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NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | NOTES | EXAMPLES | SEE ALSO | NOTES | COLOPHON |
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SD_BUS_...ONTAINER(3) sd_bus_message_open_container SD_BUS_...ONTAINER(3)
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 over them. Use sd_bus_message_skip(3) to skip over fields
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.
Errors
Returned errors may indicate the following problems:
-EINVAL
m or contents are NULL or type is invalid.
Added in version 246.
-EBADMSG
Message m has invalid structure.
Added in version 254.
-ENXIO
Message m does not have a container of type type at the
current position, or the contents do not match contents.
Added in version 254.
-EPERM
The message m is already sealed.
Added in version 246.
-ESTALE
The message m is in an invalid state.
Added in version 246.
-ENOMEM
Memory allocation failed.
Added in version 246.
-EBUSY
sd_bus_message_exit_container() was called but there are
unread members left in the container.
Added in version 247.
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.
Example 1. Append an array of strings to a message
/* SPDX-License-Identifier: MIT-0 */
#include <systemd/sd-bus.h>
int append_strings_to_message(sd_bus_message *m, const char *const *arr) {
const char *s;
int r;
r = sd_bus_message_open_container(m, 'a', "s");
if (r < 0)
return r;
for (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: MIT-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);
}
systemd(1), sd-bus(3), sd_bus_message_append(3),
sd_bus_message_read(3), sd_bus_message_skip(3), The D-Bus
specification[1]
1. The D-Bus specification
https://dbus.freedesktop.org/doc/dbus-specification.html
This page is part of the systemd (systemd system and service
manager) project. Information about the project can be found at
⟨http://www.freedesktop.org/wiki/Software/systemd⟩. If you have a
bug report for this manual page, see
⟨http://www.freedesktop.org/wiki/Software/systemd/#bugreports⟩.
This page was obtained from the project's upstream Git repository
⟨https://github.com/systemd/systemd.git⟩ on 2025-08-11. (At that
time, the date of the most recent commit that was found in the
repository was 2025-08-11.) If you discover any rendering
problems in this HTML version of the page, or you believe there is
a better or more up-to-date source for the page, or you have
corrections or improvements to the information in this COLOPHON
(which is not part of the original manual page), send a mail to
man-pages@man7.org
systemd 258~rc2 SD_BUS_...ONTAINER(3)
Pages that refer to this page: sd-bus(3), sd_bus_message_append(3), sd_bus_message_read(3), systemd.directives(7), systemd.index(7)
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HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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