sd_bus_message_open_container, sd_bus_message_close_container, sd_bus_message_enter_container, sd_bus_message_exit_container — Create and move between containers in D-Bus messages
#include <systemd/sd-bus.h>
int sd_bus_message_open_container( | sd_bus_message *m, |
| char type, | |
const char *contents); |
int sd_bus_message_close_container( | sd_bus_message *m); |
int sd_bus_message_enter_container( | sd_bus_message *m, |
| char type, | |
const char *contents); |
int sd_bus_message_exit_container( | sd_bus_message *m); |
sd_bus_message_open_container() appends a new container to the message
m. After opening a new container, it can be filled with content using
sd_bus_message_append(3)
and similar functions. Containers behave like a stack. To nest containers inside each other, call
sd_bus_message_open_container() multiple times without calling
sd_bus_message_close_container() in between. Each container will be nested inside the
previous container. type represents the container type and should be one of
"r", "a", "v" or "e" as described in
sd_bus_message_append(3).
Instead of literals, the corresponding constants SD_BUS_TYPE_STRUCT,
SD_BUS_TYPE_ARRAY, SD_BUS_TYPE_VARIANT or
SD_BUS_TYPE_DICT_ENTRY can also be used. contents describes
the type of the container's elements and should be a D-Bus type string following the rules described in
sd_bus_message_append(3).
sd_bus_message_close_container() closes the last container opened with
sd_bus_message_open_container(). On success, the write pointer of the message
m is positioned after the closed container in its parent container or in
m itself if there is no parent container.
sd_bus_message_enter_container() enters the next container of the message
m for reading. It behaves mostly the same as
sd_bus_message_open_container(). Entering a container allows reading its contents
with
sd_bus_message_read(3)
and similar functions. type and contents are the same as in
sd_bus_message_open_container().
sd_bus_message_exit_container() exits the scope of the last container entered
with sd_bus_message_enter_container(). It behaves mostly the same as
sd_bus_message_close_container(). Note that
sd_bus_message_exit_container() may only be called after iterating through all
members of the container, i.e. reading or skipping them. Use
sd_bus_message_skip(3)
to skip over felds of a container in order to be able to exit the container with
sd_bus_message_exit_container() without reading all members.
On success, these functions return a non-negative integer.
sd_bus_message_open_container() and sd_bus_message_close_container()
return 0.
sd_bus_message_enter_container() returns 1 if it successfully opened a new container, and 0 if
that was not possible because the end of the currently open container or message was reached.
sd_bus_message_exit_container() returns 1 on success.
On failure, all of these functions return a negative errno-style error code.
These APIs are implemented as a shared
library, which can be compiled and linked to with the
libsystemd pkg-config(1)
file.
Example 1. Append an array of strings to a message
/* SPDX-License-Identifier: CC0-1.0 */
#include <systemd/sd-bus.h>
int append_strings_to_message(sd_bus_message *m, const char *const *arr) {
int r;
r = sd_bus_message_open_container(m, 'a', "s");
if (r < 0)
return r;
for (const char *s = *arr; *s; s++) {
r = sd_bus_message_append(m, "s", s);
if (r < 0)
return r;
}
return sd_bus_message_close_container(m);
}
Example 2. Read an array of strings from a message
/* SPDX-License-Identifier: CC0-1.0 */
#include <stdio.h>
#include <systemd/sd-bus.h>
int read_strings_from_message(sd_bus_message *m) {
int r;
r = sd_bus_message_enter_container(m, 'a', "s");
if (r < 0)
return r;
for (;;) {
const char *s;
r = sd_bus_message_read(m, "s", &s);
if (r < 0)
return r;
if (r == 0)
break;
printf("%s\n", s);
}
return sd_bus_message_exit_container(m);
}