hello.c program for internationalization. Change the previous code so it reads as follows:

/* hello.c */
#include <stdio.h>
#include <libintl.h>
#include <locale.h>
int main(int argc, char* argv[])
{
  bindtextdomain("hello", "locale");
  setlocale(LC_ALL, "");
  textdomain("hello");
  printf(gettext("Hello world\n"));
  return 0;
}
    

Detailed recipes for such conversion can be found at http://www.gnu.org/software/gettext/manual/gettext.html#Sources. The gettext("...") has two purposes. First, it marks messages for the xgettext(1) program, which we will use to extract from the sources the messages for localization. Second, it calls the gettext library internals to translate the message at runtime.

Now we shall instruct SCons how to generate and maintain translation files. For that, use the Translate builder and MOFiles builder. The first one takes source files, extracts internationalized messages from them, creates so-called POT file (translation template), and then creates PO translation files, one for each requested language. Later, during the development lifecycle, the builder keeps all these files up-to date. The MOFiles builder compiles the PO files to binary form. Then install the MO files under directory called locale.

The completed SConstruct is as follows:

# SConstruct
env = Environment( tools = ['default', 'gettext'] )
hello = env.Program(["hello.c"])
env['XGETTEXTFLAGS'] = [
  '--package-name=%s' % 'hello',
  '--package-version=%s' % '1.0',
]
po = env.Translate(["pl","en", "de"], ["hello.c"], POAUTOINIT = 1)
mo = env.MOFiles(po)
InstallAs(["locale/en/LC_MESSAGES/hello.mo"], ["en.mo"])
InstallAs(["locale/pl/LC_MESSAGES/hello.mo"], ["pl.mo"])
InstallAs(["locale/de/LC_MESSAGES/hello.mo"], ["de.mo"])
    

Generate the translation files with scons po-update. You should see the output from SCons simillar to this:

user@host:$ scons po-update
scons: Reading SConscript files ...
scons: done reading SConscript files.
scons: Building targets ...
Entering '/home/ptomulik/projects/tmp'
xgettext --package-name=hello --package-version=1.0 -o - hello.c
Leaving '/home/ptomulik/projects/tmp'
Writting 'messages.pot' (new file)
msginit --no-translator -l pl -i messages.pot -o pl.po
Created pl.po.
msginit --no-translator -l en -i messages.pot -o en.po
Created en.po.
msginit --no-translator -l de -i messages.pot -o de.po
Created de.po.
scons: done building targets.
    

If everything is right, you should see following new files.

user@host:$ ls *.po*
de.po  en.po  messages.pot  pl.po
    

Open en.po in poedit and provide the English translation to message "Hello world\n". Do the same for de.po (deutsch) and pl.po (polish). Let the translations be, for example:

  • en: "Welcome to beautiful world!\n"

  • de: "Hallo Welt!\n"

  • pl: "Witaj swiecie!\n"

Now compile the project by executing scons. The output should be similar to this:

user@host:$ scons
scons: Reading SConscript files ...
scons: done reading SConscript files.
scons: Building targets ...
msgfmt -c -o de.mo de.po
msgfmt -c -o en.mo en.po
gcc -o hello.o -c hello.c
gcc -o hello hello.o
Install file: "de.mo" as "locale/de/LC_MESSAGES/hello.mo"
Install file: "en.mo" as "locale/en/LC_MESSAGES/hello.mo"
msgfmt -c -o pl.mo pl.po
Install file: "pl.mo" as "locale/pl/LC_MESSAGES/hello.mo"
scons: done building targets.
    

SCons automatically compiled the PO files to binary format MO, and the InstallAs lines installed these files under locale folder.

Your program should be now ready. You may try it as follows (linux):

user@host:$ LANG=en_US.UTF-8 ./hello
Welcome to beautiful world
    

user@host:$ LANG=de_DE.UTF-8 ./hello
Hallo Welt
    

user@host:$ LANG=pl_PL.UTF-8 ./hello
Witaj swiecie
    

To demonstrate the further life of translation files, let's change Polish translation (poedit pl.po) to "Witaj drogi swiecie\n". Run scons to see how scons reacts to this

user@host:$scons
scons: Reading SConscript files ...
scons: done reading SConscript files.
scons: Building targets ...
msgfmt -c -o pl.mo pl.po
Install file: "pl.mo" as "locale/pl/LC_MESSAGES/hello.mo"
scons: done building targets.
    

Now, open hello.c and add another one printf line with new message.

/* hello.c */
#include <stdio.h>
#include <libintl.h>
#include <locale.h>
int main(int argc, char* argv[])
{
  bindtextdomain("hello", "locale");
  setlocale(LC_ALL, "");
  textdomain("hello");
  printf(gettext("Hello world\n"));
  printf(gettext("and good bye\n"));
  return 0;
}
    

Compile project with scons. This time, the msgmerge(1) program is used by SCons to update PO file. The output from compilation is like:

user@host:$scons
scons: Reading SConscript files ...
scons: done reading SConscript files.
scons: Building targets ...
Entering '/home/ptomulik/projects/tmp'
xgettext --package-name=hello --package-version=1.0 -o - hello.c
Leaving '/home/ptomulik/projects/tmp'
Writting 'messages.pot' (messages in file were outdated)
msgmerge --update de.po messages.pot
... done.
msgfmt -c -o de.mo de.po
msgmerge --update en.po messages.pot
... done.
msgfmt -c -o en.mo en.po
gcc -o hello.o -c hello.c
gcc -o hello hello.o
Install file: "de.mo" as "locale/de/LC_MESSAGES/hello.mo"
Install file: "en.mo" as "locale/en/LC_MESSAGES/hello.mo"
msgmerge --update pl.po messages.pot
... done.
msgfmt -c -o pl.mo pl.po
Install file: "pl.mo" as "locale/pl/LC_MESSAGES/hello.mo"
scons: done building targets.
    

The next example demonstrates what happens if we change the source code in such way that the internationalized messages do not change. The answer is that none of translation files (POT, PO) are touched (i.e. no content changes, no creation/modification time changed and so on). Let's append another line to the program (after the last printf), so its code becomes:

/* hello.c */
#include <stdio.h>
#include <libintl.h>
#include <locale.h>
int main(int argc, char* argv[])
{
  bindtextdomain("hello", "locale");
  setlocale(LC_ALL, "");
  textdomain("hello");
  printf(gettext("Hello world\n"));
  printf(gettext("and good bye\n"));
  printf("----------------\n");
  return a;
}
    

Compile the project. You'll see on your screen

user@host:$scons
scons: Reading SConscript files ...
scons: done reading SConscript files.
scons: Building targets ...
Entering '/home/ptomulik/projects/tmp'
xgettext --package-name=hello --package-version=1.0 -o - hello.c
Leaving '/home/ptomulik/projects/tmp'
Not writting 'messages.pot' (messages in file found to be up-to-date)
gcc -o hello.o -c hello.c
gcc -o hello hello.o
scons: done building targets.
    

As you see, the internationalized messages ditn't change, so the POT and the rest of translation files have not even been touched.

SCons supports a lot of additional functionality that doesn't readily fit into the other chapters.

The FindFile function searches for a file in a list of directories. If there is only one directory, it can be given as a simple string. The function returns a File node if a matching file exists, or None if no file is found. (See the documentation for the Glob function for an alternative way of searching for entries in a directory.)

# one directory
print("%s"%FindFile('missing', '.'))
t = FindFile('exists', '.')
print("%s %s"%(t.__class__, t))
      
% scons -Q
None
<class 'SCons.Node.FS.File'> exists
scons: `.' is up to date.
# several directories
includes = [ '.', 'include', 'src/include']
headers = [ 'nonesuch.h', 'config.h', 'private.h', 'dist.h']
for hdr in headers:
    print('%-12s: %s'%(hdr, FindFile(hdr, includes)))
      
% scons -Q
nonesuch.h  : None
config.h    : config.h
private.h   : src/include/private.h
dist.h      : include/dist.h
scons: `.' is up to date.

If the file exists in more than one directory, only the first occurrence is returned.

print(FindFile('multiple', ['sub1', 'sub2', 'sub3']))
print(FindFile('multiple', ['sub2', 'sub3', 'sub1']))
print(FindFile('multiple', ['sub3', 'sub1', 'sub2']))
      
% scons -Q
sub1/multiple
sub2/multiple
sub3/multiple
scons: `.' is up to date.

In addition to existing files, FindFile will also find derived files (that is, non-leaf files) that haven't been built yet. (Leaf files should already exist, or the build will fail!)

# Neither file exists, so build will fail
Command('derived', 'leaf', 'cat >$TARGET $SOURCE')
print(FindFile('leaf', '.'))
print(FindFile('derived', '.'))
      
% scons -Q
leaf
derived
cat > derived leaf
# Only 'leaf' exists
Command('derived', 'leaf', 'cat >$TARGET $SOURCE')
print(FindFile('leaf', '.'))
print(FindFile('derived', '.'))
      
% scons -Q
leaf
derived
cat > derived leaf

If a source file exists, FindFile will correctly return the name in the build directory.

# Only 'src/leaf' exists
VariantDir('build', 'src')
print(FindFile('leaf', 'build'))
      
% scons -Q
build/leaf
scons: `.' is up to date.

SCons supports a Flatten function which takes an input Python sequence (list or tuple) and returns a flattened list containing just the individual elements of the sequence. This can be handy when trying to examine a list composed of the lists returned by calls to various Builders. For example, you might collect object files built in different ways into one call to the Program Builder by just enclosing them in a list, as follows:

objects = [
    Object('prog1.c'),
    Object('prog2.c', CCFLAGS='-DFOO'),
]
Program(objects)
      

Because the Builder calls in SCons flatten their input lists, this works just fine to build the program:

% scons -Q
cc -o prog1.o -c prog1.c
cc -o prog2.o -c -DFOO prog2.c
cc -o prog1 prog1.o prog2.o

But if you were debugging your build and wanted to print the absolute path of each object file in the objects list, you might try the following simple approach, trying to print each Node's abspath attribute:

objects = [
    Object('prog1.c'),
    Object('prog2.c', CCFLAGS='-DFOO'),
]
Program(objects)

for object_file in objects:
    print(object_file.abspath)
      

This does not work as expected because each call to str is operating an embedded list returned by each Object call, not on the underlying Nodes within those lists:

% scons -Q
AttributeError: 'NodeList' object has no attribute 'abspath':
  File "/home/my/project/SConstruct", line 8:
    print(object_file.abspath)

The solution is to use the Flatten function so that you can pass each Node to the str separately:

objects = [
    Object('prog1.c'),
    Object('prog2.c', CCFLAGS='-DFOO'),
]
Program(objects)

for object_file in Flatten(objects):
    print(object_file.abspath)
      
% scons -Q
/home/me/project/prog1.o
/home/me/project/prog2.o
cc -o prog1.o -c prog1.c
cc -o prog2.o -c -DFOO prog2.c
cc -o prog1 prog1.o prog2.o
    

Sometimes the way an action is defined causes effects on files that SCons does not recognize as targets. The SideEffect method can be used to informs SCons about such files. This can be used just to flag a dependency for use in subsequent build steps, although there is usually a better way to do that. The primary use for the SideEffect method is to prevent two build steps from simultaneously modifying or accessing the same file in a way that could impact each other.

In this example, the rule to build file1 will also put data into log, which is used as a source for the command to generate file2, but log is unknown to SCons on a clean build: it neither exists, nor is it a target output by any builder. The SConscript uses SideEffect to inform SCons about the additional output file.

env = Environment()
f2 = env.Command(
    target='file2',
    source='log',
    action=Copy('$TARGET', '$SOURCE')
)
f1 = env.Command(
    target='file1',
    source=[],
    action='echo >$TARGET data1; echo >log updated file1'
)
env.SideEffect('log', f1)
   

Without the SideEffect, this build would fail with a message Source `log' not found, needed by target `file2', but now it can proceed:

% scons -Q
echo > file1 data1; echo >log updated file1
Copy("file2", "log")

However, it is better to actually identify log as a target, since in this case that's what it is:

env = Environment()
f2 = env.Command(
    target='file2',
    source='log',
    action=Copy('$TARGET', '$SOURCE')
)
f1 = env.Command(
    target=['file1', 'log'],
    source=[],
    action='echo >$TARGET data1; echo >log updated file1'
)
   
% scons -Q
echo > file1 data1; echo >log updated file1
Copy("file2", "log")

In general, SideEffect is not intended for the case when a command produces extra target files (that is, files which will be used as sources to other build steps). For example, the the Microsoft Visual C++ compiler is capable of performing incremental linking, for which it uses a status file - such that linking foo.exe also produces a foo.ilk, or uses it if it was already present, if the /INCREMENTAL option was supplied. Specifying foo.ilk as a side-effect of foo.exe is not a recommended use of SideEffect since foo.ilk is used by the link. SCons handles side-effect files slightly differently in its analysis of the dependency graph. When a command produces multiple output files, they should be specified as multiple targets of the call to the relevant builder function. The SideEffect function itself should really only be used when it's important to ensure that commands are not executed in parallel, such as when a "peripheral" file (such as a log file) may actually be updated by more than one command invocation.

Unfortunately, the tool which sets up the Program builder for the Microsoft Visual C++ compiler chain does not come prebuilt with an understanding of the details of the .ilk example - that the target list would need to change in the presence of that specific option flag. Unlike the trivial example above where we could simply tell the Command builder there were two targets of the action, modifying the chain of events for a builder like Program, though not inherently complex, is definitely an advanced SCons topic. It's okay to use SideEffect here to get started, as long as it comes with an understanding that it's "not quite right". Perhaps leave a comment in the file as a reminder, if it does turn out to cause problems later.

So if the main use is to prevent parallelism problems, here is an example to illustrate. Say a program that you need to call to build a target file will also update a log file describing what the program does while building the target. The following configuration would have SCons invoke a hypothetical script named build (in the local directory) with command-line arguments telling it to write log information to a common logfile.txt file:

env = Environment()
env.Command(
    target='file1.out',
    source='file1.in',
    action='./build --log logfile.txt $SOURCE $TARGET'
)
env.Command(
    target='file2.out',
    source='file2.in',
    action='./build --log logfile.txt $SOURCE $TARGET'
)
 

This can cause problems when running the build in parallel if SCons decides to update both targets by running both program invocations at the same time. The multiple program invocations may interfere with each other writing to the common log file, leading at best to intermixed output in the log file, and at worst to an actual failed build (on a system like Windows, for example, where only one process at a time can open the log file for writing).

We can make sure that SCons does not run these build commands at the same time by using the SideEffect function to specify that updating the logfile.txt file is a side effect of building the specified file1 and file2 target files:

env = Environment()
f1 = env.Command(
    target='file1.out',
    source='file1.in',
    action='./build --log logfile.txt $SOURCE $TARGET'
)
f2 = env.Command(
    target='file2.out',
    source='file2.in',
    action='./build --log logfile.txt $SOURCE $TARGET'
)
env.SideEffect('logfile.txt', f1 + f2)
   

This makes sure the the two ./build steps are run sequentially, even with the --jobs=2 in the command line:

% scons -Q --jobs=2
./build --log logfile.txt file1.in file1.out
./build --log logfile.txt file2.in file2.out

The SideEffect function can be called multiple times for the same side-effect file. In fact, the name used as a SideEffect does not even need to actually exist as a file on disk - SCons will still make sure that the relevant targets will be executed sequentially, not in parallel. The side effect is actually a pseudo-target, and SCons mainly cares whether nodes are listed as depending on it, not about its contents.

env = Environment()
f1 = env.Command('file1.out', [], action='echo >$TARGET data1')
env.SideEffect('not_really_updated', f1)
f2 = env.Command('file2.out', [], action='echo >$TARGET data2')
env.SideEffect('not_really_updated', f2)
   
% scons -Q --jobs=2
echo > file1.out data1
echo > file2.out data2

Virtualenv is a tool to create isolated Python environments. A python application (such as SCons) may be executed within an activated virtualenv. The activation of virtualenv modifies current environment by defining some virtualenv-specific variables and modifying search PATH, such that executables installed within virtualenv's home directory are preferred over the ones installed outside of it.

Normally, SCons uses hard-coded PATH when searching for external executables, so it always picks-up executables from these pre-defined locations. This applies also to python interpreter, which is invoked by some custom SCons tools or test suites. This means, when running SCons in a virtualenv, an eventual invocation of python interpreter from SCons script will most probably jump out of virtualenv and execute python executable found in hard-coded SCons PATH, not the one which is executing SCons. Some users may consider this as an inconsistency.

This issue may be overcome by using the --enable-virtualenv option. The option automatically imports virtualenv-related environment variables to all created construction environment env['ENV'], and modifies SCons PATH appropriately to prefer virtualenv's executables. Setting environment variable SCONS_ENABLE_VIRTUALENV=1 will have same effect. If virtualenv support is enabled system-vide by the environment variable, it may be suppressed with the --ignore-virtualenv option.

Inside of SConscript, a global function Virtualenv is available. It returns a path to virtualenv's home directory, or None if scons is not running from virtualenv. Note that this function returns a path even if scons is run from an unactivated virtualenv.

Sometimes a project needs to interact with other projects in various ways. For example, many open source projects make use of components from other open source projects, and want to use those in their released form, not recode their builds into SCons. As another example, sometimes the flexibility and power of SCons is useful for managing the overall project, but developers might like faster incremental builds when making small changes by using a different tool.

This chapter shows some techniques for interacting with other projects and tools effectively from within SCons.

Tooling to perform analysis and modification of source code often needs to know not only the source code itself, but also how it will be compiled, as the compilation line affects the behavior of macros, includes, etc. SCons has a record of this information once it has run, in the form of Actions associated with the sources, and can emit this information so tools can use it.

The Clang project has defined a JSON Compilation Database. This database is in common use as input into Clang tools and many IDEs and editors as well. See JSON Compilation Database Format Specification for complete information. SCons can emit a compilation database in this format by enabling the compilation_db tool and calling the CompilationDatabase builder (available since scons 4.0).

The compilation database can be populated with source and output files either with paths relative to the top of the build, or using absolute paths. This is controlled by COMPILATIONDB_USE_ABSPATH=(True|False) which defaults to False. The entries in this file can be filtered by using COMPILATIONDB_PATH_FILTER='pattern' where the filter pattern is a string following the Python fnmatch syntax. This filtering can be used for outputting different build variants to different compilation database files.

The following example illustrates generating a compilation database containing absolute paths:

env = Environment(COMPILATIONDB_USE_ABSPATH=True)
env.Tool('compilation_db')
env.CompilationDatabase()
env.Program('hello.c')
            
% scons -Q
Building compilation database compile_commands.json
cc -o hello.o -c hello.c
cc -o hello hello.o

compile_commands.json contains:

[
    {
        "command": "gcc -o hello.o -c hello.c",
        "directory": "/home/user/sandbox",
        "file": "/home/user/sandbox/hello.c",
        "output": "/home/user/sandbox/hello.o"
    }
]
        

Notice that the generated database contains only an entry for the hello.c/hello.o pairing, and nothing for the generation of the final executable hello - the transformation of hello.o to hello does not have any information that affects interpretation of the source code, so it is not interesting to the compilation database.

Although it can be a little surprising at first glance, a compilation database target is, like any other target, subject to scons target selection rules. This means if you set a default target (that does not include the compilation database), or use command-line targets, it might not be selected for building. This can actually be an advantage, since you don't necessarily want to regenerate the compilation database every build. The following example shows selecting relative paths (the default) for output and source, and also giving a non-default name to the database. In order to be able to generate the database separately from building, an alias is set referring to the database, which can then be used as a target - here we are only building the compilation database target, not the code.

env = Environment()
env.Tool('compilation_db')
cdb = env.CompilationDatabase('compile_database.json')
Alias('cdb', cdb)
env.Program('test_main.c')
            
% scons -Q cdb
Building compilation database compile_database.json

compile_database.json contains:

[
    {
        "command": "gcc -o test_main.o -c test_main.c",
        "directory": "/home/user/sandbox",
        "file": "test_main.c",
        "output": "test_main.o"
    }
]
        

The following (incomplete) example shows using filtering to separate build variants. In the case of using variants, you want different compilation databases for each, since the build parameters differ, so the code analysis needs to see the correct build lines for the 32-bit build and 64-bit build hinted at here. For simplicity of presentation, the example omits the setup details of the variant directories:

env = Environment()
env.Tool("compilation_db")

env1 = env.Clone()
env1["COMPILATIONDB_PATH_FILTER"] = "build/linux32/*"
env1.CompilationDatabase("compile_commands-linux32.json")

env2 = env.Clone()
env2["COMPILATIONDB_PATH_FILTER"] = "build/linux64/*"
env2.CompilationDatabase('compile_commands-linux64.json')
        

compile_commands-linux32.json contains:

[
    {
        "command": "gcc -o hello.o -c hello.c",
        "directory": "/home/mats/github/scons/exp/compdb",
        "file": "hello.c",
        "output": "hello.o"
    }
]
        

compile_commands-linux64.json contains:

[
    {
        "command": "gcc -m64 -o build/linux64/test_main.o -c test_main.c",
        "directory": "/home/user/sandbox",
        "file": "test_main.c",
        "output": "build/linux64/test_main.o"
    }
]
        

Note

This is an experimental new feature. It is subject to change and/or removal without a depreciation cycle.

Loading the ninja tool into SCons will make significant changes in SCons' normal functioning.

  • SCons will no longer execute any commands directly and will only create the build.ninja and run ninja.

  • Any targets specified on the command line will be passed along to ninja

To enable this feature you'll need to use one of the following:

# On the command line --experimental=ninja

# Or in your SConstruct
SetOption('experimental', 'ninja')
    

Ninja is a small build system that tries to be fast by not making decisions. SCons can at times be slow because it makes lots of decisions to carry out its goal of "correctness". The two tools can be paired to benefit some build scenarios: by using the ninja tool, SCons can generate the build file ninja uses (basically doing the decision-making ahead of time and recording that for ninja), and can invoke ninja to perform a build. For situations where relationships are not changing, such as edit/build/debug iterations, this works fine and should provide considerable speedups for more complex builds. The implication is if there are larger changes taking place, ninja is not as appropriate - but you can always use SCons to regenerate the build file. You are NOT advised to use this for production builds.

