sd_bus_message_append_array_memfd(3) — Linux manual page



NAME         top

       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         top

       #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(char type, size_t size,
                                             void **ptr);

DESCRIPTION         top

       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 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

       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

       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

       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         top

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

       Returned errors may indicate the following problems:

           Specified parameter is invalid.

           Message has been sealed.

           Message is in invalid state.

           Message cannot be appended to.

           Memory allocation failed.

NOTES         top

       These APIs are implemented as a shared library, which can be compiled
       and linked to with the libsystemd pkg-config(1) file.

SEE ALSO         top

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

NOTES         top

        1. Basic Types

        2. The D-Bus specification

COLOPHON         top

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       ⟨⟩.  This
       page was obtained from the project's upstream Git repository
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systemd 246                                   SD_BUS_MESSAGE_APPEND_ARRAY(3)

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