sd_bus_message_new_method_call(3) — Linux manual page


SD_BUS_...OD_CALL(3) sd_bus_message_new_method_call SD_BUS_...OD_CALL(3)

NAME         top

       sd_bus_message_new_method_call, sd_bus_message_new_method_return
       - Create a method call message

SYNOPSIS         top

       #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);

DESCRIPTION         top

       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

       Note that this is a low level interface. See
       sd_bus_call_method(3) for a more convenient way of calling D-Bus

       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

RETURN VALUE         top

       On success, these functions return a non-negative integer. On
       failure, they return a negative errno-style error code.

       Returned errors may indicate the following problems:

           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 (""), or the member parameter
           is not a valid D-Bus member ("Name").

           The call parameter is not a method call object.

           The bus parameter bus is NULL or the bus is not connected.

           Memory allocation failed.

           The call parameter is not sealed.

           The call message does not have a cookie.

NOTES         top

       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

EXAMPLES         top

       Example 1. Make a call to a D-Bus method that takes a single

           /* 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) {
             errno = -error;
             fprintf(stderr, "%s: %m\n", message);
             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.

HISTORY         top

       sd_bus_message_new_method_call() and
       sd_bus_message_new_method_return() were added in version 246.

SEE ALSO         top

       systemd(1), sd-bus(3), sd_bus_call(3), sd_bus_call_method(3),

NOTES         top

        1. D-Bus Tutorial

COLOPHON         top

       This page is part of the systemd (systemd system and service
       manager) project.  Information about the project can be found at
       ⟨⟩.  If you have
       a bug report for this manual page, see
       This page was obtained from the project's upstream Git repository
       ⟨⟩ on 2023-12-22.  (At that
       time, the date of the most recent commit that was found in the
       repository was 2023-12-22.)  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

systemd 255                                         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_sensitive(3)sd_bus_message_set_destination(3)sd_bus_reply_method_return(3)systemd.directives(7)systemd.index(7)