To use the ninja tool you'll need to first install the Python ninja package, as the tool depends on being able to do an import of the package. This can be done via:

# In a virtualenv, or "python" is the native executable:
python -m pip install ninja

# Windows using Python launcher:
py -m pip install ninja

# Anaconda:
conda install -c conda-forge ninja
        

Reminder that like any non-default tool, you need to initialize it before use (e.g. env.Tool('ninja')).

It is not expected that the Ninja builder will work for all builds at this point. It is still under active development. If you find that your build doesn't work with ninja please bring this to the users mailing list or #scons-help channel on our Discord server.

Specifically if your build has many (or even any) Python function actions you may find that the ninja build will be slower as it will run ninja, which will then run SCons for each target created by a Python action. To alleviate some of these, especially those Python based actions built into SCons there is special logic to implement those actions via shell commands in the ninja build file.

When ninja runs the generated ninja build file, ninja will launch scons as a daemon and feed commands to that scons process which ninja is unable to build directly. This daemon will stay alive until explicitly killed, or it times out. The timeout is set by $NINJA_SCONS_DAEMON_KEEP_ALIVE .

The daemon will be restarted if any SConscript file(s) change or the build changes in a way that ninja determines it needs to regenerate the build.ninja file

See:

Ninja Build System
Ninja File Format Specification

The experience of configuring any software build tool to build a large code base usually, at some point, involves trying to figure out why the tool is behaving a certain way, and how to get it to behave the way you want. SCons is no different. This appendix contains a number of different ways in which you can get some additional insight into SCons' behavior.

Note that we're always interested in trying to improve how you can troubleshoot configuration problems. If you run into a problem that has you scratching your head, and which there just doesn't seem to be a good way to debug, odds are pretty good that someone else will run into the same problem, too. If so, please let the SCons development team know using the contact information at https://scons.org/contact.html so that we can use your feedback to try to come up with a better way to help you, and others, get the necessary insight into SCons behavior to help identify and fix configuration issues.

Let's look at a simple example of a misconfigured build that causes a target to be rebuilt every time SCons is run:

# Intentionally misspell the output file name in the
# command used to create the file:
Command('file.out', 'file.in', 'cp $SOURCE file.oout')
      

(Note to Windows users: The POSIX cp command copies the first file named on the command line to the second file. In our example, it copies the file.in file to the file.out file.)

Now if we run SCons multiple times on this example, we see that it re-runs the cp command every time:

% scons -Q
cp file.in file.oout
% scons -Q
cp file.in file.oout
% scons -Q
cp file.in file.oout

In this example, the underlying cause is obvious: we've intentionally misspelled the output file name in the cp command, so the command doesn't actually build the file.out file that we've told SCons to expect. But if the problem weren't obvious, it would be helpful to specify the --debug=explain option on the command line to have SCons tell us very specifically why it's decided to rebuild the target:

% scons -Q --debug=explain
scons: building `file.out' because it doesn't exist
cp file.in file.oout

If this had been a more complicated example involving a lot of build output, having SCons tell us that it's trying to rebuild the target file because it doesn't exist would be an important clue that something was wrong with the command that we invoked to build it.

Note that you can also use --warn=target-not-built which checks whether or not expected targets exist after a build rule is executed.

% scons -Q --warn=target-not-built
cp file.in file.oout

scons: warning: Cannot find target file.out after building
File "/Users/bdbaddog/devel/scons/git/as_scons/scripts/scons.py", line 97, in <module>

The --debug=explain option also comes in handy to help figure out what input file changed. Given a simple configuration that builds a program from three source files, changing one of the source files and rebuilding with the --debug=explain option shows very specifically why SCons rebuilds the files that it does:

% scons -Q
cc -o file1.o -c file1.c
cc -o file2.o -c file2.c
cc -o file3.o -c file3.c
cc -o prog file1.o file2.o file3.o
%     [CHANGE THE CONTENTS OF file2.c]
% scons -Q --debug=explain
scons: rebuilding `file2.o' because `file2.c' changed
cc -o file2.o -c file2.c
scons: rebuilding `prog' because `file2.o' changed
cc -o prog file1.o file2.o file3.o

This becomes even more helpful in identifying when a file is rebuilt due to a change in an implicit dependency, such as an incuded .h file. If the file1.c and file3.c files in our example both included a hello.h file, then changing that included file and re-running SCons with the --debug=explain option will pinpoint that it's the change to the included file that starts the chain of rebuilds:

% scons -Q
cc -o file1.o -c -I. file1.c
cc -o file2.o -c -I. file2.c
cc -o file3.o -c -I. file3.c
cc -o prog file1.o file2.o file3.o
%     [CHANGE THE CONTENTS OF hello.h]
% scons -Q --debug=explain
scons: rebuilding `file1.o' because `hello.h' changed
cc -o file1.o -c -I. file1.c
scons: rebuilding `file3.o' because `hello.h' changed
cc -o file3.o -c -I. file3.c
scons: rebuilding `prog' because:
           `file1.o' changed
           `file3.o' changed
cc -o prog file1.o file2.o file3.o

(Note that the --debug=explain option will only tell you why SCons decided to rebuild necessary targets. It does not tell you what files it examined when deciding not to rebuild a target file, which is often a more valuable question to answer.)

When you create a construction environment, SCons populates it with construction variables that are set up for various compilers, linkers and utilities that it finds on your system. Although this is usually helpful and what you want, it might be frustrating if SCons doesn't set certain variables that you expect to be set. In situations like this, it's sometimes helpful to use the construction environment Dump method to print all or some of the construction variables. Note that the Dump method returns the representation of the variables in the environment for you to print (or otherwise manipulate):

env = Environment()
print(env.Dump())
      

On a POSIX system with gcc installed, this might generate:

% scons
scons: Reading SConscript files ...
{ 'BUILDERS': { '_InternalInstall': <function InstallBuilderWrapper at 0x700000>,
                '_InternalInstallAs': <function InstallAsBuilderWrapper at 0x700000>,
                '_InternalInstallVersionedLib': <function InstallVersionedBuilderWrapper at 0x700000>},
  'CONFIGUREDIR': '#/.sconf_temp',
  'CONFIGURELOG': '#/config.log',
  'CPPSUFFIXES': [ '.c',
                   '.C',
                   '.cxx',
                   '.cpp',
                   '.c++',
                   '.cc',
                   '.h',
                   '.H',
                   '.hxx',
                   '.hpp',
                   '.hh',
                   '.F',
                   '.fpp',
                   '.FPP',
                   '.m',
                   '.mm',
                   '.S',
                   '.spp',
                   '.SPP',
                   '.sx'],
  'DSUFFIXES': ['.d'],
  'Dir': <SCons.Defaults.Variable_Method_Caller object at 0x700000>,
  'Dirs': <SCons.Defaults.Variable_Method_Caller object at 0x700000>,
  'ENV': {'PATH': '/usr/local/bin:/opt/bin:/bin:/usr/bin:/snap/bin'},
  'ESCAPE': <function escape at 0x700000>,
  'File': <SCons.Defaults.Variable_Method_Caller object at 0x700000>,
  'HOST_ARCH': 'arm64',
  'HOST_OS': 'posix',
  'IDLSUFFIXES': ['.idl', '.IDL'],
  'INSTALL': <function copyFunc at 0x700000>,
  'INSTALLVERSIONEDLIB': <function copyFuncVersionedLib at 0x700000>,
  'LIBLITERALPREFIX': '',
  'LIBPREFIX': 'lib',
  'LIBPREFIXES': ['$LIBPREFIX'],
  'LIBSUFFIX': '.a',
  'LIBSUFFIXES': ['$LIBSUFFIX', '$SHLIBSUFFIX'],
  'MAXLINELENGTH': 128072,
  'OBJPREFIX': '',
  'OBJSUFFIX': '.o',
  'PLATFORM': 'posix',
  'PROGPREFIX': '',
  'PROGSUFFIX': '',
  'PSPAWN': <function piped_env_spawn at 0x700000>,
  'RDirs': <SCons.Defaults.Variable_Method_Caller object at 0x700000>,
  'SCANNERS': [<SCons.Scanner.ScannerBase object at 0x700000>],
  'SHELL': 'sh',
  'SHLIBPREFIX': '$LIBPREFIX',
  'SHLIBSUFFIX': '.so',
  'SHOBJPREFIX': '$OBJPREFIX',
  'SHOBJSUFFIX': '$OBJSUFFIX',
  'SPAWN': <function subprocess_spawn at 0x700000>,
  'TARGET_ARCH': None,
  'TARGET_OS': None,
  'TEMPFILE': <class 'SCons.Platform.TempFileMunge'>,
  'TEMPFILEARGESCFUNC': <function quote_spaces at 0x700000>,
  'TEMPFILEARGJOIN': ' ',
  'TEMPFILEPREFIX': '@',
  'TOOLS': ['install', 'install'],
  '_CPPDEFFLAGS': '${_defines(CPPDEFPREFIX, CPPDEFINES, CPPDEFSUFFIX, __env__, '
                  'TARGET, SOURCE)}',
  '_CPPINCFLAGS': '${_concat(INCPREFIX, CPPPATH, INCSUFFIX, __env__, RDirs, '
                  'TARGET, SOURCE, affect_signature=False)}',
  '_LIBDIRFLAGS': '${_concat(LIBDIRPREFIX, LIBPATH, LIBDIRSUFFIX, __env__, '
                  'RDirs, TARGET, SOURCE, affect_signature=False)}',
  '_LIBFLAGS': '${_concat(LIBLINKPREFIX, LIBS, LIBLINKSUFFIX, __env__)}',
  '__DRPATH': '$_DRPATH',
  '__DSHLIBVERSIONFLAGS': '${__libversionflags(__env__,"DSHLIBVERSION","_DSHLIBVERSIONFLAGS")}',
  '__LDMODULEVERSIONFLAGS': '${__libversionflags(__env__,"LDMODULEVERSION","_LDMODULEVERSIONFLAGS")}',
  '__RPATH': '$_RPATH',
  '__SHLIBVERSIONFLAGS': '${__libversionflags(__env__,"SHLIBVERSION","_SHLIBVERSIONFLAGS")}',
  '__lib_either_version_flag': <function __lib_either_version_flag at 0x700000>,
  '__libversionflags': <function __libversionflags at 0x700000>,
  '_concat': <function _concat at 0x700000>,
  '_defines': <function _defines at 0x700000>,
  '_stripixes': <function _stripixes at 0x700000>}
scons: done reading SConscript files.
scons: Building targets ...
scons: `.' is up to date.
scons: done building targets.

On a Windows system with Microsoft Visual C++ the output might look like:

C:\>scons
scons: Reading SConscript files ...
{ 'BUILDERS': { 'Object': <SCons.Builder.CompositeBuilder object at 0x700000>,
                'PCH': <SCons.Builder.BuilderBase object at 0x700000>,
                'RES': <SCons.Builder.BuilderBase object at 0x700000>,
                'SharedObject': <SCons.Builder.CompositeBuilder object at 0x700000>,
                'StaticObject': <SCons.Builder.CompositeBuilder object at 0x700000>,
                '_InternalInstall': <function InstallBuilderWrapper at 0x700000>,
                '_InternalInstallAs': <function InstallAsBuilderWrapper at 0x700000>,
                '_InternalInstallVersionedLib': <function InstallVersionedBuilderWrapper at 0x700000>},
  'CC': 'cl',
  'CCCOM': <SCons.Action.FunctionAction object at 0x700000>,
  'CCDEPFLAGS': '/showIncludes',
  'CCFLAGS': ['/nologo'],
  'CCPCHFLAGS': <function gen_ccpchflags at 0x700000>,
  'CCPDBFLAGS': ['${(PDB and "/Z7") or ""}'],
  'CFILESUFFIX': '.c',
  'CFLAGS': [],
  'CONFIGUREDIR': '#/.sconf_temp',
  'CONFIGURELOG': '#/config.log',
  'CPPDEFPREFIX': '/D',
  'CPPDEFSUFFIX': '',
  'CPPSUFFIXES': [ '.c',
                   '.C',
                   '.cxx',
                   '.cpp',
                   '.c++',
                   '.cc',
                   '.h',
                   '.H',
                   '.hxx',
                   '.hpp',
                   '.hh',
                   '.F',
                   '.fpp',
                   '.FPP',
                   '.m',
                   '.mm',
                   '.S',
                   '.spp',
                   '.SPP',
                   '.sx'],
  'CXX': '$CC',
  'CXXCOM': '${TEMPFILE("$CXX $_MSVC_OUTPUT_FLAG /c $CHANGED_SOURCES $CXXFLAGS '
            '$CCFLAGS $_CCCOMCOM","$CXXCOMSTR")}',
  'CXXFILESUFFIX': '.cc',
  'CXXFLAGS': ['$(', '/TP', '$)'],
  'DSUFFIXES': ['.d'],
  'Dir': <SCons.Defaults.Variable_Method_Caller object at 0x700000>,
  'Dirs': <SCons.Defaults.Variable_Method_Caller object at 0x700000>,
  'ENV': { 'PATH': 'C:\\WINDOWS\\System32',
           'PATHEXT': '.COM;.EXE;.BAT;.CMD',
           'SystemRoot': 'C:\\WINDOWS'},
  'ESCAPE': <function escape at 0x700000>,
  'File': <SCons.Defaults.Variable_Method_Caller object at 0x700000>,
  'HOST_ARCH': 'arm64',
  'HOST_OS': 'win32',
  'IDLSUFFIXES': ['.idl', '.IDL'],
  'INCPREFIX': '/I',
  'INCSUFFIX': '',
  'INSTALL': <function copyFunc at 0x700000>,
  'INSTALLVERSIONEDLIB': <function copyFuncVersionedLib at 0x700000>,
  'LEXUNISTD': ['--nounistd'],
  'LIBLITERALPREFIX': '',
  'LIBPREFIX': '',
  'LIBPREFIXES': ['$LIBPREFIX'],
  'LIBSUFFIX': '.lib',
  'LIBSUFFIXES': ['$LIBSUFFIX'],
  'MAXLINELENGTH': 2048,
  'MSVC_SETUP_RUN': True,
  'NINJA_DEPFILE_PARSE_FORMAT': 'msvc',
  'OBJPREFIX': '',
  'OBJSUFFIX': '.obj',
  'PCHCOM': '$CXX /Fo${TARGETS[1]} $CXXFLAGS $CCFLAGS $CPPFLAGS $_CPPDEFFLAGS '
            '$_CPPINCFLAGS /c $SOURCES /Yc$PCHSTOP /Fp${TARGETS[0]} '
            '$CCPDBFLAGS $PCHPDBFLAGS',
  'PCHPDBFLAGS': ['${(PDB and "/Yd") or ""}'],
  'PLATFORM': 'win32',
  'PROGPREFIX': '',
  'PROGSUFFIX': '.exe',
  'PSPAWN': <function piped_spawn at 0x700000>,
  'RC': 'rc',
  'RCCOM': <SCons.Action.FunctionAction object at 0x700000>,
  'RCFLAGS': ['/nologo'],
  'RCSUFFIXES': ['.rc', '.rc2'],
  'RDirs': <SCons.Defaults.Variable_Method_Caller object at 0x700000>,
  'SCANNERS': [<SCons.Scanner.ScannerBase object at 0x700000>],
  'SHCC': '$CC',
  'SHCCCOM': <SCons.Action.FunctionAction object at 0x700000>,
  'SHCCFLAGS': ['$CCFLAGS'],
  'SHCFLAGS': ['$CFLAGS'],
  'SHCXX': '$CXX',
  'SHCXXCOM': '${TEMPFILE("$SHCXX $_MSVC_OUTPUT_FLAG /c $CHANGED_SOURCES '
              '$SHCXXFLAGS $SHCCFLAGS $_CCCOMCOM","$SHCXXCOMSTR")}',
  'SHCXXFLAGS': ['$CXXFLAGS'],
  'SHELL': 'command',
  'SHLIBPREFIX': '',
  'SHLIBSUFFIX': '.dll',
  'SHOBJPREFIX': '$OBJPREFIX',
  'SHOBJSUFFIX': '$OBJSUFFIX',
  'SPAWN': <function spawn at 0x700000>,
  'STATIC_AND_SHARED_OBJECTS_ARE_THE_SAME': 1,
  'TARGET_ARCH': None,
  'TARGET_OS': None,
  'TEMPFILE': <class 'SCons.Platform.TempFileMunge'>,
  'TEMPFILEARGESCFUNC': <function quote_spaces at 0x700000>,
  'TEMPFILEARGJOIN': '\n',
  'TEMPFILEPREFIX': '@',
  'TOOLS': ['msvc', 'install', 'install'],
  'VSWHERE': None,
  '_CCCOMCOM': '$CPPFLAGS $_CPPDEFFLAGS $_CPPINCFLAGS $CCPCHFLAGS $CCPDBFLAGS',
  '_CPPDEFFLAGS': '${_defines(CPPDEFPREFIX, CPPDEFINES, CPPDEFSUFFIX, __env__, '
                  'TARGET, SOURCE)}',
  '_CPPINCFLAGS': '${_concat(INCPREFIX, CPPPATH, INCSUFFIX, __env__, RDirs, '
                  'TARGET, SOURCE, affect_signature=False)}',
  '_LIBDIRFLAGS': '${_concat(LIBDIRPREFIX, LIBPATH, LIBDIRSUFFIX, __env__, '
                  'RDirs, TARGET, SOURCE, affect_signature=False)}',
  '_LIBFLAGS': '${_concat(LIBLINKPREFIX, LIBS, LIBLINKSUFFIX, __env__)}',
  '_MSVC_OUTPUT_FLAG': <function msvc_output_flag at 0x700000>,
  '__DSHLIBVERSIONFLAGS': '${__libversionflags(__env__,"DSHLIBVERSION","_DSHLIBVERSIONFLAGS")}',
  '__LDMODULEVERSIONFLAGS': '${__libversionflags(__env__,"LDMODULEVERSION","_LDMODULEVERSIONFLAGS")}',
  '__SHLIBVERSIONFLAGS': '${__libversionflags(__env__,"SHLIBVERSION","_SHLIBVERSIONFLAGS")}',
  '__lib_either_version_flag': <function __lib_either_version_flag at 0x700000>,
  '__libversionflags': <function __libversionflags at 0x700000>,
  '_concat': <function _concat at 0x700000>,
  '_defines': <function _defines at 0x700000>,
  '_stripixes': <function _stripixes at 0x700000>}
scons: done reading SConscript files.
scons: Building targets ...
scons: `.' is up to date.
scons: done building targets.

The construction environments in these examples have actually been restricted to just gcc and Microsoft Visual C++ respectively. In a real-life situation, the construction environments will likely contain a great many more variables. Also note that we've massaged the example output above to make the memory address of all objects a constant 0x700000. In reality, you would see a different hexadecimal number for each object.

To make it easier to see just what you're interested in, the Dump method allows you to specify a specific construction variable that you want to disply. For example, it's not unusual to want to verify the external environment used to execute build commands, to make sure that the PATH and other environment variables are set up the way they should be. You can do this as follows:

env = Environment()
print(env.Dump('ENV'))
      

Which might display the following when executed on a POSIX system:

% scons
scons: Reading SConscript files ...
{'PATH': '/usr/local/bin:/opt/bin:/bin:/usr/bin:/snap/bin'}
scons: done reading SConscript files.
scons: Building targets ...
scons: `.' is up to date.
scons: done building targets.

And the following when executed on a Windows system:

C:\>scons
scons: Reading SConscript files ...
{ 'PATH': 'C:\\WINDOWS\\System32:/usr/bin',
  'PATHEXT': '.COM;.EXE;.BAT;.CMD',
  'SystemRoot': 'C:\\WINDOWS'}
scons: done reading SConscript files.
scons: Building targets ...
scons: `.' is up to date.
scons: done building targets.

Sometimes the best way to try to figure out what SCons is doing is simply to take a look at the dependency graph that it constructs based on your SConscript files. The --tree option will display all or part of the SCons dependency graph in an "ASCII art" graphical format that shows the dependency hierarchy.

For example, given the following input SConstruct file:

env = Environment(CPPPATH = ['.'])
env.Program('prog', ['f1.c', 'f2.c', 'f3.c'])
      

Running SCons with the --tree=all option yields:

% scons -Q --tree=all
cc -o f1.o -c -I. f1.c
cc -o f2.o -c -I. f2.c
cc -o f3.o -c -I. f3.c
cc -o prog f1.o f2.o f3.o
+-.
  +-SConstruct
  +-f1.c
  +-f1.o
  | +-f1.c
  | +-inc.h
  +-f2.c
  +-f2.o
  | +-f2.c
  | +-inc.h
  +-f3.c
  +-f3.o
  | +-f3.c
  | +-inc.h
  +-inc.h
  +-prog
    +-f1.o
    | +-f1.c
    | +-inc.h
    +-f2.o
    | +-f2.c
    | +-inc.h
    +-f3.o
      +-f3.c
      +-inc.h

The tree will also be printed when the -n (no execute) option is used, which allows you to examine the dependency graph for a configuration without actually rebuilding anything in the tree.

