SD_BUS_MESSAGE_NEW_METHOD_CALL(3)

  .SH "NAME" sd_bus_message_new_method_call, sd_bus_message_new_method_return - Create a method call message
  .SH "SYNOPSIS"

#include <systemd/sd-bus.h>

      .HP u 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);
      .HP u int sd_bus_message_new_method_return(sd_bus_message *call, sd_bus_message **m);

  .SH "DESCRIPTION"

    .PP 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.
    .PP 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.
    .PP 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.
    .PP The sd_bus_message_new_method_call() 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.

  .SH "RETURN VALUE"

    .PP This function returns 0 if the message object was successfully created, and a negative errno-style error code otherwise.

  .SH "ERRORS"

    .PP Returned errors may indicate the following problems:

      .PP -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.

          .RE
          .PP -ENOTCONN

                The bus parameter bus is NULL or the bus is not connected.
              .RE
              .PP -ENOMEM

                    Memory allocation failed.
                  .RE
                  .PP -EPERM

                        The call parameter is not sealed.

                      .RE
                      .PP -EOPNOTSUPP

                            The call message does not have a cookie.

                          .RE

                      .SH "NOTES"

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

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

                        #include <stdio.h>
                        #include <string.h>
                        #include <unistd.h>
                        #include <sys/types.h>
                        #include <systemd/sd-bus.h>
                        #define _cleanup_(f) __attribute__((cleanup(f)))
                        /* This is equivalent to:
                         * busctl call org.freedesktop.systemd1 /org/freedesktop/systemd1 \
                         *       org.freedesktop.systemd1.Manager GetUnitByPID $$
                         *
                         * Compile with 'cc -lsystemd print-unit-path.c'
                         */
                        #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;
                        }
                        static int print_unit_path(sd_bus *bus) {
                          _cleanup_(sd_bus_message_unrefp) sd_bus_message *m = NULL;
                          _cleanup_(sd_bus_error_free) sd_bus_error error = SD_BUS_ERROR_NULL;
                          _cleanup_(sd_bus_message_unrefp) sd_bus_message *reply = NULL;
                          int r;
                          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, "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;
                        }
                        int main(int argc, char **argv) {
                          _cleanup_(sd_bus_flush_close_unrefp) sd_bus *bus = NULL;
                          int r;
                          r = sd_bus_open_system(&bus);
                          if (r < 0)
                            return log_error(r, "Failed to acquire bus");
                          print_unit_path(bus);
                        }

                          .PP 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.

                      .SH "SEE ALSO"

                        .PP systemd(1), sd-bus(3), sd_bus_path_encode(3)

NOTES

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