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HTML rendering created 2026-09-09 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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NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | NOTES | EXAMPLES | HISTORY | SEE ALSO | NOTES | COLOPHON |
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SD_BUS_...OD_CALL(3) sd_bus_message_new_method_call SD_BUS_...OD_CALL(3)
sd_bus_message_new_method_call, sd_bus_message_new_method_return -
Create a method call message
#include <systemd/sd-bus.h>
int sd_bus_message_new_method_call(sd_bus *bus,
sd_bus_message **m,
const char *destination,
const char *path,
const char *interface,
const char *member);
int sd_bus_message_new_method_return(sd_bus_message *call,
sd_bus_message **m);
The sd_bus_message_new_method_call() function creates a new bus
message object that encapsulates a D-Bus method call, and returns
it in the m output parameter. The call will be made on the
destination destination, path path, on the interface interface,
member member.
Briefly, the destination is a dot-separated name that identifies a
service connected to the bus. The path is a slash-separated
identifier of an object within the destination that resembles a
file system path. The meaning of this path is defined by the
destination. The interface is a dot-separated name that resembles
a Java interface name that identifies a group of methods and
signals supported by the object identified by path. Methods and
signals are collectively called members and are identified by a
simple name composed of ASCII letters, numbers, and underscores.
See the D-Bus Tutorial[1] for an in-depth explanation.
The destination parameter may be NULL. The interface parameter may
be NULL, if the destination has only a single member with the
given name and there is no ambiguity if the interface name is
omitted.
Note that this is a low level interface. See sd_bus_call_method(3)
for a more convenient way of calling D-Bus methods.
The sd_bus_message_new_method_return() function creates a new bus
message object that is a reply to the method call call and returns
it in the m output parameter. The call parameter must be a method
call message. The sender of call is used as the destination.
On success, these functions return a non-negative integer. On
failure, they return a negative errno-style error code.
Errors
Returned errors may indicate the following problems:
-EINVAL
The output parameter m is NULL.
The destination parameter is non-null and is not a valid D-Bus
service name ("org.somewhere.Something"), the path parameter
is not a valid D-Bus path ("/an/object/path"), the interface
parameter is non-null and is not a valid D-Bus interface name
("an.interface.name"), or the member parameter is not a valid
D-Bus member ("Name").
The call parameter is not a method call object.
-ENOTCONN
The bus parameter bus is NULL or the bus is not connected.
-ENOMEM
Memory allocation failed.
-EPERM
The call parameter is not sealed.
-EOPNOTSUPP
The call message does not have a cookie.
Functions described here are available as a shared library, which
can be compiled against and linked to with the
libsystemd pkg-config(1) file.
All functions listed here are thread-agnostic and only a single
thread may operate on a given object at any given time. Different
threads may access the same object at different times. Multiple
independent objects may be used from different threads in
parallel.
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. Make a call to a D-Bus method that takes a single
parameter
/* SPDX-License-Identifier: MIT-0 */
/* This is equivalent to:
* busctl call org.freedesktop.systemd1 /org/freedesktop/systemd1 \
* org.freedesktop.systemd1.Manager GetUnitByPID $$
*
* Compile with 'cc print-unit-path.c -lsystemd'
*/
#include <errno.h>
#include <stdio.h>
#include <sys/types.h>
#include <unistd.h>
#include <systemd/sd-bus.h>
#define _cleanup_(f) __attribute__((cleanup(f)))
#define DESTINATION "org.freedesktop.systemd1"
#define PATH "/org/freedesktop/systemd1"
#define INTERFACE "org.freedesktop.systemd1.Manager"
#define MEMBER "GetUnitByPID"
static int log_error(int error, const char *message) {
fprintf(stderr, "%s: %s\n", message, strerror(-error));
return error;
}
int main(int argc, char **argv) {
_cleanup_(sd_bus_flush_close_unrefp) sd_bus *bus = NULL;
_cleanup_(sd_bus_error_free) sd_bus_error error = SD_BUS_ERROR_NULL;
_cleanup_(sd_bus_message_unrefp) sd_bus_message *reply = NULL, *m = NULL;
int r;
r = sd_bus_open_system(&bus);
if (r < 0)
return log_error(r, "Failed to acquire bus");
r = sd_bus_message_new_method_call(bus, &m,
DESTINATION, PATH, INTERFACE, MEMBER);
if (r < 0)
return log_error(r, "Failed to create bus message");
r = sd_bus_message_append(m, "u", (unsigned) getpid());
if (r < 0)
return log_error(r, "Failed to append to bus message");
r = sd_bus_call(bus, m, -1, &error, &reply);
if (r < 0)
return log_error(r, MEMBER " call failed");
const char *ans;
r = sd_bus_message_read(reply, "o", &ans);
if (r < 0)
return log_error(r, "Failed to read reply");
printf("Unit path is \"%s\".\n", ans);
return 0;
}
This defines a minimally useful program that will open a
connection to the bus, create a message object, send it, wait for
the reply, and finally extract and print the answer. It does error
handling and proper memory management.
sd_bus_message_new_method_call() and
sd_bus_message_new_method_return() were added in version 246.
systemd(1), sd-bus(3), sd_bus_call(3), sd_bus_call_method(3),
sd_bus_path_encode(3)
1. D-Bus Tutorial
https://dbus.freedesktop.org/doc/dbus-tutorial.html#concepts
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 2026-08-04. (At that
time, the date of the most recent commit that was found in the
repository was 2026-08-03.) 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 262~devel SD_BUS_...OD_CALL(3)
Pages that refer to this page: sd-bus(3), sd_bus_call(3), sd_bus_call_method(3), sd_bus_message_get_type(3), sd_bus_message_new(3), sd_bus_message_new_signal(3), sd_bus_message_sensitive(3), sd_bus_message_set_destination(3), sd_bus_reply_method_return(3), systemd.directives(7), systemd.index(7)
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HTML rendering created 2026-09-09 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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