By default SCons uses "ASCII art" to draw the tree. It is possible to use line-drawing characters (Unicode calls these Box Drawing) to make a nicer display. To do this, add the linedraw qualifier:

% scons -Q --tree=all,linedraw
cc -o f1.o -c -I. f1.c
cc -o f2.o -c -I. f2.c
cc -o f3.o -c -I. f3.c
cc -o prog f1.o f2.o f3.o
└─┬.
  ├─SConstruct
  ├─f1.c
  ├─┬f1.o
  │ ├─f1.c
  │ └─inc.h
  ├─f2.c
  ├─┬f2.o
  │ ├─f2.c
  │ └─inc.h
  ├─f3.c
  ├─┬f3.o
  │ ├─f3.c
  │ └─inc.h
  ├─inc.h
  └─┬prog
    ├─┬f1.o
    │ ├─f1.c
    │ └─inc.h
    ├─┬f2.o
    │ ├─f2.c
    │ └─inc.h
    └─┬f3.o
      ├─f3.c
      └─inc.h

The --tree option only prints the dependency graph for the specified targets (or the default target(s) if none are specified on the command line). So if you specify a target like f2.o on the command line, the --tree option will only print the dependency graph for that file:

% scons -Q --tree=all f2.o
cc -o f2.o -c -I. f2.c
+-f2.o
  +-f2.c
  +-inc.h

This is, of course, useful for restricting the output from a very large build configuration to just a portion in which you're interested. Multiple targets are fine, in which case a tree will be printed for each specified target:

% scons -Q --tree=all f1.o f3.o
cc -o f1.o -c -I. f1.c
+-f1.o
  +-f1.c
  +-inc.h
cc -o f3.o -c -I. f3.c
+-f3.o
  +-f3.c
  +-inc.h

The status argument may be used to tell SCons to print status information about each file in the dependency graph:

% scons -Q --tree=status
cc -o f1.o -c -I. f1.c
cc -o f2.o -c -I. f2.c
cc -o f3.o -c -I. f3.c
cc -o prog f1.o f2.o f3.o
 E         = exists
  R        = exists in repository only
   b       = implicit builder
   B       = explicit builder
    S      = side effect
     P     = precious
      A    = always build
       C   = current
        N  = no clean
         H = no cache

[E b      ]+-.
[E     C  ]  +-SConstruct
[E     C  ]  +-f1.c
[E B   C  ]  +-f1.o
[E     C  ]  | +-f1.c
[E     C  ]  | +-inc.h
[E     C  ]  +-f2.c
[E B   C  ]  +-f2.o
[E     C  ]  | +-f2.c
[E     C  ]  | +-inc.h
[E     C  ]  +-f3.c
[E B   C  ]  +-f3.o
[E     C  ]  | +-f3.c
[E     C  ]  | +-inc.h
[E     C  ]  +-inc.h
[E B   C  ]  +-prog
[E B   C  ]    +-f1.o
[E     C  ]    | +-f1.c
[E     C  ]    | +-inc.h
[E B   C  ]    +-f2.o
[E     C  ]    | +-f2.c
[E     C  ]    | +-inc.h
[E B   C  ]    +-f3.o
[E     C  ]      +-f3.c
[E     C  ]      +-inc.h

Note that --tree=all,status is equivalent; the all is assumed if only status is present. As an alternative to all, you can specify --tree=derived to have SCons only print derived targets in the tree output, skipping source files (like .c and .h files):

% scons -Q --tree=derived
cc -o f1.o -c -I. f1.c
cc -o f2.o -c -I. f2.c
cc -o f3.o -c -I. f3.c
cc -o prog f1.o f2.o f3.o
+-.
  +-f1.o
  +-f2.o
  +-f3.o
  +-prog
    +-f1.o
    +-f2.o
    +-f3.o

You can use the status modifier with derived as well:

% scons -Q --tree=derived,status
cc -o f1.o -c -I. f1.c
cc -o f2.o -c -I. f2.c
cc -o f3.o -c -I. f3.c
cc -o prog f1.o f2.o f3.o
 E         = exists
  R        = exists in repository only
   b       = implicit builder
   B       = explicit builder
    S      = side effect
     P     = precious
      A    = always build
       C   = current
        N  = no clean
         H = no cache

[E b      ]+-.
[E B   C  ]  +-f1.o
[E B   C  ]  +-f2.o
[E B   C  ]  +-f3.o
[E B   C  ]  +-prog
[E B   C  ]    +-f1.o
[E B   C  ]    +-f2.o
[E B   C  ]    +-f3.o

Note that the order of the --tree= arguments doesn't matter; --tree=status,derived is completely equivalent.

The default behavior of the --tree option is to repeat all of the dependencies each time the library dependency (or any other dependency file) is encountered in the tree. If certain target files share other target files, such as two programs that use the same library:

env = Environment(CPPPATH = ['.'],
                  LIBS = ['foo'],
                  LIBPATH = ['.'])
env.Library('foo', ['f1.c', 'f2.c', 'f3.c'])
env.Program('prog1.c')
env.Program('prog2.c')
      

Then there can be a lot of repetition in the --tree= output:

% scons -Q --tree=all
cc -o f1.o -c -I. f1.c
cc -o f2.o -c -I. f2.c
cc -o f3.o -c -I. f3.c
ar rc libfoo.a f1.o f2.o f3.o
ranlib libfoo.a
cc -o prog1.o -c -I. prog1.c
cc -o prog1 prog1.o -L. -lfoo
cc -o prog2.o -c -I. prog2.c
cc -o prog2 prog2.o -L. -lfoo
+-.
  +-SConstruct
  +-f1.c
  +-f1.o
  | +-f1.c
  | +-inc.h
  +-f2.c
  +-f2.o
  | +-f2.c
  | +-inc.h
  +-f3.c
  +-f3.o
  | +-f3.c
  | +-inc.h
  +-inc.h
  +-libfoo.a
  | +-f1.o
  | | +-f1.c
  | | +-inc.h
  | +-f2.o
  | | +-f2.c
  | | +-inc.h
  | +-f3.o
  |   +-f3.c
  |   +-inc.h
  +-prog1
  | +-prog1.o
  | | +-prog1.c
  | | +-inc.h
  | +-libfoo.a
  |   +-f1.o
  |   | +-f1.c
  |   | +-inc.h
  |   +-f2.o
  |   | +-f2.c
  |   | +-inc.h
  |   +-f3.o
  |     +-f3.c
  |     +-inc.h
  +-prog1.c
  +-prog1.o
  | +-prog1.c
  | +-inc.h
  +-prog2
  | +-prog2.o
  | | +-prog2.c
  | | +-inc.h
  | +-libfoo.a
  |   +-f1.o
  |   | +-f1.c
  |   | +-inc.h
  |   +-f2.o
  |   | +-f2.c
  |   | +-inc.h
  |   +-f3.o
  |     +-f3.c
  |     +-inc.h
  +-prog2.c
  +-prog2.o
    +-prog2.c
    +-inc.h

In a large configuration with many internal libraries and include files, this can very quickly lead to huge output trees. To help make this more manageable, a prune modifier may be added to the option list, in which case SCons will print the name of a target that has already been visited during the tree-printing in square brackets ([]) as an indication that the dependencies of the target file may be found by looking farther up the tree:

% scons -Q --tree=prune
cc -o f1.o -c -I. f1.c
cc -o f2.o -c -I. f2.c
cc -o f3.o -c -I. f3.c
ar rc libfoo.a f1.o f2.o f3.o
ranlib libfoo.a
cc -o prog1.o -c -I. prog1.c
cc -o prog1 prog1.o -L. -lfoo
cc -o prog2.o -c -I. prog2.c
cc -o prog2 prog2.o -L. -lfoo
+-.
  +-SConstruct
  +-f1.c
  +-f1.o
  | +-f1.c
  | +-inc.h
  +-f2.c
  +-f2.o
  | +-f2.c
  | +-inc.h
  +-f3.c
  +-f3.o
  | +-f3.c
  | +-inc.h
  +-inc.h
  +-libfoo.a
  | +-[f1.o]
  | +-[f2.o]
  | +-[f3.o]
  +-prog1
  | +-prog1.o
  | | +-prog1.c
  | | +-inc.h
  | +-[libfoo.a]
  +-prog1.c
  +-[prog1.o]
  +-prog2
  | +-prog2.o
  | | +-prog2.c
  | | +-inc.h
  | +-[libfoo.a]
  +-prog2.c
  +-[prog2.o]

Like the status keyword, the prune argument by itself is equivalent to --tree=all,prune.

In general, SCons tries to keep its error messages short and informative. That means we usually try to avoid showing the stack traces that are familiar to experienced Python programmers, since they usually contain much more information than is useful to most people.

For example, the following SConstruct file:

Program('prog.c')
      

Generates the following error if the prog.c file does not exist:

% scons -Q
scons: *** [prog.o] Source `prog.c' not found, needed by target `prog.o'.

In this case, the error is pretty obvious. But if it weren't, and you wanted to try to get more information about the error, the --debug=stacktrace option would show you exactly where in the SCons source code the problem occurs:

% scons -Q --debug=stacktrace
scons: *** [prog.o] Source `prog.c' not found, needed by target `prog.o'.
scons: internal stack trace:
  File "SCons/Taskmaster/Job.py", line 671, in _work
    task.prepare()
  File "SCons/Script/Main.py", line 201, in prepare
    return SCons.Taskmaster.OutOfDateTask.prepare(self)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "SCons/Taskmaster/__init__.py", line 195, in prepare
    executor.prepare()
  File "SCons/Executor.py", line 420, in prepare
    raise SCons.Errors.StopError(msg % (s, self.batches[0].targets[0]))

Of course, if you do need to dive into the SCons source code, we'd like to know if, or how, the error messages or troubleshooting options could have been improved to avoid that. Not everyone has the necessary time or Python skill to dive into the source code, and we'd like to improve SCons for those people as well...

The internal SCons subsystem that handles walking the dependency graph and controls the decision-making about what to rebuild is the Taskmaster. SCons supports a --taskmastertrace option that tells the Taskmaster to print information about the children (dependencies) of the various Nodes on its walk down the graph, which specific dependent Nodes are being evaluated, and in what order.

The --taskmastertrace option takes as an argument the name of a file in which to put the trace output, with - (a single hyphen) indicating that the trace messages should be printed to the standard output:

env = Environment(CPPPATH = ['.'])
env.Program('prog.c')
      
% scons -Q --taskmastertrace=- prog
Job.NewParallel._work(): [Thread:8149967104] Gained exclusive access
Job.NewParallel._work(): [Thread:8149967104] Starting search
Job.NewParallel._work(): [Thread:8149967104] Found 0 completed tasks to process
Job.NewParallel._work(): [Thread:8149967104] Searching for new tasks

Taskmaster: Looking for a node to evaluate
Taskmaster:     Considering node <no_state   0   'prog'> and its children:
Taskmaster:        <no_state   0   'prog.o'>
Taskmaster:      adjusted ref count: <pending    1   'prog'>, child 'prog.o'
Taskmaster:     Considering node <no_state   0   'prog.o'> and its children:
Taskmaster:        <no_state   0   'prog.c'>
Taskmaster:        <no_state   0   'inc.h'>
Taskmaster:      adjusted ref count: <pending    1   'prog.o'>, child 'prog.c'
Taskmaster:      adjusted ref count: <pending    2   'prog.o'>, child 'inc.h'
Taskmaster:     Considering node <no_state   0   'prog.c'> and its children:
Taskmaster: Evaluating <pending    0   'prog.c'>

Task.make_ready_current(): node <pending    0   'prog.c'>
Task.prepare():      node <up_to_date 0   'prog.c'>
Job.NewParallel._work(): [Thread:8149967104] Found internal task
Task.executed_with_callbacks(): node <up_to_date 0   'prog.c'>
Task.postprocess():  node <up_to_date 0   'prog.c'>
Task.postprocess():  removing <up_to_date 0   'prog.c'>
Task.postprocess():  adjusted parent ref count <pending    1   'prog.o'>
Job.NewParallel._work(): [Thread:8149967104] Searching for new tasks

Taskmaster: Looking for a node to evaluate
Taskmaster:     Considering node <no_state   0   'inc.h'> and its children:
Taskmaster: Evaluating <pending    0   'inc.h'>

Task.make_ready_current(): node <pending    0   'inc.h'>
Task.prepare():      node <up_to_date 0   'inc.h'>
Job.NewParallel._work(): [Thread:8149967104] Found internal task
Task.executed_with_callbacks(): node <up_to_date 0   'inc.h'>
Task.postprocess():  node <up_to_date 0   'inc.h'>
Task.postprocess():  removing <up_to_date 0   'inc.h'>
Task.postprocess():  adjusted parent ref count <pending    0   'prog.o'>
Job.NewParallel._work(): [Thread:8149967104] Searching for new tasks

Taskmaster: Looking for a node to evaluate
Taskmaster:     Considering node <pending    0   'prog.o'> and its children:
Taskmaster:        <up_to_date 0   'prog.c'>
Taskmaster:        <up_to_date 0   'inc.h'>
Taskmaster: Evaluating <pending    0   'prog.o'>

Task.make_ready_current(): node <pending    0   'prog.o'>
Task.prepare():      node <executing  0   'prog.o'>
Job.NewParallel._work(): [Thread:8149967104] Found task requiring execution
Job.NewParallel._work(): [Thread:8149967104] Executing task
Task.execute():      node <executing  0   'prog.o'>
cc -o prog.o -c -I. prog.c
Job.NewParallel._work(): [Thread:8149967104] Enqueueing executed task results
Job.NewParallel._work(): [Thread:8149967104] Gained exclusive access
Job.NewParallel._work(): [Thread:8149967104] Starting search
Job.NewParallel._work(): [Thread:8149967104] Found 1 completed tasks to process
Task.executed_with_callbacks(): node <executing  0   'prog.o'>
Task.postprocess():  node <executed   0   'prog.o'>
Task.postprocess():  removing <executed   0   'prog.o'>
Task.postprocess():  adjusted parent ref count <pending    0   'prog'>
Job.NewParallel._work(): [Thread:8149967104] Searching for new tasks

Taskmaster: Looking for a node to evaluate
Taskmaster:     Considering node <pending    0   'prog'> and its children:
Taskmaster:        <executed   0   'prog.o'>
Taskmaster: Evaluating <pending    0   'prog'>

Task.make_ready_current(): node <pending    0   'prog'>
Task.prepare():      node <executing  0   'prog'>
Job.NewParallel._work(): [Thread:8149967104] Found task requiring execution
Job.NewParallel._work(): [Thread:8149967104] Executing task
Task.execute():      node <executing  0   'prog'>
cc -o prog prog.o
Job.NewParallel._work(): [Thread:8149967104] Enqueueing executed task results
Job.NewParallel._work(): [Thread:8149967104] Gained exclusive access
Job.NewParallel._work(): [Thread:8149967104] Starting search
Job.NewParallel._work(): [Thread:8149967104] Found 1 completed tasks to process
Task.executed_with_callbacks(): node <executing  0   'prog'>
Task.postprocess():  node <executed   0   'prog'>
Job.NewParallel._work(): [Thread:8149967104] Searching for new tasks

Taskmaster: Looking for a node to evaluate
Taskmaster: No candidate anymore.
Job.NewParallel._work(): [Thread:8149967104] Found no task requiring execution, and have no jobs: marking complete
Job.NewParallel._work(): [Thread:8149967104] Gained exclusive access
Job.NewParallel._work(): [Thread:8149967104] Completion detected, breaking from main loop

The --taskmastertrace option doesn't provide information about the actual calculations involved in deciding if a file is up-to-date, but it does show all of the dependencies it knows about for each Node, and the order in which those dependencies are evaluated. This can be useful as an alternate way to determine whether or not your SCons configuration, or the implicit dependency scan, has actually identified all the correct dependencies you want it to.

This appendix contains descriptions of all of the construction variables that are potentially available "out of the box" in this version of SCons. Whether or not setting a construction variable in a construction environment will actually have an effect depends on whether any of the Tools and/or Builders that use the variable have been included in the construction environment.

In this appendix, we have appended the initial $ (dollar sign) to the beginning of each variable name when it appears in the text, but left off the dollar sign in the left-hand column where the name appears for each entry.

__LDMODULEVERSIONFLAGS

This construction variable automatically introduces $_LDMODULEVERSIONFLAGS if $LDMODULEVERSION is set. Othervise it evaluates to an empty string.

__SHLIBVERSIONFLAGS

This construction variable automatically introduces $_SHLIBVERSIONFLAGS if $SHLIBVERSION is set. Othervise it evaluates to an empty string.

APPLELINK_COMPATIBILITY_VERSION

On Mac OS X this is used to set the linker flag: -compatibility_version

The value is specified as X[.Y[.Z]] where X is between 1 and 65535, Y can be omitted or between 1 and 255, Z can be omitted or between 1 and 255. This value will be derived from $SHLIBVERSION if not specified. The lowest digit will be dropped and replaced by a 0.

If the $APPLELINK_NO_COMPATIBILITY_VERSION is set then no -compatibility_version will be output.

See MacOS's ld manpage for more details

_APPLELINK_COMPATIBILITY_VERSION

A macro (by default a generator function) used to create the linker flags to specify apple's linker's -compatibility_version flag. The default generator uses $APPLELINK_COMPATIBILITY_VERSION and $APPLELINK_NO_COMPATIBILITY_VERSION and $SHLIBVERSION to determine the correct flag.

APPLELINK_CURRENT_VERSION

On Mac OS X this is used to set the linker flag: -current_version

The value is specified as X[.Y[.Z]] where X is between 1 and 65535, Y can be omitted or between 1 and 255, Z can be omitted or between 1 and 255. This value will be set to $SHLIBVERSION if not specified.

If the $APPLELINK_NO_CURRENT_VERSION is set then no -current_version will be output.

See MacOS's ld manpage for more details

_APPLELINK_CURRENT_VERSION

A macro (by default a generator function) used to create the linker flags to specify apple's linker's -current_version flag. The default generator uses $APPLELINK_CURRENT_VERSION and $APPLELINK_NO_CURRENT_VERSION and $SHLIBVERSION to determine the correct flag.

APPLELINK_NO_COMPATIBILITY_VERSION

Set this to any True (1|True|non-empty string) value to disable adding -compatibility_version flag when generating versioned shared libraries.

This overrides $APPLELINK_COMPATIBILITY_VERSION.

APPLELINK_NO_CURRENT_VERSION

Set this to any True (1|True|non-empty string) value to disable adding -current_version flag when generating versioned shared libraries.

This overrides $APPLELINK_CURRENT_VERSION.

AR

The static library archiver.

ARCHITECTURE

Specifies the system architecture for which the package is being built. The default is the system architecture of the machine on which SCons is running. This is used to fill in the Architecture: field in an Ipkg control file, and the BuildArch: field in the RPM .spec file, as well as forming part of the name of a generated RPM package file.

See the Package builder.

ARCOM

The command line used to generate a static library from object files.

ARCOMSTR

The string displayed when a static library is generated from object files. If this is not set, then $ARCOM (the command line) is displayed.

env = Environment(ARCOMSTR = "Archiving $TARGET")
ARFLAGS

General options passed to the static library archiver.

AS

The assembler.

ASCOM

The command line used to generate an object file from an assembly-language source file.

ASCOMSTR

The string displayed when an object file is generated from an assembly-language source file. If this is not set, then $ASCOM (the command line) is displayed.

env = Environment(ASCOMSTR = "Assembling $TARGET")
ASFLAGS

General options passed to the assembler.

ASPPCOM

The command line used to assemble an assembly-language source file into an object file after first running the file through the C preprocessor. Any options specified in the $ASFLAGS and $CPPFLAGS construction variables are included on this command line.

ASPPCOMSTR

The string displayed when an object file is generated from an assembly-language source file after first running the file through the C preprocessor. If this is not set, then $ASPPCOM (the command line) is displayed.

env = Environment(ASPPCOMSTR = "Assembling $TARGET")
ASPPFLAGS

General options when an assembling an assembly-language source file into an object file after first running the file through the C preprocessor. The default is to use the value of $ASFLAGS.

BIBTEX

The bibliography generator for the TeX formatter and typesetter and the LaTeX structured formatter and typesetter.

BIBTEXCOM

The command line used to call the bibliography generator for the TeX formatter and typesetter and the LaTeX structured formatter and typesetter.

BIBTEXCOMSTR

The string displayed when generating a bibliography for TeX or LaTeX. If this is not set, then $BIBTEXCOM (the command line) is displayed.

env = Environment(BIBTEXCOMSTR = "Generating bibliography $TARGET")
BIBTEXFLAGS

General options passed to the bibliography generator for the TeX formatter and typesetter and the LaTeX structured formatter and typesetter.

BUILDERS

A dictionary mapping the names of the builders available through the construction environment to underlying Builder objects. Custom builders need to be added to this to make them available.

A platform-dependent default list of builders such as Program, Library etc. is used to populate this construction variable when the construction environment is initialized via the presence/absence of the tools those builders depend on. $BUILDERS can be examined to learn which builders will actually be available at run-time.

Note that if you initialize this construction variable through assignment when the construction environment is created, that value for $BUILDERS will override any defaults:

bld = Builder(action='foobuild < $SOURCE > $TARGET')
env = Environment(BUILDERS={'NewBuilder': bld})

To instead use a new Builder object in addition to the default Builders, add your new Builder object like this:

env = Environment()
env.Append(BUILDERS={'NewBuilder': bld})

or this:

env = Environment()
env['BUILDERS']['NewBuilder'] = bld
CACHEDIR_CLASS

The class type that SCons should use when instantiating a new CacheDir in this construction environment. Must be a subclass of the SCons.CacheDir.CacheDir class.

CC

The C compiler.

CCCOM

The command line used to compile a C source file to a (static) object file. Any options specified in the $CFLAGS, $CCFLAGS and $CPPFLAGS construction variables are included on this command line. See also $SHCCCOM for compiling to shared objects.

CCCOMSTR

If set, the string displayed when a C source file is compiled to a (static) object file. If not set, then $CCCOM (the command line) is displayed. See also $SHCCCOMSTR for compiling to shared objects.

env = Environment(CCCOMSTR = "Compiling static object $TARGET")
CCDEPFLAGS

Options to pass to C or C++ compiler to generate list of dependency files.

This is set only by compilers which support this functionality. (gcc, clang, and msvc currently)

CCFLAGS

General options that are passed to the C and C++ compilers. See also $SHCCFLAGS for compiling to shared objects.

CCPCHFLAGS

Options added to the compiler command line to support building with precompiled headers. The default value expands expands to the appropriate Microsoft Visual C++ command-line options when the $PCH construction variable is set.

