NAME

       sd_bus_message_new_method_call, sd_bus_message_new_method_return - Create a method call
       message

SYNOPSIS

       #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

       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.

RETURN VALUE

       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.

NOTES

       These APIs are implemented as a shared library, which can be compiled and linked to with
       the libsystemd pkg-config(1) file.

EXAMPLES

       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) {
             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.

SEE ALSO

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

NOTES

        1. D-Bus Tutorial
           https://dbus.freedesktop.org/doc/dbus-tutorial.html#concepts