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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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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.
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 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
[email protected]
systemd 258~rc2 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 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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