CCPDBFLAGS

Options added to the compiler command line to support storing debugging information in a Microsoft Visual C++ PDB file. The default value expands expands to appropriate Microsoft Visual C++ command-line options when the $PDB construction variable is set.

The Microsoft Visual C++ compiler option that SCons uses by default to generate PDB information is /Z7. This works correctly with parallel (-j) builds because it embeds the debug information in the intermediate object files, as opposed to sharing a single PDB file between multiple object files. This is also the only way to get debug information embedded into a static library. Using the /Zi instead may yield improved link-time performance, although parallel builds will no longer work.

You can generate PDB files with the /Zi switch by overriding the default $CCPDBFLAGS variable as follows:

env['CCPDBFLAGS'] = ['${(PDB and "/Zi /Fd%s" % File(PDB)) or ""}']

An alternative would be to use the /Zi to put the debugging information in a separate .pdb file for each object file by overriding the $CCPDBFLAGS variable as follows:

env['CCPDBFLAGS'] = '/Zi /Fd${TARGET}.pdb'
CCVERSION

The version number of the C compiler. This may or may not be set, depending on the specific C compiler being used.

CFILESUFFIX

The suffix for C source files. This is used by the internal CFile builder when generating C files from Lex (.l) or YACC (.y) input files. The default suffix, of course, is .c (lower case). On case-insensitive systems (like Windows), SCons also treats .C (upper case) files as C files.

CFLAGS

General options that are passed to the C compiler (C only; not C++). See also $SHCFLAGS for compiling to shared objects.

CHANGE_SPECFILE

A hook for modifying the file that controls the packaging build (the .spec for RPM, the control for Ipkg, the .wxs for MSI). If set, the function will be called after the SCons template for the file has been written.

See the Package builder.

CHANGED_SOURCES

A reserved variable name that may not be set or used in a construction environment. (See the manpage section "Variable Substitution" for more information).

CHANGED_TARGETS

A reserved variable name that may not be set or used in a construction environment. (See the manpage section "Variable Substitution" for more information).

CHANGELOG

The name of a file containing the change log text to be included in the package. This is included as the %changelog section of the RPM .spec file.

See the Package builder.

COMPILATIONDB_COMSTR

The string displayed when the CompilationDatabase builder's action is run.

COMPILATIONDB_PATH_FILTER

A string which instructs CompilationDatabase to only include entries where the output member matches the pattern in the filter string using fnmatch, which uses glob style wildcards.

The default value is an empty string '', which disables filtering.

COMPILATIONDB_USE_ABSPATH

A boolean flag to instruct CompilationDatabase whether to write the file and output members in the compilation database using absolute or relative paths.

The default value is False (use relative paths)

_concat

A function used to produce variables like $_CPPINCFLAGS. It takes four mandatory arguments, and up to 4 additional optional arguments: 1) a prefix to concatenate onto each element, 2) a list of elements, 3) a suffix to concatenate onto each element, 4) an environment for variable interpolation, 5) an optional function that will be called to transform the list before concatenation, 6) an optionally specified target (Can use TARGET), 7) an optionally specified source (Can use SOURCE), 8) optional affect_signature flag which will wrap non-empty returned value with $( and $) to indicate the contents should not affect the signature of the generated command line.

        env['_CPPINCFLAGS'] = '${_concat(INCPREFIX, CPPPATH, INCSUFFIX, __env__, RDirs, TARGET, SOURCE, affect_signature=False)}'
    
CONFIGUREDIR

The name of the directory in which Configure context test files are written. The default is .sconf_temp in the top-level directory containing the SConstruct file.

If variant directories are in use, and the configure check results should not be shared between variants, you can set $CONFIGUREDIR and $CONFIGURELOG so they are unique per variant directory.

CONFIGURELOG

The name of the Configure context log file. The default is config.log in the top-level directory containing the SConstruct file.

If variant directories are in use, and the configure check results should not be shared between variants, you can set $CONFIGUREDIR and $CONFIGURELOG so they are unique per variant directory.

_CPPDEFFLAGS

An automatically-generated construction variable containing the C preprocessor command-line options to define values. The value of $_CPPDEFFLAGS is created by respectively prepending and appending $CPPDEFPREFIX and $CPPDEFSUFFIX to each definition in $CPPDEFINES.

CPPDEFINES

A platform independent specification of C preprocessor macro definitions. The definitions are added to command lines through the automatically-generated $_CPPDEFFLAGS construction variable, which is constructed according to the contents of $CPPDEFINES:

  • If $CPPDEFINES is a string, the values of the $CPPDEFPREFIX and $CPPDEFSUFFIX construction variables are respectively prepended and appended to each definition in $CPPDEFINES, split on whitespace.

    # Adds -Dxyz to POSIX compiler command lines,
    # and /Dxyz to Microsoft Visual C++ command lines.
    env = Environment(CPPDEFINES='xyz')
    
  • If $CPPDEFINES is a list, the values of the $CPPDEFPREFIX and $CPPDEFSUFFIX construction variables are respectively prepended and appended to each element in the list. If any element is a tuple (or list) then the first item of the tuple is the macro name and the second is the macro definition. If the definition is not omitted or None, the name and definition are combined into a single name=definition item before the preending/appending.

    # Adds -DB=2 -DA to POSIX compiler command lines,
    # and /DB=2 /DA to Microsoft Visual C++ command lines.
    env = Environment(CPPDEFINES=[('B', 2), 'A'])
    
  • If $CPPDEFINES is a dictionary, the values of the $CPPDEFPREFIX and $CPPDEFSUFFIX construction variables are respectively prepended and appended to each key from the dictionary. If the value for a key is not None, then the key (macro name) and the value (macros definition) are combined into a single name=definition item before the prepending/appending.

    # Adds -DA -DB=2 to POSIX compiler command lines,
    # or /DA /DB=2 to Microsoft Visual C++ command lines.
    env = Environment(CPPDEFINES={'B':2, 'A':None})
    

Depending on how contents are added to $CPPDEFINES, it may be transformed into a compound type, for example a list containing strings, tuples and/or dictionaries. SCons can correctly expand such a compound type.

Note that SCons may call the compiler via a shell. If a macro definition contains characters such as spaces that have meaning to the shell, or is intended to be a string value, you may need to use the shell's quoting syntax to avoid interpretation by the shell before the preprocessor sees it. Function-like macros are not supported via this mechanism (and some compilers do not even implement that functionality via the command lines). When quoting, note that one set of quote characters are used to define a Python string, then quotes embedded inside that would be consumed by the shell unless escaped. These examples may help illustrate:

env = Environment(CPPDEFINES=['USE_ALT_HEADER=\\"foo_alt.h\\"'])
env = Environment(CPPDEFINES=[('USE_ALT_HEADER', '\\"foo_alt.h\\"')])

:Changed in version 4.5: SCons no longer sorts $CPPDEFINES values entered in dictionary form. Python now preserves dictionary keys in the order they are entered, so it is no longer necessary to sort them to ensure a stable command line.

CPPDEFPREFIX

The prefix used to specify preprocessor macro definitions on the C compiler command line. This will be prepended to each definition in the $CPPDEFINES construction variable when the $_CPPDEFFLAGS variable is automatically generated.

CPPDEFSUFFIX

The suffix used to specify preprocessor macro definitions on the C compiler command line. This will be appended to each definition in the $CPPDEFINES construction variable when the $_CPPDEFFLAGS variable is automatically generated.

CPPFLAGS

User-specified C preprocessor options. These will be included in any command that uses the C preprocessor, including not just compilation of C and C++ source files via the $CCCOM, $SHCCCOM, $CXXCOM and $SHCXXCOM command lines, but also the $FORTRANPPCOM, $SHFORTRANPPCOM, $F77PPCOM and $SHF77PPCOM command lines used to compile a Fortran source file, and the $ASPPCOM command line used to assemble an assembly language source file, after first running each file through the C preprocessor. Note that this variable does not contain -I (or similar) include search path options that scons generates automatically from $CPPPATH. See $_CPPINCFLAGS, below, for the variable that expands to those options.

_CPPINCFLAGS

An automatically-generated construction variable containing the C preprocessor command-line options for specifying directories to be searched for include files. The value of $_CPPINCFLAGS is created by respectively prepending and appending $INCPREFIX and $INCSUFFIX to each directory in $CPPPATH.

CPPPATH

The list of directories that the C preprocessor will search for include directories. The C/C++ implicit dependency scanner will search these directories for include files. In general it's not advised to put include directory directives directly into $CCFLAGS or $CXXFLAGS as the result will be non-portable and the directories will not be searched by the dependency scanner. $CPPPATH should be a list of path strings, or a single string, not a pathname list joined by Python's os.pathsep.

Note: directory names in $CPPPATH will be looked-up relative to the directory of the SConscript file when they are used in a command. To force scons to look-up a directory relative to the root of the source tree use the # prefix:

env = Environment(CPPPATH='#/include')

The directory look-up can also be forced using the Dir function:

include = Dir('include')
env = Environment(CPPPATH=include)

The directory list will be added to command lines through the automatically-generated $_CPPINCFLAGS construction variable, which is constructed by respectively prepending and appending the values of the $INCPREFIX and $INCSUFFIX construction variables to each directory in $CPPPATH. Any command lines you define that need the $CPPPATH directory list should include $_CPPINCFLAGS:

env = Environment(CCCOM="my_compiler $_CPPINCFLAGS -c -o $TARGET $SOURCE")
CPPSUFFIXES

The list of suffixes of files that will be scanned for C preprocessor implicit dependencies (#include lines). The default list is:

[".c", ".C", ".cxx", ".cpp", ".c++", ".cc",
 ".h", ".H", ".hxx", ".hpp", ".hh",
 ".F", ".fpp", ".FPP",
 ".m", ".mm",
 ".S", ".spp", ".SPP"]
CXX

The C++ compiler. See also $SHCXX for compiling to shared objects..

CXXCOM

The command line used to compile a C++ source file to an object file. Any options specified in the $CXXFLAGS and $CPPFLAGS construction variables are included on this command line. See also $SHCXXCOM for compiling to shared objects..

CXXCOMSTR

If set, the string displayed when a C++ source file is compiled to a (static) object file. If not set, then $CXXCOM (the command line) is displayed. See also $SHCXXCOMSTR for compiling to shared objects..

env = Environment(CXXCOMSTR = "Compiling static object $TARGET")
CXXFILESUFFIX

The suffix for C++ source files. This is used by the internal CXXFile builder when generating C++ files from Lex (.ll) or YACC (.yy) input files. The default suffix is .cc. SCons also treats files with the suffixes .cpp, .cxx, .c++, and .C++ as C++ files, and files with .mm suffixes as Objective C++ files. On case-sensitive systems (Linux, UNIX, and other POSIX-alikes), SCons also treats .C (upper case) files as C++ files.

CXXFLAGS

General options that are passed to the C++ compiler. By default, this includes the value of $CCFLAGS, so that setting $CCFLAGS affects both C and C++ compilation. If you want to add C++-specific flags, you must set or override the value of $CXXFLAGS. See also $SHCXXFLAGS for compiling to shared objects..

CXXVERSION

The version number of the C++ compiler. This may or may not be set, depending on the specific C++ compiler being used.

DC

The D compiler to use. See also $SHDC for compiling to shared objects.

DCOM

The command line used to compile a D file to an object file. Any options specified in the $DFLAGS construction variable is included on this command line. See also $SHDCOM for compiling to shared objects.

DCOMSTR

If set, the string displayed when a D source file is compiled to a (static) object file. If not set, then $DCOM (the command line) is displayed. See also $SHDCOMSTR for compiling to shared objects.

DDEBUG

List of debug tags to enable when compiling.

DDEBUGPREFIX

DDEBUGPREFIX.

DDEBUGSUFFIX

DDEBUGSUFFIX.

DESCRIPTION

A long description of the project being packaged. This is included in the relevant section of the file that controls the packaging build.

See the Package builder.

DESCRIPTION_lang

A language-specific long description for the specified lang. This is used to populate a %description -l section of an RPM .spec file.

See the Package builder.

DFILESUFFIX

DFILESUFFIX.

DFLAGPREFIX

DFLAGPREFIX.

DFLAGS

General options that are passed to the D compiler.

DFLAGSUFFIX

DFLAGSUFFIX.

DI_FILE_DIR

Path where .di files will be generated

DI_FILE_DIR_PREFIX

Prefix to send the di path argument to compiler

DI_FILE_DIR_SUFFFIX

Suffix to send the di path argument to compiler

DI_FILE_SUFFIX

Suffix of d include files default is .di

DINCPREFIX

DINCPREFIX.

DINCSUFFIX

DLIBFLAGSUFFIX.

Dir

A function that converts a string into a Dir instance relative to the target being built.

Dirs

A function that converts a list of strings into a list of Dir instances relative to the target being built.

DLIB

Name of the lib tool to use for D codes.

DLIBCOM

The command line to use when creating libraries.

DLIBDIRPREFIX

DLIBLINKPREFIX.

DLIBDIRSUFFIX

DLIBLINKSUFFIX.

DLIBFLAGPREFIX

DLIBFLAGPREFIX.

DLIBFLAGSUFFIX

DLIBFLAGSUFFIX.

DLIBLINKPREFIX

DLIBLINKPREFIX.

DLIBLINKSUFFIX

DLIBLINKSUFFIX.

DLINK

Name of the linker to use for linking systems including D sources. See also $SHDLINK for linking shared objects.

DLINKCOM

The command line to use when linking systems including D sources. See also $SHDLINKCOM for linking shared objects.

DLINKFLAGPREFIX

DLINKFLAGPREFIX.

DLINKFLAGS

List of linker flags. See also $SHDLINKFLAGS for linking shared objects.

DLINKFLAGSUFFIX

DLINKFLAGSUFFIX.

DOCBOOK_DEFAULT_XSL_EPUB

The default XSLT file for the DocbookEpub builder within the current environment, if no other XSLT gets specified via keyword.

DOCBOOK_DEFAULT_XSL_HTML

The default XSLT file for the DocbookHtml builder within the current environment, if no other XSLT gets specified via keyword.

DOCBOOK_DEFAULT_XSL_HTMLCHUNKED

The default XSLT file for the DocbookHtmlChunked builder within the current environment, if no other XSLT gets specified via keyword.

DOCBOOK_DEFAULT_XSL_HTMLHELP

The default XSLT file for the DocbookHtmlhelp builder within the current environment, if no other XSLT gets specified via keyword.

DOCBOOK_DEFAULT_XSL_MAN

The default XSLT file for the DocbookMan builder within the current environment, if no other XSLT gets specified via keyword.

DOCBOOK_DEFAULT_XSL_PDF

The default XSLT file for the DocbookPdf builder within the current environment, if no other XSLT gets specified via keyword.

DOCBOOK_DEFAULT_XSL_SLIDESHTML

The default XSLT file for the DocbookSlidesHtml builder within the current environment, if no other XSLT gets specified via keyword.

DOCBOOK_DEFAULT_XSL_SLIDESPDF

The default XSLT file for the DocbookSlidesPdf builder within the current environment, if no other XSLT gets specified via keyword.

DOCBOOK_FOP

The path to the PDF renderer fop or xep, if one of them is installed (fop gets checked first).

DOCBOOK_FOPCOM

The full command-line for the PDF renderer fop or xep.

DOCBOOK_FOPCOMSTR

The string displayed when a renderer like fop or xep is used to create PDF output from an XML file.

DOCBOOK_FOPFLAGS

Additonal command-line flags for the PDF renderer fop or xep.

DOCBOOK_XMLLINT

The path to the external executable xmllint, if it's installed. Note, that this is only used as last fallback for resolving XIncludes, if no lxml Python binding can be imported in the current system.

DOCBOOK_XMLLINTCOM

The full command-line for the external executable xmllint.

DOCBOOK_XMLLINTCOMSTR

The string displayed when xmllint is used to resolve XIncludes for a given XML file.

DOCBOOK_XMLLINTFLAGS

Additonal command-line flags for the external executable xmllint.

DOCBOOK_XSLTPROC

The path to the external executable xsltproc (or saxon, xalan), if one of them is installed. Note, that this is only used as last fallback for XSL transformations, if no lxml Python binding can be imported in the current system.

DOCBOOK_XSLTPROCCOM

The full command-line for the external executable xsltproc (or saxon, xalan).

DOCBOOK_XSLTPROCCOMSTR

The string displayed when xsltproc is used to transform an XML file via a given XSLT stylesheet.

DOCBOOK_XSLTPROCFLAGS

Additonal command-line flags for the external executable xsltproc (or saxon, xalan).

DOCBOOK_XSLTPROCPARAMS

Additonal parameters that are not intended for the XSLT processor executable, but the XSL processing itself. By default, they get appended at the end of the command line for saxon and saxon-xslt, respectively.

DPATH

List of paths to search for import modules.

DRPATHPREFIX

DRPATHPREFIX.

DRPATHSUFFIX

DRPATHSUFFIX.

DSUFFIXES

The list of suffixes of files that will be scanned for imported D package files. The default list is ['.d'].

DVERPREFIX

DVERPREFIX.

DVERSIONS

List of version tags to enable when compiling.

DVERSUFFIX

DVERSUFFIX.

DVIPDF

The TeX DVI file to PDF file converter.

DVIPDFCOM

The command line used to convert TeX DVI files into a PDF file.

DVIPDFCOMSTR

The string displayed when a TeX DVI file is converted into a PDF file. If this is not set, then $DVIPDFCOM (the command line) is displayed.

DVIPDFFLAGS

General options passed to the TeX DVI file to PDF file converter.

DVIPS

The TeX DVI file to PostScript converter.

DVIPSFLAGS

General options passed to the TeX DVI file to PostScript converter.

ENV

The execution environment - a dictionary of environment variables used when SCons invokes external commands to build targets defined in this construction environment. When $ENV is passed to a command, all list values are assumed to be path lists and are joined using the search path separator. Any other non-string values are coerced to a string.

Note that by default SCons does not propagate the environment in effect when you execute scons (the "shell environment") to the execution environment. This is so that builds will be guaranteed repeatable regardless of the environment variables set at the time scons is invoked. If you want to propagate a shell environment variable to the commands executed to build target files, you must do so explicitly. A common example is the system PATH environment variable, so that scons will find utilities the same way as the invoking shell (or other process):

import os
env = Environment(ENV={'PATH': os.environ['PATH']})

Although it is usually not recommended, you can propagate the entire shell environment in one go:

import os
env = Environment(ENV=os.environ.copy())
ESCAPE

A function that will be called to escape shell special characters in command lines. The function should take one argument: the command line string to escape; and should return the escaped command line.

F03

The Fortran 03 compiler. You should normally set the $FORTRAN variable, which specifies the default Fortran compiler for all Fortran versions. You only need to set $F03 if you need to use a specific compiler or compiler version for Fortran 03 files.

F03COM

The command line used to compile a Fortran 03 source file to an object file. You only need to set $F03COM if you need to use a specific command line for Fortran 03 files. You should normally set the $FORTRANCOM variable, which specifies the default command line for all Fortran versions.

F03COMSTR

If set, the string displayed when a Fortran 03 source file is compiled to an object file. If not set, then $F03COM or $FORTRANCOM (the command line) is displayed.

F03FILESUFFIXES

The list of file extensions for which the F03 dialect will be used. By default, this is ['.f03']

F03FLAGS

General user-specified options that are passed to the Fortran 03 compiler. Note that this variable does not contain -I (or similar) include search path options that scons generates automatically from $F03PATH. See $_F03INCFLAGS below, for the variable that expands to those options. You only need to set $F03FLAGS if you need to define specific user options for Fortran 03 files. You should normally set the $FORTRANFLAGS variable, which specifies the user-specified options passed to the default Fortran compiler for all Fortran versions.

_F03INCFLAGS

An automatically-generated construction variable containing the Fortran 03 compiler command-line options for specifying directories to be searched for include files. The value of $_F03INCFLAGS is created by appending $INCPREFIX and $INCSUFFIX to the beginning and end of each directory in $F03PATH.

F03PATH

The list of directories that the Fortran 03 compiler will search for include directories. The implicit dependency scanner will search these directories for include files. Don't explicitly put include directory arguments in $F03FLAGS because the result will be non-portable and the directories will not be searched by the dependency scanner. Note: directory names in $F03PATH will be looked-up relative to the SConscript directory when they are used in a command. To force scons to look-up a directory relative to the root of the source tree use #: You only need to set $F03PATH if you need to define a specific include path for Fortran 03 files. You should normally set the $FORTRANPATH variable, which specifies the include path for the default Fortran compiler for all Fortran versions.

env = Environment(F03PATH='#/include')

The directory look-up can also be forced using the Dir() function:

include = Dir('include')
env = Environment(F03PATH=include)

The directory list will be added to command lines through the automatically-generated $_F03INCFLAGS construction variable, which is constructed by appending the values of the $INCPREFIX and $INCSUFFIX construction variables to the beginning and end of each directory in $F03PATH. Any command lines you define that need the F03PATH directory list should include $_F03INCFLAGS:

env = Environment(F03COM="my_compiler $_F03INCFLAGS -c -o $TARGET $SOURCE")
F03PPCOM

The command line used to compile a Fortran 03 source file to an object file after first running the file through the C preprocessor. Any options specified in the $F03FLAGS and $CPPFLAGS construction variables are included on this command line. You only need to set $F03PPCOM if you need to use a specific C-preprocessor command line for Fortran 03 files. You should normally set the $FORTRANPPCOM variable, which specifies the default C-preprocessor command line for all Fortran versions.

F03PPCOMSTR

If set, the string displayed when a Fortran 03 source file is compiled to an object file after first running the file through the C preprocessor. If not set, then $F03PPCOM or $FORTRANPPCOM (the command line) is displayed.

F03PPFILESUFFIXES

The list of file extensions for which the compilation + preprocessor pass for F03 dialect will be used. By default, this is empty.

F08

The Fortran 08 compiler. You should normally set the $FORTRAN variable, which specifies the default Fortran compiler for all Fortran versions. You only need to set $F08 if you need to use a specific compiler or compiler version for Fortran 08 files.

