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