SD_BUS_MESSAGE_APPEND_ARRAY(3)

NAME
sd_bus_message_append_array, sd_bus_message_append_array_memfd, sd_bus_message_append_array_iovec, sd_bus_message_append_array_space - Append an array of fields to a D-Bus message

SYNOPSIS

#include <systemd/sd-bus.h>

int sd_bus_message_append_array(sd_bus_message *m, char type, void *ptr, size_t size);

int sd_bus_message_append_array_memfd(sd_bus_message *m, char type, int memfd, uint64_t offset, uint64_t size);

int sd_bus_message_append_array_iovec(sd_bus_message *m, char type, const struct iovec *iov, unsigned n);

int sd_bus_message_append_array_space(sd_bus_message *m, char type, size_t size, void **ptr);

DESCRIPTION

The sd_bus_message_append_array() function appends an array to a D-Bus message m. A container will be opened, the array contents appended, and the container closed. The parameter type determines how the pointer p is interpreted. type must be one of the "trivial" types "y", "n", "q", "i", "u", "x", "t", "d" (but not "b"), as defined by the Basic D-Bus Types[1] section of the D-Bus specification, and listed in sd_bus_message_append_basic(3). Pointer p must point to an array of size size bytes containing items of the respective type. Size size must be a multiple of the size of the type type. As a special case, p may be NULL, if size is 0. The memory pointed to by p is copied into the memory area containing the message and stays in possession of the caller. The caller may hence freely change the data after this call without affecting the message the array was appended to.

The sd_bus_message_append_array_memfd() function appends an array of a trivial type to message m, similar to sd_bus_message_append_array(). The contents of the memory file descriptor memfd starting at the specified offset and of the specified size is used as the contents of the array. The offset and size must be a multiple of the size of the type type. However, as a special exception, if the offset is specified as zero and the size specified as UINT64_MAX the full memory file descriptor contents is used. The memory file descriptor is sealed by this call if it has not been sealed yet, and cannot be modified after this call. See memfd_create(2) for details about memory file descriptors. Appending arrays with memory file descriptors enables efficient zero-copy data transfer, as the memory file descriptor may be passed as-is to the destination, without copying the memory in it to the destination process. Not all protocol transports support passing memory file descriptors between participants, in which case this call will automatically fall back to copying. Also, as memory file descriptor passing is inefficient for smaller amounts of data, copying might still be enforced even where memory file descriptor passing is supported.

The sd_bus_message_append_array_iovec() function appends an array of a trivial type to the message m, similar to sd_bus_message_append_array(). Contents of the I/O vector array iov are used as the contents of the array. The total size of iov payload (the sum of iov_len fields) must be a multiple of the size of the type type. The iov argument must point to n I/O vector structures. Each structure may have the iov_base field set, in which case the memory pointed to will be copied into the message, or unset (set to zero), in which case a block of zeros of length iov_len bytes will be inserted. The memory pointed at by iov may be changed after this call.

The sd_bus_message_append_array_space() function appends space for an array of a trivial type to message m. It behaves the same as sd_bus_message_append_array(), but instead of copying items to the message, it returns a pointer to the destination area to the caller in pointer p. The caller should subsequently write the array contents to this memory. Modifications to the memory pointed to should only occur until the next operation on the bus message is invoked. Most importantly, the memory should not be altered anymore when another field has been added to the message or the message has been sealed.

RETURN VALUE

On success, these calls return 0 or a positive integer. On failure, they return a negative errno-style error code.

  Errors

Returned errors may indicate the following problems:

-EINVAL
    Specified parameter is invalid.

-EPERM
    Message has been sealed.

-ESTALE
    Message is in invalid state.

-ENXIO
    Message cannot be appended to.

-ENOMEM
    Memory allocation failed.

NOTES

Functions described here are available as a shared library, which can be compiled against and linked to with the libsystemd pkg-config(1) file.

The code described here uses getenv(3), which is declared to be not multi-thread-safe. This means that the code calling the functions described here must not call setenv(3) from a parallel thread. It is recommended to only do calls to setenv() from an early phase of the program when no other threads have been started.

SEE ALSO

systemd(1), sd-bus(3), sd_bus_message_append(3), sd_bus_message_append_basic(3), memfd_create(2), The D-Bus specification[2]

NOTES

1.  Basic D-Bus Types
    https://dbus.freedesktop.org/doc/dbus-specification.html#basic-types

2.  The D-Bus specification
    https://dbus.freedesktop.org/doc/dbus-specification.html