F08COM

The command line used to compile a Fortran 08 source file to an object file. You only need to set $F08COM if you need to use a specific command line for Fortran 08 files. You should normally set the $FORTRANCOM variable, which specifies the default command line for all Fortran versions.

F08COMSTR

If set, the string displayed when a Fortran 08 source file is compiled to an object file. If not set, then $F08COM or $FORTRANCOM (the command line) is displayed.

F08FILESUFFIXES

The list of file extensions for which the F08 dialect will be used. By default, this is ['.f08']

F08FLAGS

General user-specified options that are passed to the Fortran 08 compiler. Note that this variable does not contain -I (or similar) include search path options that scons generates automatically from $F08PATH. See $_F08INCFLAGS below, for the variable that expands to those options. You only need to set $F08FLAGS if you need to define specific user options for Fortran 08 files. You should normally set the $FORTRANFLAGS variable, which specifies the user-specified options passed to the default Fortran compiler for all Fortran versions.

_F08INCFLAGS

An automatically-generated construction variable containing the Fortran 08 compiler command-line options for specifying directories to be searched for include files. The value of $_F08INCFLAGS is created by appending $INCPREFIX and $INCSUFFIX to the beginning and end of each directory in $F08PATH.

F08PATH

The list of directories that the Fortran 08 compiler will search for include directories. The implicit dependency scanner will search these directories for include files. Don't explicitly put include directory arguments in $F08FLAGS because the result will be non-portable and the directories will not be searched by the dependency scanner. Note: directory names in $F08PATH will be looked-up relative to the SConscript directory when they are used in a command. To force scons to look-up a directory relative to the root of the source tree use #: You only need to set $F08PATH if you need to define a specific include path for Fortran 08 files. You should normally set the $FORTRANPATH variable, which specifies the include path for the default Fortran compiler for all Fortran versions.

env = Environment(F08PATH='#/include')

The directory look-up can also be forced using the Dir() function:

include = Dir('include')
env = Environment(F08PATH=include)

The directory list will be added to command lines through the automatically-generated $_F08INCFLAGS construction variable, which is constructed by appending the values of the $INCPREFIX and $INCSUFFIX construction variables to the beginning and end of each directory in $F08PATH. Any command lines you define that need the F08PATH directory list should include $_F08INCFLAGS:

env = Environment(F08COM="my_compiler $_F08INCFLAGS -c -o $TARGET $SOURCE")
F08PPCOM

The command line used to compile a Fortran 08 source file to an object file after first running the file through the C preprocessor. Any options specified in the $F08FLAGS and $CPPFLAGS construction variables are included on this command line. You only need to set $F08PPCOM if you need to use a specific C-preprocessor command line for Fortran 08 files. You should normally set the $FORTRANPPCOM variable, which specifies the default C-preprocessor command line for all Fortran versions.

F08PPCOMSTR

If set, the string displayed when a Fortran 08 source file is compiled to an object file after first running the file through the C preprocessor. If not set, then $F08PPCOM or $FORTRANPPCOM (the command line) is displayed.

F08PPFILESUFFIXES

The list of file extensions for which the compilation + preprocessor pass for F08 dialect will be used. By default, this is empty.

F77

The Fortran 77 compiler. You should normally set the $FORTRAN variable, which specifies the default Fortran compiler for all Fortran versions. You only need to set $F77 if you need to use a specific compiler or compiler version for Fortran 77 files.

F77COM

The command line used to compile a Fortran 77 source file to an object file. You only need to set $F77COM if you need to use a specific command line for Fortran 77 files. You should normally set the $FORTRANCOM variable, which specifies the default command line for all Fortran versions.

F77COMSTR

If set, the string displayed when a Fortran 77 source file is compiled to an object file. If not set, then $F77COM or $FORTRANCOM (the command line) is displayed.

F77FILESUFFIXES

The list of file extensions for which the F77 dialect will be used. By default, this is ['.f77']

F77FLAGS

General user-specified options that are passed to the Fortran 77 compiler. Note that this variable does not contain -I (or similar) include search path options that scons generates automatically from $F77PATH. See $_F77INCFLAGS below, for the variable that expands to those options. You only need to set $F77FLAGS if you need to define specific user options for Fortran 77 files. You should normally set the $FORTRANFLAGS variable, which specifies the user-specified options passed to the default Fortran compiler for all Fortran versions.

_F77INCFLAGS

An automatically-generated construction variable containing the Fortran 77 compiler command-line options for specifying directories to be searched for include files. The value of $_F77INCFLAGS is created by appending $INCPREFIX and $INCSUFFIX to the beginning and end of each directory in $F77PATH.

F77PATH

The list of directories that the Fortran 77 compiler will search for include directories. The implicit dependency scanner will search these directories for include files. Don't explicitly put include directory arguments in $F77FLAGS because the result will be non-portable and the directories will not be searched by the dependency scanner. Note: directory names in $F77PATH will be looked-up relative to the SConscript directory when they are used in a command. To force scons to look-up a directory relative to the root of the source tree use #: You only need to set $F77PATH if you need to define a specific include path for Fortran 77 files. You should normally set the $FORTRANPATH variable, which specifies the include path for the default Fortran compiler for all Fortran versions.

env = Environment(F77PATH='#/include')

The directory look-up can also be forced using the Dir() function:

include = Dir('include')
env = Environment(F77PATH=include)

The directory list will be added to command lines through the automatically-generated $_F77INCFLAGS construction variable, which is constructed by appending the values of the $INCPREFIX and $INCSUFFIX construction variables to the beginning and end of each directory in $F77PATH. Any command lines you define that need the F77PATH directory list should include $_F77INCFLAGS:

env = Environment(F77COM="my_compiler $_F77INCFLAGS -c -o $TARGET $SOURCE")
F77PPCOM

The command line used to compile a Fortran 77 source file to an object file after first running the file through the C preprocessor. Any options specified in the $F77FLAGS and $CPPFLAGS construction variables are included on this command line. You only need to set $F77PPCOM if you need to use a specific C-preprocessor command line for Fortran 77 files. You should normally set the $FORTRANPPCOM variable, which specifies the default C-preprocessor command line for all Fortran versions.

F77PPCOMSTR

If set, the string displayed when a Fortran 77 source file is compiled to an object file after first running the file through the C preprocessor. If not set, then $F77PPCOM or $FORTRANPPCOM (the command line) is displayed.

F77PPFILESUFFIXES

The list of file extensions for which the compilation + preprocessor pass for F77 dialect will be used. By default, this is empty.

F90

The Fortran 90 compiler. You should normally set the $FORTRAN variable, which specifies the default Fortran compiler for all Fortran versions. You only need to set $F90 if you need to use a specific compiler or compiler version for Fortran 90 files.

F90COM

The command line used to compile a Fortran 90 source file to an object file. You only need to set $F90COM if you need to use a specific command line for Fortran 90 files. You should normally set the $FORTRANCOM variable, which specifies the default command line for all Fortran versions.

F90COMSTR

If set, the string displayed when a Fortran 90 source file is compiled to an object file. If not set, then $F90COM or $FORTRANCOM (the command line) is displayed.

F90FILESUFFIXES

The list of file extensions for which the F90 dialect will be used. By default, this is ['.f90']

F90FLAGS

General user-specified options that are passed to the Fortran 90 compiler. Note that this variable does not contain -I (or similar) include search path options that scons generates automatically from $F90PATH. See $_F90INCFLAGS below, for the variable that expands to those options. You only need to set $F90FLAGS if you need to define specific user options for Fortran 90 files. You should normally set the $FORTRANFLAGS variable, which specifies the user-specified options passed to the default Fortran compiler for all Fortran versions.

_F90INCFLAGS

An automatically-generated construction variable containing the Fortran 90 compiler command-line options for specifying directories to be searched for include files. The value of $_F90INCFLAGS is created by appending $INCPREFIX and $INCSUFFIX to the beginning and end of each directory in $F90PATH.

F90PATH

The list of directories that the Fortran 90 compiler will search for include directories. The implicit dependency scanner will search these directories for include files. Don't explicitly put include directory arguments in $F90FLAGS because the result will be non-portable and the directories will not be searched by the dependency scanner. Note: directory names in $F90PATH will be looked-up relative to the SConscript directory when they are used in a command. To force scons to look-up a directory relative to the root of the source tree use #: You only need to set $F90PATH if you need to define a specific include path for Fortran 90 files. You should normally set the $FORTRANPATH variable, which specifies the include path for the default Fortran compiler for all Fortran versions.

env = Environment(F90PATH='#/include')

The directory look-up can also be forced using the Dir() function:

include = Dir('include')
env = Environment(F90PATH=include)

The directory list will be added to command lines through the automatically-generated $_F90INCFLAGS construction variable, which is constructed by appending the values of the $INCPREFIX and $INCSUFFIX construction variables to the beginning and end of each directory in $F90PATH. Any command lines you define that need the F90PATH directory list should include $_F90INCFLAGS:

env = Environment(F90COM="my_compiler $_F90INCFLAGS -c -o $TARGET $SOURCE")
F90PPCOM

The command line used to compile a Fortran 90 source file to an object file after first running the file through the C preprocessor. Any options specified in the $F90FLAGS and $CPPFLAGS construction variables are included on this command line. You only need to set $F90PPCOM if you need to use a specific C-preprocessor command line for Fortran 90 files. You should normally set the $FORTRANPPCOM variable, which specifies the default C-preprocessor command line for all Fortran versions.

F90PPCOMSTR

If set, the string displayed when a Fortran 90 source file is compiled after first running the file through the C preprocessor. If not set, then $F90PPCOM or $FORTRANPPCOM (the command line) is displayed.

F90PPFILESUFFIXES

The list of file extensions for which the compilation + preprocessor pass for F90 dialect will be used. By default, this is empty.

F95

The Fortran 95 compiler. You should normally set the $FORTRAN variable, which specifies the default Fortran compiler for all Fortran versions. You only need to set $F95 if you need to use a specific compiler or compiler version for Fortran 95 files.

F95COM

The command line used to compile a Fortran 95 source file to an object file. You only need to set $F95COM if you need to use a specific command line for Fortran 95 files. You should normally set the $FORTRANCOM variable, which specifies the default command line for all Fortran versions.

F95COMSTR

If set, the string displayed when a Fortran 95 source file is compiled to an object file. If not set, then $F95COM or $FORTRANCOM (the command line) is displayed.

F95FILESUFFIXES

The list of file extensions for which the F95 dialect will be used. By default, this is ['.f95']

F95FLAGS

General user-specified options that are passed to the Fortran 95 compiler. Note that this variable does not contain -I (or similar) include search path options that scons generates automatically from $F95PATH. See $_F95INCFLAGS below, for the variable that expands to those options. You only need to set $F95FLAGS if you need to define specific user options for Fortran 95 files. You should normally set the $FORTRANFLAGS variable, which specifies the user-specified options passed to the default Fortran compiler for all Fortran versions.

_F95INCFLAGS

An automatically-generated construction variable containing the Fortran 95 compiler command-line options for specifying directories to be searched for include files. The value of $_F95INCFLAGS is created by appending $INCPREFIX and $INCSUFFIX to the beginning and end of each directory in $F95PATH.

F95PATH

The list of directories that the Fortran 95 compiler will search for include directories. The implicit dependency scanner will search these directories for include files. Don't explicitly put include directory arguments in $F95FLAGS because the result will be non-portable and the directories will not be searched by the dependency scanner. Note: directory names in $F95PATH will be looked-up relative to the SConscript directory when they are used in a command. To force scons to look-up a directory relative to the root of the source tree use #: You only need to set $F95PATH if you need to define a specific include path for Fortran 95 files. You should normally set the $FORTRANPATH variable, which specifies the include path for the default Fortran compiler for all Fortran versions.

env = Environment(F95PATH='#/include')

The directory look-up can also be forced using the Dir() function:

include = Dir('include')
env = Environment(F95PATH=include)

The directory list will be added to command lines through the automatically-generated $_F95INCFLAGS construction variable, which is constructed by appending the values of the $INCPREFIX and $INCSUFFIX construction variables to the beginning and end of each directory in $F95PATH. Any command lines you define that need the F95PATH directory list should include $_F95INCFLAGS:

env = Environment(F95COM="my_compiler $_F95INCFLAGS -c -o $TARGET $SOURCE")
F95PPCOM

The command line used to compile a Fortran 95 source file to an object file after first running the file through the C preprocessor. Any options specified in the $F95FLAGS and $CPPFLAGS construction variables are included on this command line. You only need to set $F95PPCOM if you need to use a specific C-preprocessor command line for Fortran 95 files. You should normally set the $FORTRANPPCOM variable, which specifies the default C-preprocessor command line for all Fortran versions.

F95PPCOMSTR

If set, the string displayed when a Fortran 95 source file is compiled to an object file after first running the file through the C preprocessor. If not set, then $F95PPCOM or $FORTRANPPCOM (the command line) is displayed.

F95PPFILESUFFIXES

The list of file extensions for which the compilation + preprocessor pass for F95 dialect will be used. By default, this is empty.

File

A function that converts a string into a File instance relative to the target being built.

FILE_ENCODING

File encoding used for files written by Textfile and Substfile. Set to "utf-8" by default.

New in version 4.5.0.

FORTRAN

The default Fortran compiler for all versions of Fortran.

FORTRANCOM

The command line used to compile a Fortran source file to an object file. By default, any options specified in the $FORTRANFLAGS, $_FORTRANMODFLAG, and $_FORTRANINCFLAGS construction variables are included on this command line.

FORTRANCOMMONFLAGS

General user-specified options that are passed to the Fortran compiler. Similar to $FORTRANFLAGS, but this construction variable is applied to all dialects.

New in version 4.4.

FORTRANCOMSTR

If set, the string displayed when a Fortran source file is compiled to an object file. If not set, then $FORTRANCOM (the command line) is displayed.

FORTRANFILESUFFIXES

The list of file extensions for which the FORTRAN dialect will be used. By default, this is ['.f', '.for', '.ftn']

FORTRANFLAGS

General user-specified options for the FORTRAN dialect that are passed to the Fortran compiler. Note that this variable does not contain -I (or similar) include or module search path options that scons generates automatically from $FORTRANPATH. See $_FORTRANINCFLAGS and $_FORTRANMODFLAG for the construction variables that expand those options.

_FORTRANINCFLAGS

An automatically-generated construction variable containing the Fortran compiler command-line options for specifying directories to be searched for include files and module files. The value of $_FORTRANINCFLAGS is created by respectively prepending and appending $INCPREFIX and $INCSUFFIX to the beginning and end of each directory in $FORTRANPATH.

FORTRANMODDIR

Directory location where the Fortran compiler should place any module files it generates. This variable is empty, by default. Some Fortran compilers will internally append this directory in the search path for module files, as well.

FORTRANMODDIRPREFIX

The prefix used to specify a module directory on the Fortran compiler command line. This will be prepended to the beginning of the directory in the $FORTRANMODDIR construction variables when the $_FORTRANMODFLAG variables is automatically generated.

FORTRANMODDIRSUFFIX

The suffix used to specify a module directory on the Fortran compiler command line. This will be appended to the end of the directory in the $FORTRANMODDIR construction variables when the $_FORTRANMODFLAG variables is automatically generated.

_FORTRANMODFLAG

An automatically-generated construction variable containing the Fortran compiler command-line option for specifying the directory location where the Fortran compiler should place any module files that happen to get generated during compilation. The value of $_FORTRANMODFLAG is created by respectively prepending and appending $FORTRANMODDIRPREFIX and $FORTRANMODDIRSUFFIX to the beginning and end of the directory in $FORTRANMODDIR.

FORTRANMODPREFIX

The module file prefix used by the Fortran compiler. SCons assumes that the Fortran compiler follows the quasi-standard naming convention for module files of module_name.mod. As a result, this variable is left empty, by default. For situations in which the compiler does not necessarily follow the normal convention, the user may use this variable. Its value will be appended to every module file name as scons attempts to resolve dependencies.

FORTRANMODSUFFIX

The module file suffix used by the Fortran compiler. SCons assumes that the Fortran compiler follows the quasi-standard naming convention for module files of module_name.mod. As a result, this variable is set to ".mod", by default. For situations in which the compiler does not necessarily follow the normal convention, the user may use this variable. Its value will be appended to every module file name as scons attempts to resolve dependencies.

FORTRANPATH

The list of directories that the Fortran compiler will search for include files and (for some compilers) module files. The Fortran implicit dependency scanner will search these directories for include files (but not module files since they are autogenerated and, as such, may not actually exist at the time the scan takes place). Don't explicitly put include directory arguments in FORTRANFLAGS because the result will be non-portable and the directories will not be searched by the dependency scanner. Note: directory names in FORTRANPATH will be looked-up relative to the SConscript directory when they are used in a command. To force scons to look-up a directory relative to the root of the source tree use #:

env = Environment(FORTRANPATH='#/include')

The directory look-up can also be forced using the Dir() function:

include = Dir('include')
env = Environment(FORTRANPATH=include)

The directory list will be added to command lines through the automatically-generated $_FORTRANINCFLAGS construction variable, which is constructed by respectively prepending and appending the values of the $INCPREFIX and $INCSUFFIX construction variables to the beginning and end of each directory in $FORTRANPATH. Any command lines you define that need the FORTRANPATH directory list should include $_FORTRANINCFLAGS:

env = Environment(FORTRANCOM="my_compiler $_FORTRANINCFLAGS -c -o $TARGET $SOURCE")
FORTRANPPCOM

The command line used to compile a Fortran source file to an object file after first running the file through the C preprocessor. By default, any options specified in the $FORTRANFLAGS, $CPPFLAGS, $_CPPDEFFLAGS, $_FORTRANMODFLAG, and $_FORTRANINCFLAGS construction variables are included on this command line.

FORTRANPPCOMSTR

If set, the string displayed when a Fortran source file is compiled to an object file after first running the file through the C preprocessor. If not set, then $FORTRANPPCOM (the command line) is displayed.

FORTRANPPFILESUFFIXES

The list of file extensions for which the compilation + preprocessor pass for FORTRAN dialect will be used. By default, this is ['.fpp', '.FPP']

FORTRANSUFFIXES

The list of suffixes of files that will be scanned for Fortran implicit dependencies (INCLUDE lines and USE statements). The default list is:

[".f", ".F", ".for", ".FOR", ".ftn", ".FTN", ".fpp", ".FPP",
".f77", ".F77", ".f90", ".F90", ".f95", ".F95"]
FRAMEWORKPATH

On Mac OS X with gcc, a list containing the paths to search for frameworks. Used by the compiler to find framework-style includes like #include <Fmwk/Header.h>. Used by the linker to find user-specified frameworks when linking (see $FRAMEWORKS). For example:

env.AppendUnique(FRAMEWORKPATH='#myframeworkdir')
            

will add

... -Fmyframeworkdir
            

to the compiler and linker command lines.

_FRAMEWORKPATH

On Mac OS X with gcc, an automatically-generated construction variable containing the linker command-line options corresponding to $FRAMEWORKPATH.

FRAMEWORKPATHPREFIX

On Mac OS X with gcc, the prefix to be used for the FRAMEWORKPATH entries. (see $FRAMEWORKPATH). The default value is -F.

FRAMEWORKPREFIX

On Mac OS X with gcc, the prefix to be used for linking in frameworks (see $FRAMEWORKS). The default value is -framework.

FRAMEWORKS

On Mac OS X with gcc, a list of the framework names to be linked into a program or shared library or bundle. The default value is the empty list. For example:

env.AppendUnique(FRAMEWORKS=Split('System Cocoa SystemConfiguration'))
            
_FRAMEWORKS

On Mac OS X with gcc, an automatically-generated construction variable containing the linker command-line options for linking with FRAMEWORKS.

FRAMEWORKSFLAGS

On Mac OS X with gcc, general user-supplied frameworks options to be added at the end of a command line building a loadable module. (This has been largely superseded by the $FRAMEWORKPATH, $FRAMEWORKPATHPREFIX, $FRAMEWORKPREFIX and $FRAMEWORKS variables described above.)

GS

The Ghostscript program used to, for example, convert PostScript to PDF files.

GSCOM

The full Ghostscript command line used for the conversion process. Its default value is $GS $GSFLAGS -sOutputFile=$TARGET $SOURCES.

GSCOMSTR

The string displayed when Ghostscript is called for the conversion process. If this is not set (the default), then $GSCOM (the command line) is displayed.

GSFLAGS

General options passed to the Ghostscript program, when converting PostScript to PDF files for example. Its default value is -dNOPAUSE -dBATCH -sDEVICE=pdfwrite

HOST_ARCH

The name of the host hardware architecture used to create this construction environment. The platform code sets this when initializing (see $PLATFORM and the platform argument to Environment). Note the detected name of the architecture may not be identical to that returned by the Python platform.machine method.

On the win32 platform, if the Microsoft Visual C++ compiler is available, msvc tool setup is done using $HOST_ARCH and $TARGET_ARCH. Changing the values at any later time will not cause the tool to be reinitialized. Valid host arch values are x86 and arm for 32-bit hosts and amd64, arm64, and x86_64 for 64-bit hosts.

Should be considered immutable. $HOST_ARCH is not currently used by other platforms, but the option is reserved to do so in future

HOST_OS

The name of the host operating system for the platform used to create this construction environment. The platform code sets this when initializing (see $PLATFORM and the platform argument to Environment).

Should be considered immutable. $HOST_OS is not currently used by SCons, but the option is reserved to do so in future

IDLSUFFIXES

The list of suffixes of files that will be scanned for IDL implicit dependencies (#include or import lines). The default list is:

[".idl", ".IDL"]
IMPLIBNOVERSIONSYMLINKS

Used to override $SHLIBNOVERSIONSYMLINKS/$LDMODULENOVERSIONSYMLINKS when creating versioned import library for a shared library/loadable module. If not defined, then $SHLIBNOVERSIONSYMLINKS/$LDMODULENOVERSIONSYMLINKS is used to determine whether to disable symlink generation or not.

