Manpages - sd_bus_message_new_method_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.

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

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);
    }

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.

*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