IMPLIBPREFIX

The prefix used for import library names. For example, cygwin uses import libraries (libfoo.dll.a) in pair with dynamic libraries (cygfoo.dll). The cyglink linker sets $IMPLIBPREFIX to 'lib' and $SHLIBPREFIX to 'cyg'.

IMPLIBSUFFIX

The suffix used for import library names. For example, cygwin uses import libraries (libfoo.dll.a) in pair with dynamic libraries (cygfoo.dll). The cyglink linker sets $IMPLIBSUFFIX to '.dll.a' and $SHLIBSUFFIX to '.dll'.

IMPLIBVERSION

Used to override $SHLIBVERSION/$LDMODULEVERSION when generating versioned import library for a shared library/loadable module. If undefined, the $SHLIBVERSION/$LDMODULEVERSION is used to determine the version of versioned import library.

IMPLICIT_COMMAND_DEPENDENCIES

Controls whether or not SCons will add implicit dependencies for the commands executed to build targets.

By default, SCons will add to each target an implicit dependency on the command represented by the first argument of any command line it executes (which is typically the command itself). By setting such a dependency, SCons can determine that a target should be rebuilt if the command changes, such as when a compiler is upgraded to a new version. The specific file for the dependency is found by searching the PATH variable in the ENV dictionary in the construction environment used to execute the command. The default is the same as setting the construction variable $IMPLICIT_COMMAND_DEPENDENCIES to a True-like value (true, yes, or 1 - but not a number greater than one, as that has a different meaning).

Action strings can be segmented by the use of an AND operator, &&. In a segemented string, each segment is a separate command line, these are run sequentially until one fails or the entire sequence has been executed. If an action string is segmented, then the selected behavior of $IMPLICIT_COMMAND_DEPENDENCIES is applied to each segment.

If $IMPLICIT_COMMAND_DEPENDENCIES is set to a False-like value (none, false, no, 0, etc.), then the implicit dependency will not be added to the targets built with that construction environment.

If $IMPLICIT_COMMAND_DEPENDENCIES is set to 2 or higher, then that number of arguments in the command line will be scanned for relative or absolute paths. If any are present, they will be added as implicit dependencies to the targets built with that construction environment. The first argument in the command line will be searched for using the PATH variable in the ENV dictionary in the construction environment used to execute the command. The other arguments will only be found if they are absolute paths or valid paths relative to the working directory.

If $IMPLICIT_COMMAND_DEPENDENCIES is set to all, then all arguments in the command line will be scanned for relative or absolute paths. If any are present, they will be added as implicit dependencies to the targets built with that construction environment. The first argument in the command line will be searched for using the PATH variable in the ENV dictionary in the construction environment used to execute the command. The other arguments will only be found if they are absolute paths or valid paths relative to the working directory.

env = Environment(IMPLICIT_COMMAND_DEPENDENCIES=False)
INCPREFIX

The prefix used to specify an include directory on the C compiler command line. This will be prepended to each directory in the $CPPPATH and $FORTRANPATH construction variables when the $_CPPINCFLAGS and $_FORTRANINCFLAGS variables are automatically generated.

INCSUFFIX

The suffix used to specify an include directory on the C compiler command line. This will be appended to each directory in the $CPPPATH and $FORTRANPATH construction variables when the $_CPPINCFLAGS and $_FORTRANINCFLAGS variables are automatically generated.

INSTALL

A function to be called to install a file into a destination file name. The default function copies the file into the destination (and sets the destination file's mode and permission bits to match the source file's). The function takes the following arguments:

def install(dest, source, env):

dest is the path name of the destination file. source is the path name of the source file. env is the construction environment (a dictionary of construction values) in force for this file installation.

INSTALLSTR

The string displayed when a file is installed into a destination file name. The default is:

Install file: "$SOURCE" as "$TARGET"
INTEL_C_COMPILER_VERSION

Set by the intelc Tool to the major version number of the Intel C compiler selected for use.

JAR

The Java archive tool.

JARCHDIR

The directory to which the Java archive tool should change (using the -C option).

JARCOM

The command line used to call the Java archive tool.

JARCOMSTR

The string displayed when the Java archive tool is called If this is not set, then $JARCOM (the command line) is displayed.

env = Environment(JARCOMSTR="JARchiving $SOURCES into $TARGET")
JARFLAGS

General options passed to the Java archive tool. By default this is set to cf to create the necessary jar file.

JARSUFFIX

The suffix for Java archives: .jar by default.

JAVABOOTCLASSPATH

Specifies the location of the bootstrap class files. Can be specified as a string or Node object, or as a list of strings or Node objects.

The value will be added to the JDK command lines via the -bootclasspath option, which requires a system-specific search path separator. This will be supplied by SCons as needed when it constructs the command line if $JAVABOOTCLASSPATH is provided in list form. If $JAVABOOTCLASSPATH is a single string containing search path separator characters (: for POSIX systems or ; for Windows), it will not be modified; and so is inherently system-specific; to supply the path in a system-independent manner, give $JAVABOOTCLASSPATH as a list of paths instead.

Note

Can only be used when compiling for releases prior to JDK 9.

JAVAC

The Java compiler.

JAVACCOM

The command line used to compile a directory tree containing Java source files to corresponding Java class files. Any options specified in the $JAVACFLAGS construction variable are included on this command line.

JAVACCOMSTR

The string displayed when compiling a directory tree of Java source files to corresponding Java class files. If this is not set, then $JAVACCOM (the command line) is displayed.

env = Environment(JAVACCOMSTR="Compiling class files $TARGETS from $SOURCES")
            
JAVACFLAGS

General options that are passed to the Java compiler.

JAVACLASSDIR

The directory in which Java class files may be found. This is stripped from the beginning of any Java .class file names supplied to the JavaH builder.

JAVACLASSPATH

Specifies the class search path for the JDK tools. Can be specified as a string or Node object, or as a list of strings or Node objects. Class path entries may be directory names to search for class files or packages, pathnames to archives (.jar or .zip) containing classes, or paths ending in a "base name wildcard" character (*), which matches files in that directory with a .jar suffix. See the Java documentation for more details.

The value will be added to the JDK command lines via the -classpath option, which requires a system-specific search path separator. This will be supplied by SCons as needed when it constructs the command line if $JAVACLASSPATH is provided in list form. If $JAVACLASSPATH is a single string containing search path separator characters (: for POSIX systems or ; for Windows), it will be split on the separator into a list of individual paths for dependency scanning purposes. It will not be modified for JDK command-line usage, so such a string is inherently system-specific; to supply the path in a system-independent manner, give $JAVACLASSPATH as a list of paths instead.

Note

SCons always supplies a -sourcepath when invoking the Java compiler javac, regardless of the setting of $JAVASOURCEPATH, as it passes the path(s) to the source(s) supplied in the call to the Java builder via -sourcepath . From the documentation of the standard Java toolkit for javac: If not compiling code for modules, if the --source-path or -sourcepath option is not specified, then the user class path is also searched for source files. Since -sourcepath is always supplied, javac will not use the contents of the value of $JAVACLASSPATH when searching for sources.

JAVACLASSSUFFIX

The suffix for Java class files; .class by default.

JAVAH

The Java generator for C header and stub files.

JAVAHCOM

The command line used to generate C header and stub files from Java classes. Any options specified in the $JAVAHFLAGS construction variable are included on this command line.

JAVAHCOMSTR

The string displayed when C header and stub files are generated from Java classes. If this is not set, then $JAVAHCOM (the command line) is displayed.

env = Environment(JAVAHCOMSTR="Generating header/stub file(s) $TARGETS from $SOURCES")
JAVAHFLAGS

General options passed to the C header and stub file generator for Java classes.

JAVAINCLUDES

Include path for Java header files (such as jni.h).

JAVAPROCESSORPATH

Specifies the location of the annotation processor class files. Can be specified as a string or Node object, or as a list of strings or Node objects.

The value will be added to the JDK command lines via the -processorpath option, which requires a system-specific search path separator. This will be supplied by SCons as needed when it constructs the command line if $JAVAPROCESSORPATH is provided in list form. If $JAVAPROCESSORPATH is a single string containing search path separator characters (: for POSIX systems or ; for Windows), it will not be modified; and so is inherently system-specific; to supply the path in a system-independent manner, give $JAVAPROCESSORPATH as a list of paths instead.

New in version 4.5.0

JAVASOURCEPATH

Specifies the list of directories that will be searched for input (source) .java files. Can be specified as a string or Node object, or as a list of strings or Node objects.

The value will be added to the JDK command lines via the -sourcepath option, which requires a system-specific search path separator, This will be supplied by SCons as needed when it constructs the command line if $JAVASOURCEPATH is provided in list form. If $JAVASOURCEPATH is a single string containing search path separator characters (: for POSIX systems or ; for Windows), it will not be modified, and so is inherently system-specific; to supply the path in a system-independent manner, give $JAVASOURCEPATH as a list of paths instead.

Note that the specified directories are only added to the command line via the -sourcepath option. SCons does not currently search the $JAVASOURCEPATH directories for dependent .java files.

JAVASUFFIX

The suffix for Java files; .java by default.

JAVAVERSION

Specifies the Java version being used by the Java builder. Set this to specify the version of Java targeted by the javac compiler. This is sometimes necessary because Java 1.5 changed the file names that are created for nested anonymous inner classes, which can cause a mismatch with the files that SCons expects will be generated by the javac compiler. Setting $JAVAVERSION to a version greater than 1.4 makes SCons realize that a build with such a compiler is actually up to date. The default is 1.4.

While this is not primarily intended for selecting one version of the Java compiler vs. another, it does have that effect on the Windows platform. A more precise approach is to set $JAVAC (and related construction variables for related utilities) to the path to the specific Java compiler you want, if that is not the default compiler. On non-Windows platforms, the alternatives system may provide a way to adjust the default Java compiler without having to specify explicit paths.

LATEX

The LaTeX structured formatter and typesetter.

LATEXCOM

The command line used to call the LaTeX structured formatter and typesetter.

LATEXCOMSTR

The string displayed when calling the LaTeX structured formatter and typesetter. If this is not set, then $LATEXCOM (the command line) is displayed.

env = Environment(LATEXCOMSTR = "Building $TARGET from LaTeX input $SOURCES")
LATEXFLAGS

General options passed to the LaTeX structured formatter and typesetter.

LATEXRETRIES

The maximum number of times that LaTeX will be re-run if the .log generated by the $LATEXCOM command indicates that there are undefined references. The default is to try to resolve undefined references by re-running LaTeX up to three times.

LATEXSUFFIXES

The list of suffixes of files that will be scanned for LaTeX implicit dependencies (\include or \import files). The default list is:

[".tex", ".ltx", ".latex"]
LDMODULE

The linker for building loadable modules. By default, this is the same as $SHLINK.

LDMODULECOM

The command line for building loadable modules. On Mac OS X, this uses the $LDMODULE, $LDMODULEFLAGS and $FRAMEWORKSFLAGS variables. On other systems, this is the same as $SHLINK.

LDMODULECOMSTR

If set, the string displayed when building loadable modules. If not set, then $LDMODULECOM (the command line) is displayed.

LDMODULEEMITTER

Contains the emitter specification for the LoadableModule builder. The manpage section "Builder Objects" contains general information on specifying emitters.

LDMODULEFLAGS

General user options passed to the linker for building loadable modules.

LDMODULENOVERSIONSYMLINKS

Instructs the LoadableModule builder to not automatically create symlinks for versioned modules. Defaults to $SHLIBNOVERSIONSYMLINKS

LDMODULEPREFIX

The prefix used for loadable module file names. On Mac OS X, this is null; on other systems, this is the same as $SHLIBPREFIX.

_LDMODULESONAME

A macro that automatically generates loadable module's SONAME based on $TARGET, $LDMODULEVERSION and $LDMODULESUFFIX. Used by LoadableModule builder when the linker tool supports SONAME (e.g. gnulink).

LDMODULESUFFIX

The suffix used for loadable module file names. On Mac OS X, this is null; on other systems, this is the same as $SHLIBSUFFIX.

LDMODULEVERSION

When this construction variable is defined, a versioned loadable module is created by LoadableModule builder. This activates the $_LDMODULEVERSIONFLAGS and thus modifies the $LDMODULECOM as required, adds the version number to the library name, and creates the symlinks that are needed. $LDMODULEVERSION versions should exist in the same format as $SHLIBVERSION.

_LDMODULEVERSIONFLAGS

This macro automatically introduces extra flags to $LDMODULECOM when building versioned LoadableModule (that is when $LDMODULEVERSION is set). _LDMODULEVERSIONFLAGS usually adds $SHLIBVERSIONFLAGS and some extra dynamically generated options (such as -Wl,-soname=$_LDMODULESONAME). It is unused by plain (unversioned) loadable modules.

LDMODULEVERSIONFLAGS

Extra flags added to $LDMODULECOM when building versioned LoadableModule. These flags are only used when $LDMODULEVERSION is set.

LEX

The lexical analyzer generator.

LEX_HEADER_FILE

If supplied, generate a C header file with the name taken from this variable. Will be emitted as a --header-file= command-line option. Use this in preference to including --header-file= in $LEXFLAGS directly.

LEX_TABLES_FILE

If supplied, write the lex tables to a file with the name taken from this variable. Will be emitted as a --tables-file= command-line option. Use this in preference to including --tables-file= in $LEXFLAGS directly.

LEXCOM

The command line used to call the lexical analyzer generator to generate a source file.

LEXCOMSTR

The string displayed when generating a source file using the lexical analyzer generator. If this is not set, then $LEXCOM (the command line) is displayed.

env = Environment(LEXCOMSTR="Lex'ing $TARGET from $SOURCES")
LEXFLAGS

General options passed to the lexical analyzer generator. In addition to passing the value on during invocation, the lex tool also examines this construction variable for options which cause additional output files to be generated, and adds those to the target list. Recognized for this purpose are GNU flex options --header-file= and --tables-file=; the output file is named by the option argument.

Note that files specified by --header-file= and --tables-file= may not be properly handled by SCons in all situations. Consider using $LEX_HEADER_FILE and $LEX_TABLES_FILE instead.

LEXUNISTD

Used only on windows environments to set a lex flag to prevent 'unistd.h' from being included. The default value is '--nounistd'.

_LIBDIRFLAGS

An automatically-generated construction variable containing the linker command-line options for specifying directories to be searched for library. The value of $_LIBDIRFLAGS is created by respectively prepending and appending $LIBDIRPREFIX and $LIBDIRSUFFIX to each directory in $LIBPATH.

LIBDIRPREFIX

The prefix used to specify a library directory on the linker command line. This will be prepended to each directory in the $LIBPATH construction variable when the $_LIBDIRFLAGS variable is automatically generated.

LIBDIRSUFFIX

The suffix used to specify a library directory on the linker command line. This will be appended to each directory in the $LIBPATH construction variable when the $_LIBDIRFLAGS variable is automatically generated.

LIBEMITTER

Contains the emitter specification for the StaticLibrary builder. The manpage section "Builder Objects" contains general information on specifying emitters.

_LIBFLAGS

An automatically-generated construction variable containing the linker command-line options for specifying libraries to be linked with the resulting target. The value of $_LIBFLAGS is created by respectively prepending and appending $LIBLINKPREFIX and $LIBLINKSUFFIX to each filename in $LIBS.

LIBLINKPREFIX

The prefix used to specify a library to link on the linker command line. This will be prepended to each library in the $LIBS construction variable when the $_LIBFLAGS variable is automatically generated.

LIBLINKSUFFIX

The suffix used to specify a library to link on the linker command line. This will be appended to each library in the $LIBS construction variable when the $_LIBFLAGS variable is automatically generated.

LIBLITERALPREFIX

If the linker supports command line syntax directing that the argument specifying a library should be searched for literally (without modification), $LIBLITERALPREFIX can be set to that indicator. For example, the GNU linker follows this rule: -l:foo searches the library path for a filename called foo, without converting it to libfoo.so or libfoo.a. If $LIBLITERALPREFIX is set, SCons will not transform a string-valued entry in $LIBS that starts with that string. The entry will still be surrounded with $LIBLINKPREFIX and $LIBLINKSUFFIX on the command line. This is useful, for example, in directing that a static library be used when both a static and dynamic library are available and linker policy is to prefer dynamic libraries. Compared to the example in $LIBS,

env.Append(LIBS=":libmylib.a")

will let the linker select that specific (static) library name if found in the library search path. This differs from using a File object to specify the static library, as the latter bypasses the library search path entirely.

LIBPATH

The list of directories that will be searched for libraries specified by the $LIBS construction variable. $LIBPATH should be a list of path strings, or a single string, not a pathname list joined by Python's os.pathsep. Do not put library search directives directly into $LINKFLAGS or $SHLINKFLAGS as the result will be non-portable.

Note: directory names in $LIBPATH will be looked-up relative to the directory of the SConscript file when they are used in a command. To force scons to look-up a directory relative to the root of the source tree use the # prefix:

env = Environment(LIBPATH='#/libs')

The directory look-up can also be forced using the Dir function:

libs = Dir('libs')
env = Environment(LIBPATH=libs)

The directory list will be added to command lines through the automatically-generated $_LIBDIRFLAGS construction variable, which is constructed by respectively prepending and appending the values of the $LIBDIRPREFIX and $LIBDIRSUFFIX construction variables to each directory in $LIBPATH. Any command lines you define that need the $LIBPATH directory list should include $_LIBDIRFLAGS:

env = Environment(LINKCOM="my_linker $_LIBDIRFLAGS $_LIBFLAGS -o $TARGET $SOURCE")
LIBPREFIX

The prefix used for (static) library file names. A default value is set for each platform (posix, win32, os2, etc.), but the value is overridden by individual tools (ar, mslib, sgiar, sunar, tlib, etc.) to reflect the names of the libraries they create.

LIBPREFIXES

A list of all legal prefixes for library file names on the current platform. When searching for library dependencies, SCons will look for files with these prefixes, the base library name, and suffixes from the $LIBSUFFIXES list.

LIBS

The list of libraries that will be added to the link line for linking with any executable program, shared library, or loadable module created by the construction environment or override.

For portability, a string-valued library name should include only the base library name, without prefixes such as lib or suffixes such as .so or .dll. SCons will attempt to strip prefixes from the $LIBPREFIXES list and suffixes from the $LIBSUFFIXES list, but depending on that behavior will make the build less portable: for example, on a POSIX system, no attempt will be made to strip a suffix like .dll. Library name strings in $LIBS should not include a path component: instead use $LIBPATH to direct the compiler to look for libraries in those paths, plus any default paths the linker searches in. If $LIBLITERALPREFIX is set to a non-empty string, then a string-valued $LIBS entry that starts with $LIBLITERALPREFIX will cause the rest of the entry to be searched for for unmodified, but respecting normal library search paths (this is an exception to the guideline above about leaving off the prefix/suffix from the library name).

If a $LIBS entry is a Node object (either as returned by a previous Builder call, or as the result of an explicit call to File), the pathname from that Node will be added to $_LIBFLAGS, and thus to the link line, unmodified - without adding $LIBLINKPREFIX or $LIBLINKSUFFIX. Such entries are searched for literally (including any path component); the library search paths are not used. For example:

env.Append(LIBS=File('/tmp/mylib.so'))

For each Builder call that causes linking with libraries, SCons will add the libraries in the setting of $LIBS in effect at that moment to the dependecy graph as dependencies of the target being generated.

The library list will transformed to command line arguments through the automatically-generated $_LIBFLAGS construction variable which is constructed by respectively prepending and appending the values of the $LIBLINKPREFIX and $LIBLINKSUFFIX construction variables to each library name.

Any command lines you define yourself that need the libraries from $LIBS should include $_LIBFLAGS (as well as $_LIBDIRFLAGS) rather than $LIBS. For example:

env = Environment(LINKCOM="my_linker $_LIBDIRFLAGS $_LIBFLAGS -o $TARGET $SOURCE")
LIBSUFFIX

The suffix used for (static) library file names. A default value is set for each platform (posix, win32, os2, etc.), but the value is overridden by individual tools (ar, mslib, sgiar, sunar, tlib, etc.) to reflect the names of the libraries they create.

LIBSUFFIXES

A list of all legal suffixes for library file names. on the current platform. When searching for library dependencies, SCons will look for files with prefixes from the $LIBPREFIXES list, the base library name, and these suffixes.

LICENSE

The abbreviated name, preferably the SPDX code, of the license under which this project is released (GPL-3.0, LGPL-2.1, BSD-2-Clause etc.). See http://www.opensource.org/licenses/alphabetical for a list of license names and SPDX codes.

See the Package builder.

LINESEPARATOR

The separator used by the Substfile and Textfile builders. This value is used between sources when constructing the target. It defaults to the current system line separator.

LINGUAS_FILE

The $LINGUAS_FILE defines file(s) containing list of additional linguas to be processed by POInit, POUpdate or MOFiles builders. It also affects Translate builder. If the variable contains a string, it defines name of the list file. The $LINGUAS_FILE may be a list of file names as well. If $LINGUAS_FILE is set to True (or non-zero numeric value), the list will be read from default file named LINGUAS.

LINK

The linker. See also $SHLINK for linking shared objects.

On POSIX systems (those using the link tool), you should normally not change this value as it defaults to a "smart" linker tool which selects a compiler driver matching the type of source files in use. So for example, if you set $CXX to a specific compiler name, and are compiling C++ sources, the smartlink function will automatically select the same compiler for linking.

LINKCOM

The command line used to link object files into an executable. See also $SHLINKCOM for linking shared objects.

LINKCOMSTR

If set, the string displayed when object files are linked into an executable. If not set, then $LINKCOM (the command line) is displayed. See also $SHLINKCOMSTR. for linking shared objects.

env = Environment(LINKCOMSTR = "Linking $TARGET")
LINKFLAGS

General user options passed to the linker. Note that this variable should not contain -l (or similar) options for linking with the libraries listed in $LIBS, nor -L (or similar) library search path options that scons generates automatically from $LIBPATH. See $_LIBFLAGS above, for the variable that expands to library-link options, and $_LIBDIRFLAGS above, for the variable that expands to library search path options. See also $SHLINKFLAGS. for linking shared objects.

M4

The M4 macro preprocessor.

M4COM

The command line used to pass files through the M4 macro preprocessor.

M4COMSTR

The string displayed when a file is passed through the M4 macro preprocessor. If this is not set, then $M4COM (the command line) is displayed.

M4FLAGS

General options passed to the M4 macro preprocessor.

MAKEINDEX

The makeindex generator for the TeX formatter and typesetter and the LaTeX structured formatter and typesetter.

MAKEINDEXCOM

The command line used to call the makeindex generator for the TeX formatter and typesetter and the LaTeX structured formatter and typesetter.

MAKEINDEXCOMSTR

The string displayed when calling the makeindex generator for the TeX formatter and typesetter and the LaTeX structured formatter and typesetter. If this is not set, then $MAKEINDEXCOM (the command line) is displayed.

MAKEINDEXFLAGS

General options passed to the makeindex generator for the TeX formatter and typesetter and the LaTeX structured formatter and typesetter.

MAXLINELENGTH

The maximum number of characters allowed on an external command line. On Win32 systems, link lines longer than this many characters are linked via a temporary file name.

MIDL

The Microsoft IDL compiler.

MIDLCOM

The command line used to pass files to the Microsoft IDL compiler.

MIDLCOMSTR

The string displayed when the Microsoft IDL compiler is called. If this is not set, then $MIDLCOM (the command line) is displayed.

MIDLFLAGS

General options passed to the Microsoft IDL compiler.

MOSUFFIX

Suffix used for MO files (default: '.mo'). See msgfmt tool and MOFiles builder.

MSGFMT

Absolute path to msgfmt(1) binary, found by Detect(). See msgfmt tool and MOFiles builder.

MSGFMTCOM

Complete command line to run msgfmt(1) program. See msgfmt tool and MOFiles builder.

MSGFMTCOMSTR

String to display when msgfmt(1) is invoked (default: '', which means ``print $MSGFMTCOM''). See msgfmt tool and MOFiles builder.

MSGFMTFLAGS

Additional flags to msgfmt(1). See msgfmt tool and MOFiles builder.

MSGINIT

Path to msginit(1) program (found via Detect()). See msginit tool and POInit builder.

MSGINITCOM

Complete command line to run msginit(1) program. See msginit tool and POInit builder.

MSGINITCOMSTR

String to display when msginit(1) is invoked (default: '', which means ``print $MSGINITCOM''). See msginit tool and POInit builder.

MSGINITFLAGS

List of additional flags to msginit(1) (default: []). See msginit tool and POInit builder.

_MSGINITLOCALE

Internal ``macro''. Computes locale (language) name based on target filename (default: '${TARGET.filebase}' ).

See msginit tool and POInit builder.

MSGMERGE

Absolute path to msgmerge(1) binary as found by Detect(). See msgmerge tool and POUpdate builder.

MSGMERGECOM

Complete command line to run msgmerge(1) command. See msgmerge tool and POUpdate builder.

MSGMERGECOMSTR

String to be displayed when msgmerge(1) is invoked (default: '', which means ``print $MSGMERGECOM''). See msgmerge tool and POUpdate builder.

MSGMERGEFLAGS

Additional flags to msgmerge(1) command. See msgmerge tool and POUpdate builder.

MSSDK_DIR

The directory containing the Microsoft SDK (either Platform SDK or Windows SDK) to be used for compilation.

MSSDK_VERSION

The version string of the Microsoft SDK (either Platform SDK or Windows SDK) to be used for compilation. Supported versions include 6.1, 6.0A, 6.0, 2003R2 and 2003R1.

MSVC_BATCH

When set to any true value, specifies that SCons should batch compilation of object files when calling the Microsoft Visual C++ compiler. All compilations of source files from the same source directory that generate target files in a same output directory and were configured in SCons using the same construction environment will be built in a single call to the compiler. Only source files that have changed since their object files were built will be passed to each compiler invocation (via the $CHANGED_SOURCES construction variable). Any compilations where the object (target) file base name (minus the .obj) does not match the source file base name will be compiled separately.

MSVC_NOTFOUND_POLICY

Specify the scons behavior when the Microsoft Visual C++ compiler is not detected.

The $MSVC_NOTFOUND_POLICY specifies the scons behavior when no msvc versions are detected or when the requested msvc version is not detected.

The valid values for $MSVC_NOTFOUND_POLICY and the corresponding scons behavior are:

'Error' or 'Exception'

Raise an exception when no msvc versions are detected or when the requested msvc version is not detected.

'Warning' or 'Warn'

Issue a warning and continue when no msvc versions are detected or when the requested msvc version is not detected. Depending on usage, this could result in build failure(s).

'Ignore' or 'Suppress'

Take no action and continue when no msvc versions are detected or when the requested msvc version is not detected. Depending on usage, this could result in build failure(s).

Note: in addition to the camel case values shown above, lower case and upper case values are accepted as well.

The $MSVC_NOTFOUND_POLICY is applied when any of the following conditions are satisfied:

  • $MSVC_VERSION is specified, the default tools list is implicitly defined (i.e., the tools list is not specified), and the default tools list contains one or more of the msvc tools.

  • $MSVC_VERSION is specified, the default tools list is explicitly specified (e.g., tools=['default']), and the default tools list contains one or more of the msvc tools.

  • A non-default tools list is specified that contains one or more of the msvc tools (e.g., tools=['msvc', 'mslink']).

The $MSVC_NOTFOUND_POLICY is ignored when any of the following conditions are satisfied:

  • $MSVC_VERSION is not specified and the default tools list is implicitly defined (i.e., the tools list is not specified).

  • $MSVC_VERSION is not specified and the default tools list is explicitly specified (e.g., tools=['default']).

  • A non-default tool list is specified that does not contain any of the msvc tools (e.g., tools=['mingw']).

Important usage details:

When $MSVC_NOTFOUND_POLICY is not specified, the default scons behavior is to issue a warning and continue subject to the conditions listed above. The default scons behavior may change in the future.

New in version 4.4

MSVC_SCRIPT_ARGS

Pass user-defined arguments to the Microsoft Visual C++ batch file determined via autodetection.

$MSVC_SCRIPT_ARGS is available for msvc batch file arguments that do not have first-class support via construction variables or when there is an issue with the appropriate construction variable validation. When available, it is recommended to use the appropriate construction variables (e.g., $MSVC_TOOLSET_VERSION) rather than $MSVC_SCRIPT_ARGS arguments.

The valid values for $MSVC_SCRIPT_ARGS are: None, a string, or a list of strings.

The $MSVC_SCRIPT_ARGS value is converted to a scalar string (i.e., "flattened"). The resulting scalar string, if not empty, is passed as an argument to the msvc batch file determined via autodetection subject to the validation conditions listed below.

$MSVC_SCRIPT_ARGS is ignored when the value is None and when the result from argument conversion is an empty string. The validation conditions below do not apply.

An exception is raised when any of the following conditions are satisfied:

  • $MSVC_SCRIPT_ARGS is specified for Visual Studio 2013 and earlier.

  • Multiple SDK version arguments (e.g., '10.0.20348.0') are specified in $MSVC_SCRIPT_ARGS.

  • $MSVC_SDK_VERSION is specified and an SDK version argument (e.g., '10.0.20348.0') is specified in $MSVC_SCRIPT_ARGS. Multiple SDK version declarations via $MSVC_SDK_VERSION and $MSVC_SCRIPT_ARGS are not allowed.

  • Multiple toolset version arguments (e.g., '-vcvars_ver=14.29') are specified in $MSVC_SCRIPT_ARGS.

  • $MSVC_TOOLSET_VERSION is specified and a toolset version argument (e.g., '-vcvars_ver=14.29') is specified in $MSVC_SCRIPT_ARGS. Multiple toolset version declarations via $MSVC_TOOLSET_VERSION and $MSVC_SCRIPT_ARGS are not allowed.

  • Multiple spectre library arguments (e.g., '-vcvars_spectre_libs=spectre') are specified in $MSVC_SCRIPT_ARGS.

  • $MSVC_SPECTRE_LIBS is enabled and a spectre library argument (e.g., '-vcvars_spectre_libs=spectre') is specified in $MSVC_SCRIPT_ARGS. Multiple spectre library declarations via $MSVC_SPECTRE_LIBS and $MSVC_SCRIPT_ARGS are not allowed.

  • Multiple UWP arguments (e.g., uwp or store) are specified in $MSVC_SCRIPT_ARGS.

  • $MSVC_UWP_APP is enabled and a UWP argument (e.g., uwp or store) is specified in $MSVC_SCRIPT_ARGS. Multiple UWP declarations via $MSVC_UWP_APP and $MSVC_SCRIPT_ARGS are not allowed.

Example 1 - A Visual Studio 2022 build with an SDK version and a toolset version specified with a string argument:

env = Environment(MSVC_VERSION='14.3', MSVC_SCRIPT_ARGS='10.0.20348.0 -vcvars_ver=14.29.30133')

Example 2 - A Visual Studio 2022 build with an SDK version and a toolset version specified with a list argument:

env = Environment(MSVC_VERSION='14.3', MSVC_SCRIPT_ARGS=['10.0.20348.0', '-vcvars_ver=14.29.30133'])

Important usage details:

  • $MSVC_SCRIPT_ARGS must be passed as an argument to the Environment constructor when an msvc tool (e.g., msvc, msvs, etc.) is loaded via the default tools list or via a tools list passed to the Environment constructor. Otherwise, $MSVC_SCRIPT_ARGS must be set before the first msvc tool is loaded into the environment.

  • Other than checking for multiple declarations as described above, $MSVC_SCRIPT_ARGS arguments are not validated.

  • Erroneous, inconsistent, and/or version incompatible $MSVC_SCRIPT_ARGS arguments are likely to result in build failures for reasons that are not readily apparent and may be difficult to diagnose. The burden is on the user to ensure that the arguments provided to the msvc batch file are valid, consistent and compatible with the version of msvc selected.

New in version 4.4

MSVC_SCRIPTERROR_POLICY

Specify the scons behavior when Microsoft Visual C++ batch file errors are detected.

The $MSVC_SCRIPTERROR_POLICY specifies the scons behavior when msvc batch file errors are detected. When $MSVC_SCRIPTERROR_POLICY is not specified, the default scons behavior is to suppress msvc batch file error messages.

The root cause of msvc build failures may be difficult to diagnose. In these situations, setting the scons behavior to issue a warning when msvc batch file errors are detected may produce additional diagnostic information.

The valid values for $MSVC_SCRIPTERROR_POLICY and the corresponding scons behavior are:

'Error' or 'Exception'

Raise an exception when msvc batch file errors are detected.

'Warning' or 'Warn'

Issue a warning when msvc batch file errors are detected.

'Ignore' or 'Suppress'

Suppress msvc batch file error messages.

New in version 4.4

Note: in addition to the camel case values shown above, lower case and upper case values are accepted as well.

Example 1 - A Visual Studio 2022 build with user-defined script arguments:

env = environment(MSVC_VERSION='14.3', MSVC_SCRIPT_ARGS=['8.1', 'store', '-vcvars_ver=14.1'])
env.Program('hello', ['hello.c'], CCFLAGS='/MD', LIBS=['kernel32', 'user32', 'runtimeobject'])

Example 1 - Output fragment:

...
link /nologo /OUT:_build001\hello.exe kernel32.lib user32.lib runtimeobject.lib _build001\hello.obj
LINK : fatal error LNK1104: cannot open file 'MSVCRT.lib'
...

Example 2 - A Visual Studio 2022 build with user-defined script arguments and the script error policy set to issue a warning when msvc batch file errors are detected:

env = environment(MSVC_VERSION='14.3', MSVC_SCRIPT_ARGS=['8.1', 'store', '-vcvars_ver=14.1'], MSVC_SCRIPTERROR_POLICY='warn')
env.Program('hello', ['hello.c'], CCFLAGS='/MD', LIBS=['kernel32', 'user32', 'runtimeobject'])

Example 2 - Output fragment:

...
scons: warning: vc script errors detected:
[ERROR:vcvars.bat] The UWP Application Platform requires a Windows 10 SDK.
[ERROR:vcvars.bat] WindowsSdkDir = "C:\Program Files (x86)\Windows Kits\8.1\"
[ERROR:vcvars.bat] host/target architecture is not supported : { x64 , x64 }
...
link /nologo /OUT:_build001\hello.exe kernel32.lib user32.lib runtimeobject.lib _build001\hello.obj
LINK : fatal error LNK1104: cannot open file 'MSVCRT.lib'

Important usage details:

  • $MSVC_SCRIPTERROR_POLICY must be passed as an argument to the Environment constructor when an msvc tool (e.g., msvc, msvs, etc.) is loaded via the default tools list or via a tools list passed to the Environment constructor. Otherwise, $MSVC_SCRIPTERROR_POLICY must be set before the first msvc tool is loaded into the environment.

  • Due to scons implementation details, not all Windows system environment variables are propagated to the environment in which the msvc batch file is executed. Depending on Visual Studio version and installation options, non-fatal msvc batch file error messages may be generated for ancillary tools which may not affect builds with the msvc compiler. For this reason, caution is recommended when setting the script error policy to raise an exception (e.g., 'Error').

New in version 4.4

MSVC_SDK_VERSION

Build with a specific version of the Microsoft Software Development Kit (SDK).

The valid values for $MSVC_SDK_VERSION are: None or a string containing the requested SDK version (e.g., '10.0.20348.0').

$MSVC_SDK_VERSION is ignored when the value is None and when the value is an empty string. The validation conditions below do not apply.

An exception is raised when any of the following conditions are satisfied:

Example 1 - A Visual Studio 2022 build with a specific Windows SDK version:

env = Environment(MSVC_VERSION='14.3', MSVC_SDK_VERSION='10.0.20348.0')

Example 2 - A Visual Studio 2022 build with a specific SDK version for the Universal Windows Platform:

env = Environment(MSVC_VERSION='14.3', MSVC_SDK_VERSION='10.0.20348.0', MSVC_UWP_APP=True)

Important usage details:

  • $MSVC_SDK_VERSION must be passed as an argument to the Environment constructor when an msvc tool (e.g., msvc, msvs, etc.) is loaded via the default tools list or via a tools list passed to the Environment constructor. Otherwise, $MSVC_SDK_VERSION must be set before the first msvc tool is loaded into the environment.

  • Should a SDK 10.0 version be installed that does not follow the naming scheme above, the SDK version will need to be specified via $MSVC_SCRIPT_ARGS until the version number validation format can be extended.

  • Should an exception be raised indicating that the SDK version is not found, verify that the requested SDK version is installed with the necessary platform type components.

  • There is a known issue with the Microsoft libraries when the target architecture is ARM64 and a Windows 11 SDK (version '10.0.22000.0' and later) is used with the v141 build tools and older v142 toolsets (versions '14.28.29333' and earlier). Should build failures arise with these combinations of settings due to unresolved symbols in the Microsoft libraries, $MSVC_SDK_VERSION may be employed to specify a Windows 10 SDK (e.g., '10.0.20348.0') for the build.

New in version 4.4

MSVC_SPECTRE_LIBS

Build with the spectre-mitigated Microsoft Visual C++ libraries.

The valid values for $MSVC_SPECTRE_LIBS are: True, False, or None.

When $MSVC_SPECTRE_LIBS is enabled (i.e., True), the Microsoft Visual C++ environment will include the paths to the spectre-mitigated implementations of the Microsoft Visual C++ libraries.

An exception is raised when any of the following conditions are satisfied:

Example - A Visual Studio 2022 build with spectre mitigated Microsoft Visual C++ libraries:

env = Environment(MSVC_VERSION='14.3', MSVC_SPECTRE_LIBS=True)

Important usage details:

  • $MSVC_SPECTRE_LIBS must be passed as an argument to the Environment constructor when an msvc tool (e.g., msvc, msvs, etc.) is loaded via the default tools list or via a tools list passed to the Environment constructor. Otherwise, $MSVC_SPECTRE_LIBS must be set before the first msvc tool is loaded into the environment.

  • Additional compiler switches (e.g., /Qspectre) are necessary for including spectre mitigations when building user artifacts. Refer to the Visual Studio documentation for details.

  • The existence of the spectre libraries host architecture and target architecture folders are not verified when $MSVC_SPECTRE_LIBS is enabled which could result in build failures. The burden is on the user to ensure the requisite libraries with spectre mitigations are installed.

New in version 4.4

MSVC_TOOLSET_VERSION

Build with a specific Microsoft Visual C++ toolset version.

Specifying $MSVC_TOOLSET_VERSION does not affect the autodetection and selection of msvc instances. The $MSVC_TOOLSET_VERSION is applied after an msvc instance is selected. This could be the default version of msvc if $MSVC_VERSION is not specified.

The valid values for $MSVC_TOOLSET_VERSION are: None or a string containing the requested toolset version (e.g., '14.29').

$MSVC_TOOLSET_VERSION is ignored when the value is None and when the value is an empty string. The validation conditions below do not apply.

An exception is raised when any of the following conditions are satisfied:

  • $MSVC_TOOLSET_VERSION is specified for Visual Studio 2015 and earlier.

  • $MSVC_TOOLSET_VERSION is specified and a toolset version argument is specified in $MSVC_SCRIPT_ARGS. Multiple toolset version declarations via $MSVC_TOOLSET_VERSION and $MSVC_SCRIPT_ARGS are not allowed.

  • The $MSVC_TOOLSET_VERSION specified does not match any of the supported formats:

    • 'XX.Y'

    • 'XX.YY'

    • 'XX.YY.ZZZZZ'

    • 'XX.YY.Z' to 'XX.YY.ZZZZ' [scons extension not directly supported by the msvc batch files and may be removed in the future]

    • 'XX.YY.ZZ.N' [SxS format]

    • 'XX.YY.ZZ.NN' [SxS format]

  • The major msvc version prefix (i.e., 'XX.Y') of the $MSVC_TOOLSET_VERSION specified is for Visual Studio 2013 and earlier (e.g., '12.0').

  • The major msvc version prefix (i.e., 'XX.Y') of the $MSVC_TOOLSET_VERSION specified is greater than the msvc version selected (e.g., '99.0').

  • A system folder for the corresponding $MSVC_TOOLSET_VERSION version is not found. The requested toolset version does not appear to be installed.

Toolset selection details:

  • When $MSVC_TOOLSET_VERSION is not an SxS version number or a full toolset version number: the first toolset version, ranked in descending order, that matches the $MSVC_TOOLSET_VERSION prefix is selected.

  • When $MSVC_TOOLSET_VERSION is specified using the major msvc version prefix (i.e., 'XX.Y') and the major msvc version is that of the latest release of Visual Studio, the selected toolset version may not be the same as the default Microsoft Visual C++ toolset version.

    In the latest release of Visual Studio, the default Microsoft Visual C++ toolset version is not necessarily the toolset with the largest version number.

Example 1 - A default Visual Studio build with a partial toolset version specified:

env = Environment(MSVC_TOOLSET_VERSION='14.2')

Example 2 - A default Visual Studio build with a partial toolset version specified:

env = Environment(MSVC_TOOLSET_VERSION='14.29')

Example 3 - A Visual Studio 2022 build with a full toolset version specified:

env = Environment(MSVC_VERSION='14.3', MSVC_TOOLSET_VERSION='14.29.30133')

Example 4 - A Visual Studio 2022 build with an SxS toolset version specified:

env = Environment(MSVC_VERSION='14.3', MSVC_TOOLSET_VERSION='14.29.16.11')

Important usage details:

  • $MSVC_TOOLSET_VERSION must be passed as an argument to the Environment constructor when an msvc tool (e.g., msvc, msvs, etc.) is loaded via the default tools list or via a tools list passed to the Environment constructor. Otherwise, $MSVC_TOOLSET_VERSION must be set before the first msvc tool is loaded into the environment.

  • The existence of the toolset host architecture and target architecture folders are not verified when $MSVC_TOOLSET_VERSION is specified which could result in build failures. The burden is on the user to ensure the requisite toolset target architecture build tools are installed.

New in version 4.4

MSVC_USE_SCRIPT

Use a batch script to set up the Microsoft Visual C++ compiler.

If set to the name of a Visual Studio .bat file (e.g. vcvars.bat), SCons will run that batch file instead of the auto-detected one, and extract the relevant variables from the result (typically %INCLUDE%, %LIB%, and %PATH%) for supplying to the build. This can be useful to force the use of a compiler version that SCons does not detect. $MSVC_USE_SCRIPT_ARGS provides arguments passed to this script.

Setting $MSVC_USE_SCRIPT to None bypasses the Visual Studio autodetection entirely; use this if you are running SCons in a Visual Studio cmd window and importing the shell's environment variables - that is, if you are sure everything is set correctly already and you don't want SCons to change anything.

$MSVC_USE_SCRIPT ignores $MSVC_VERSION and $TARGET_ARCH.

Changed in version 4.4: new $MSVC_USE_SCRIPT_ARGS provides a way to pass arguments.

MSVC_USE_SCRIPT_ARGS

Provides arguments passed to the script $MSVC_USE_SCRIPT.

New in version 4.4

MSVC_USE_SETTINGS

Use a dictionary to set up the Microsoft Visual C++ compiler.

$MSVC_USE_SETTINGS is ignored when $MSVC_USE_SCRIPT is defined and/or when $MSVC_USE_SETTINGS is set to None.

The dictionary is used to populate the environment with the relevant variables (typically %INCLUDE%, %LIB%, and %PATH%) for supplying to the build. This can be useful to force the use of a compiler environment that SCons does not configure correctly. This is an alternative to manually configuring the environment when bypassing Visual Studio autodetection entirely by setting $MSVC_USE_SCRIPT to None.

Here is an example of configuring a build environment using the Microsoft Visual C++ compiler included in the Microsoft SDK on a 64-bit host and building for a 64-bit architecture:

# Microsoft SDK 6.0 (MSVC 8.0): 64-bit host and 64-bit target
msvc_use_settings = {
    "PATH": [
        "C:\\Program Files\\Microsoft SDKs\\Windows\\v6.0\\VC\\Bin\\x64",
        "C:\\Program Files\\Microsoft SDKs\\Windows\\v6.0\\Bin\\x64",
        "C:\\Program Files\\Microsoft SDKs\\Windows\\v6.0\\Bin",
        "C:\\Windows\\Microsoft.NET\\Framework\\v2.0.50727",
        "C:\\Windows\\system32",
        "C:\\Windows",
        "C:\\Windows\\System32\\Wbem",
        "C:\\Windows\\System32\\WindowsPowerShell\\v1.0\\"
    ],
    "INCLUDE": [
        "C:\\Program Files\\Microsoft SDKs\\Windows\\v6.0\\VC\\Include",
        "C:\\Program Files\\Microsoft SDKs\\Windows\\v6.0\\VC\\Include\\Sys",
        "C:\\Program Files\\Microsoft SDKs\\Windows\\v6.0\\Include",
        "C:\\Program Files\\Microsoft SDKs\\Windows\\v6.0\\Include\\gl",
    ],
    "LIB": [
        "C:\\Program Files\\Microsoft SDKs\\Windows\\v6.0\\VC\\Lib\\x64",
        "C:\\Program Files\\Microsoft SDKs\\Windows\\v6.0\\Lib\\x64",
    ],
    "LIBPATH": [],
    "VSCMD_ARG_app_plat": [],
    "VCINSTALLDIR": [],
    "VCToolsInstallDir": []
}

# Specifying MSVC_VERSION is recommended
env = Environment(MSVC_VERSION='8.0', MSVC_USE_SETTINGS=msvc_use_settings)

Important usage details:

  • $MSVC_USE_SETTINGS must be passed as an argument to the Environment constructor when an msvc tool (e.g., msvc, msvs, etc.) is loaded via the default tools list or via a tools list passed to the Environment constructor. Otherwise, $MSVC_USE_SETTINGS must be set before the first msvc tool is loaded into the environment.

  • The dictionary content requirements are based on the internal msvc implementation and therefore may change at any time. The burden is on the user to ensure the dictionary contents are minimally sufficient to ensure successful builds.

New in version 4.4

MSVC_UWP_APP

Build with the Universal Windows Platform (UWP) application Microsoft Visual C++ libraries.

The valid values for $MSVC_UWP_APP are: True, '1', False, '0', or None.

When $MSVC_UWP_APP is enabled (i.e., True or '1'), the Microsoft Visual C++ environment will be set up to point to the Windows Store compatible libraries and Microsoft Visual C++ runtimes. In doing so, any libraries that are built will be able to be used in a UWP App and published to the Windows Store.

An exception is raised when any of the following conditions are satisfied:

Example - A Visual Studio 2022 build for the Universal Windows Platform:

env = Environment(MSVC_VERSION='14.3', MSVC_UWP_APP=True)

Important usage details:

  • $MSVC_UWP_APP must be passed as an argument to the Environment constructor when an msvc tool (e.g., msvc, msvs, etc.) is loaded via the default tools list or via a tools list passed to the Environment constructor. Otherwise, $MSVC_UWP_APP must be set before the first msvc tool is loaded into the environment.

  • The existence of the UWP libraries is not verified when $MSVC_UWP_APP is enabled which could result in build failures. The burden is on the user to ensure the requisite UWP libraries are installed.

MSVC_VERSION

A string to select the preferred version of Microsoft Visual C++. If the specified version is unavailable and/or unknown to SCons, a warning is issued showing the versions actually discovered, and the build will eventually fail indicating a missing compiler binary. If $MSVC_VERSION is not set, SCons will (by default) select the latest version of Microsoft Visual C++ installed on your system. The valid values for $MSVC_VERSION represent major versions of the compiler, except that versions ending in Exp refer to "Express" or "Express for Desktop" Visual Studio editions, which require distinct entries because they use a different filesystem layout and have feature limitations compared to the full version. Values that do not look like a valid compiler version string are not supported.

To have the desired effect, $MSVC_VERSION must be set by the time compiler discovery takes place. If the default tools list or an explicit tools list including msvc is used, discovery takes place as the construction environment is created, so passing it as an argument in the the Environment call is the effective solution. Otherwise, $MSVC_VERSION must be set before the first msvc tool is loaded into the environment. See the manpage section "Construction Environments" for an example.

The following table shows the correspondence of $MSVC_VERSION values to various version indicators ('x' is used as a placeholder for a single digit that can vary). Note that it is not necessary to install Visual Studio to build with SCons (for example, you can install only Build Tools), but if Visual Studio is installed, additional builders such as MSVSSolution and MSVSProject become available and will correspond to the indicated versions.

SCons Key


Visual C++
Version 

_MSVC_VER Visual Studio Product


MSBuild /
Visual Studio 

"14.3" 14.3x 193x Visual Studio 2022 17.x, 17.1x
"14.2" 14.2x 192x Visual Studio 2019 16.x, 16.1x
"14.1" 14.1 or 14.1x 191x Visual Studio 2017 15.x
"14.1Exp" 14.1 1910 Visual Studio 2017 Express 15.0
"14.0" 14.0 1900 Visual Studio 2015 14.0
"14.0Exp" 14.0 1900 Visual Studio 2015 Express 14.0
"12.0" 12.0 1800 Visual Studio 2013 12.0
"12.0Exp" 12.0 1800 Visual Studio 2013 Express 12.0
"11.0" 11.0 1700 Visual Studio 2012 11.0
"11.0Exp" 11.0 1700 Visual Studio 2012 Express 11.0
"10.0" 10.0 1600 Visual Studio 2010 10.0
"10.0Exp" 10.0 1600 Visual C++ Express 2010 10.0
"9.0" 9.0 1500 Visual Studio 2008 9.0
"9.0Exp" 9.0 1500 Visual C++ Express 2008 9.0
"8.0" 8.0 1400 Visual Studio 2005 8.0
"8.0Exp" 8.0 1400 Visual C++ Express 2005 8.0
"7.1" 7.1 1300 Visual Studio .NET 2003 7.1
"7.0" 7.0 1200 Visual Studio .NET 2002 7.0
"6.0" 6.0 1100 Visual Studio 6.0 6.0

The compilation environment can be further or more precisely specified through the use of several other construction variables: see the descriptions of $MSVC_TOOLSET_VERSION, $MSVC_SDK_VERSION, $MSVC_USE_SCRIPT, $MSVC_USE_SCRIPT_ARGS, and $MSVC_USE_SETTINGS.

MSVS

When the Microsoft Visual Studio tools are initialized, they set up this dictionary with the following keys:

VERSION

the version of MSVS being used (can be set via $MSVC_VERSION)

VERSIONS

the available versions of MSVS installed

VCINSTALLDIR

installed directory of Microsoft Visual C++

VSINSTALLDIR

installed directory of Visual Studio

FRAMEWORKDIR

installed directory of the .NET framework

FRAMEWORKVERSIONS

list of installed versions of the .NET framework, sorted latest to oldest.

FRAMEWORKVERSION

latest installed version of the .NET framework

FRAMEWORKSDKDIR

installed location of the .NET SDK.

PLATFORMSDKDIR

installed location of the Platform SDK.

PLATFORMSDK_MODULES

dictionary of installed Platform SDK modules, where the dictionary keys are keywords for the various modules, and the values are 2-tuples where the first is the release date, and the second is the version number.

If a value is not set, it was not available in the registry. Visual Studio 2017 and later do not use the registry for primary storage of this information, so typically for these versions only PROJECTSUFFIX and SOLUTIONSUFFIX will be set.

MSVS_ARCH

Sets the architecture for which the generated project(s) should build.

The default value is x86. amd64 is also supported by SCons for most Visual Studio versions. Since Visual Studio 2015 arm is supported, and since Visual Studio 2017 arm64 is supported. Trying to set $MSVS_ARCH to an architecture that's not supported for a given Visual Studio version will generate an error.

MSVS_PROJECT_GUID

The string placed in a generated Microsoft Visual C++ project file as the value of the ProjectGUID attribute. There is no default value. If not defined, a new GUID is generated.

MSVS_SCC_AUX_PATH

The path name placed in a generated Microsoft Visual C++ project file as the value of the SccAuxPath attribute if the MSVS_SCC_PROVIDER construction variable is also set. There is no default value.

MSVS_SCC_CONNECTION_ROOT

The root path of projects in your SCC workspace, i.e the path under which all project and solution files will be generated. It is used as a reference path from which the relative paths of the generated Microsoft Visual C++ project and solution files are computed. The relative project file path is placed as the value of the SccLocalPath attribute of the project file and as the values of the SccProjectFilePathRelativizedFromConnection[i] (where [i] ranges from 0 to the number of projects in the solution) attributes of the GlobalSection(SourceCodeControl) section of the Microsoft Visual Studio solution file. Similarly the relative solution file path is placed as the values of the SccLocalPath[i] (where [i] ranges from 0 to the number of projects in the solution) attributes of the GlobalSection(SourceCodeControl) section of the Microsoft Visual Studio solution file. This is used only if the MSVS_SCC_PROVIDER construction variable is also set. The default value is the current working directory.

MSVS_SCC_PROJECT_NAME

The project name placed in a generated Microsoft Visual C++ project file as the value of the SccProjectName attribute if the MSVS_SCC_PROVIDER construction variable is also set. In this case the string is also placed in the SccProjectName0 attribute of the GlobalSection(SourceCodeControl) section of the Microsoft Visual Studio solution file. There is no default value.

MSVS_SCC_PROVIDER

The string placed in a generated Microsoft Visual C++ project file as the value of the SccProvider attribute. The string is also placed in the SccProvider0 attribute of the GlobalSection(SourceCodeControl) section of the Microsoft Visual Studio solution file. There is no default value.

MSVS_VERSION

Set the preferred version of Microsoft Visual Studio to use.

If $MSVS_VERSION is not set, SCons will (by default) select the latest version of Visual Studio installed on your system. So, if you have version 6 and version 7 (MSVS .NET) installed, it will prefer version 7. You can override this by specifying the $MSVS_VERSION variable when initializing the Environment, setting it to the appropriate version ('6.0' or '7.0', for example). If the specified version isn't installed, tool initialization will fail.

Deprecated since 1.3.0: $MSVS_VERSION is deprecated in favor of $MSVC_VERSION. As a transitional aid, if $MSVS_VERSION is set and $MSVC_VERSION is not, $MSVC_VERSION will be initialized to the value of $MSVS_VERSION. An error is raised if If both are set and have different values,

MSVSBUILDCOM

The build command line placed in a generated Microsoft Visual C++ project file. The default is to have Visual Studio invoke SCons with any specified build targets.

MSVSCLEANCOM

The clean command line placed in a generated Microsoft Visual C++ project file. The default is to have Visual Studio invoke SCons with the -c option to remove any specified targets.

MSVSENCODING

The encoding string placed in a generated Microsoft Visual C++ project file. The default is encoding Windows-1252.

MSVSPROJECTCOM

The action used to generate Microsoft Visual C++ project files.

MSVSPROJECTSUFFIX

The suffix used for Microsoft Visual C++ project (DSP) files. The default value is .vcxproj when using Visual Studio 2010 and later, .vcproj when using Visual Studio versions between 2002 and 2008, and .dsp when using Visual Studio 6.0.

MSVSREBUILDCOM

The rebuild command line placed in a generated Microsoft Visual C++ project file. The default is to have Visual Studio invoke SCons with any specified rebuild targets.

MSVSSCONS

The SCons used in generated Microsoft Visual C++ project files. The default is the version of SCons being used to generate the project file.

MSVSSCONSCOM

The default SCons command used in generated Microsoft Visual C++ project files.

MSVSSCONSCRIPT

The sconscript file (that is, SConstruct or SConscript file) that will be invoked by Microsoft Visual C++ project files (through the $MSVSSCONSCOM variable). The default is the same sconscript file that contains the call to MSVSProject to build the project file.

MSVSSCONSFLAGS

The SCons flags used in generated Microsoft Visual C++ project files.

MSVSSOLUTIONCOM

The action used to generate Microsoft Visual Studio solution files.

MSVSSOLUTIONSUFFIX

The suffix used for Microsoft Visual Studio solution (DSW) files. The default value is .sln when using Visual Studio version 7.x (.NET 2002) and later, and .dsw when using Visual Studio 6.0.

MT

The program used on Windows systems to embed manifests into DLLs and EXEs. See also $WINDOWS_EMBED_MANIFEST.

MTEXECOM

The Windows command line used to embed manifests into executables. See also $MTSHLIBCOM.

MTFLAGS

Flags passed to the $MT manifest embedding program (Windows only).

MTSHLIBCOM

The Windows command line used to embed manifests into shared libraries (DLLs). See also $MTEXECOM.

MWCW_VERSION

The version number of the MetroWerks CodeWarrior C compiler to be used.

MWCW_VERSIONS

A list of installed versions of the MetroWerks CodeWarrior C compiler on this system.

NAME

Specfies the name of the project to package.

See the Package builder.

NINJA_ALIAS_NAME

The name of the alias target which will cause SCons to create the ninja build file, and then (optionally) run ninja. The default value is generate-ninja.

NINJA_CMD_ARGS

A string which will pass arguments through SCons to the ninja command when scons executes ninja. Has no effect if $NINJA_DISABLE_AUTO_RUN is set.

This value can also be passed on the command line:

scons NINJA_CMD_ARGS=-v
or
scons NINJA_CMD_ARGS="-v -j 3"
            
NINJA_COMPDB_EXPAND

Boolean value to instruct ninja to expand the command line arguments normally put into response files. If true, prevents unexpanded lines in the compilation database like gcc @rsp_file and instead yields expanded lines like gcc -c -o myfile.o myfile.c -Ia -DXYZ.

Ninja's compdb tool added the -x flag in Ninja V1.9.0

NINJA_DEPFILE_PARSE_FORMAT

Determines the type of format ninja should expect when parsing header include depfiles. Can be msvc, gcc, or clang. The msvc option corresponds to /showIncludes format, and gcc or clang correspond to -MMD -MF.

NINJA_DIR

The builddir value. Propagates directly into the generated ninja build file. From Ninja's docs: A directory for some Ninja output files. ... (You can also store other build output in this directory.) The default value is .ninja.

NINJA_DISABLE_AUTO_RUN

Boolean. Default: False. If true, SCons will not run ninja automatically after creating the ninja build file.

If not explicitly set, this will be set to True if --disable_execute_ninja or SetOption('disable_execute_ninja', True) is seen.

NINJA_ENV_VAR_CACHE

A string that sets the environment for any environment variables that differ between the OS environment and the SCons execution environment.

It will be compatible with the default shell of the operating system.

If not explicitly set, SCons will generate this dynamically from the execution environment stored in the current construction environment (e.g. env['ENV']) where those values differ from the existing shell..

NINJA_FILE_NAME

The filename for the generated Ninja build file. The default is ninja.build.

NINJA_FORCE_SCONS_BUILD

If true, causes the build nodes to callback to scons instead of using ninja to build them. This is intended to be passed to the environment on the builder invocation. It is useful if you have a build node which does something which is not easily translated into ninja.

NINJA_GENERATED_SOURCE_ALIAS_NAME

A string matching the name of a user defined alias which represents a list of all generated sources. This will prevent the auto-detection of generated sources from $NINJA_GENERATED_SOURCE_SUFFIXES. Then all other source files will be made to depend on this in the ninja build file, forcing the generated sources to be built first.

NINJA_GENERATED_SOURCE_SUFFIXES

The list of source file suffixes which are generated by SCons build steps. All source files which match these suffixes will be added to the _generated_sources alias in the output ninja build file. Then all other source files will be made to depend on this in the ninja build file, forcing the generated sources to be built first.

NINJA_MSVC_DEPS_PREFIX

The msvc_deps_prefix string. Propagates directly into the generated ninja build file. From Ninja's docs: defines the string which should be stripped from msvc's /showIncludes output

NINJA_POOL

Set the ninja_pool for this or all targets in scope for this env var.

NINJA_REGENERATE_DEPS

A generator function used to create a ninja depfile which includes all the files which would require SCons to be invoked if they change. Or a list of said files.

_NINJA_REGENERATE_DEPS_FUNC

Internal value used to specify the function to call with argument env to generate the list of files which if changed would require the ninja build file to be regenerated.

NINJA_SCONS_DAEMON_KEEP_ALIVE

The number of seconds for the SCons deamon launched by ninja to stay alive. (Default: 180000)

NINJA_SCONS_DAEMON_PORT

The TCP/IP port for the SCons daemon to listen on. NOTE: You cannot use a port already being listened to on your build machine. (Default: random number between 10000,60000)

NINJA_SYNTAX

The path to a custom ninja_syntax.py file which is used in generation. The tool currently assumes you have ninja installed as a Python module and grabs the syntax file from that installation if $NINJA_SYNTAX is not explicitly set.

no_import_lib

When set to non-zero, suppresses creation of a corresponding Windows static import lib by the SharedLibrary builder when used with MinGW, Microsoft Visual Studio or Metrowerks. This also suppresses creation of an export (.exp) file when using Microsoft Visual Studio.

OBJPREFIX

The prefix used for (static) object file names.

OBJSUFFIX

The suffix used for (static) object file names.

PACKAGEROOT

Specifies the directory where all files in resulting archive will be placed if applicable. The default value is $NAME-$VERSION.

See the Package builder.

PACKAGETYPE

Selects the package type to build when using the Package builder. May be a string or list of strings. See the docuentation for the builder for the currently supported types.

$PACKAGETYPE may be overridden with the --package-type command line option.

See the Package builder.

PACKAGEVERSION

The version of the package (not the underlying project). This is currently only used by the rpm packager and should reflect changes in the packaging, not the underlying project code itself.

See the Package builder.

PCH

A node for the Microsoft Visual C++ precompiled header that will be used when compiling object files. This variable is ignored by tools other than Microsoft Visual C++. When this variable is defined, SCons will add options to the compiler command line to cause it to use the precompiled header, and will also set up the dependencies for the PCH file. Examples:

env['PCH'] = File('StdAfx.pch')
env['PCH'] = env.PCH('pch.cc')[0]
PCHCOM

The command line used by the PCH builder to generated a precompiled header.

PCHCOMSTR

The string displayed when generating a precompiled header. If not set, then $PCHCOM (the command line) is displayed.

PCHPDBFLAGS

A construction variable that, when expanded, adds the /yD flag to the command line only if the $PDB construction variable is set.

PCHSTOP

This variable specifies how much of a source file is precompiled. This variable is ignored by tools other than Microsoft Visual C++, or when the PCH variable is not being used. When this variable is define it must be a string that is the name of the header that is included at the end of the precompiled portion of the source files, or the empty string if the "#pragma hrdstop" construct is being used:

env['PCHSTOP'] = 'StdAfx.h'
PDB

The Microsoft Visual C++ PDB file that will store debugging information for object files, shared libraries, and programs. This variable is ignored by tools other than Microsoft Visual C++. When this variable is defined SCons will add options to the compiler and linker command line to cause them to generate external debugging information, and will also set up the dependencies for the PDB file. Example:

env['PDB'] = 'hello.pdb'

The Microsoft Visual C++ compiler switch that SCons uses by default to generate PDB information is /Z7. This works correctly with parallel (-j) builds because it embeds the debug information in the intermediate object files, as opposed to sharing a single PDB file between multiple object files. This is also the only way to get debug information embedded into a static library. Using the /Zi instead may yield improved link-time performance, although parallel builds will no longer work. You can generate PDB files with the /Zi switch by overriding the default $CCPDBFLAGS variable; see the entry for that variable for specific examples.

PDFLATEX

The pdflatex utility.

PDFLATEXCOM

The command line used to call the pdflatex utility.

PDFLATEXCOMSTR

The string displayed when calling the pdflatex utility. If this is not set, then $PDFLATEXCOM (the command line) is displayed.

env = Environment(PDFLATEX;COMSTR = "Building $TARGET from LaTeX input $SOURCES")
PDFLATEXFLAGS

General options passed to the pdflatex utility.

PDFPREFIX

The prefix used for PDF file names.

PDFSUFFIX

The suffix used for PDF file names.

PDFTEX

The pdftex utility.

PDFTEXCOM

The command line used to call the pdftex utility.

PDFTEXCOMSTR

The string displayed when calling the pdftex utility. If this is not set, then $PDFTEXCOM (the command line) is displayed.

env = Environment(PDFTEXCOMSTR = "Building $TARGET from TeX input $SOURCES")
PDFTEXFLAGS

General options passed to the pdftex utility.

PKGCHK

On Solaris systems, the package-checking program that will be used (along with $PKGINFO) to look for installed versions of the Sun PRO C++ compiler. The default is /usr/sbin/pgkchk.

PKGINFO

On Solaris systems, the package information program that will be used (along with $PKGCHK) to look for installed versions of the Sun PRO C++ compiler. The default is pkginfo.

PLATFORM

The name of the platform used to create this construction environment. SCons sets this when initializing the platform, which by default is auto-detected (see the platform argument to Environment).

env = Environment(tools=[])
if env['PLATFORM'] == 'cygwin':
    Tool('mingw')(env)
else:
    Tool('msvc')(env)
    
POAUTOINIT

The $POAUTOINIT variable, if set to True (on non-zero numeric value), let the msginit tool to automatically initialize missing PO files with msginit(1). This applies to both, POInit and POUpdate builders (and others that use any of them).

POCREATE_ALIAS

Common alias for all PO files created with POInit builder (default: 'po-create'). See msginit tool and POInit builder.

POSUFFIX

Suffix used for PO files (default: '.po') See msginit tool and POInit builder.

POTDOMAIN

The $POTDOMAIN defines default domain, used to generate POT filename as $POTDOMAIN.pot when no POT file name is provided by the user. This applies to POTUpdate, POInit and POUpdate builders (and builders, that use them, e.g. Translate). Normally (if $POTDOMAIN is not defined), the builders use messages.pot as default POT file name.