sd_bus_call_method - Man Page

#include <systemd/sd-bus.h>

typedef int (*sd_bus_message_handler_t)(sd_bus_message *m, void *userdata, sd_bus_error *ret_error);

int sd_bus_call_method(sd_bus *bus, const char *destination, const char *path, const char *interface, const char *member, sd_bus_error *ret_error, sd_bus_message **reply, const char *types, ...);

int sd_bus_call_methodv(sd_bus *bus, const char *destination, const char *path, const char *interface, const char *member, sd_bus_error *ret_error, sd_bus_message **reply, const char *types, va_list ap);

int sd_bus_call_method_async(sd_bus *bus, sd_bus_slot **slot, const char *destination, const char *path, const char *interface, const char *member, sd_bus_message_handler_t callback, void *userdata, const char *types, ...);

int sd_bus_call_method_asyncv(sd_bus *bus, sd_bus_slot **slot, const char *destination, const char *path, const char *interface, const char *member, sd_bus_message_handler_t callback, void *userdata, const char *types, va_list ap);

sd_bus_call_method() is a convenience function for initializing a bus message object and calling the corresponding D-Bus method. It combines the sd_bus_message_new_method_call(3), sd_bus_message_append(3) and sd_bus_call(3) functions into a single function call.

sd_bus_call_method_async() is a convenience function for initializing a bus message object and calling the corresponding D-Bus method asynchronously. It combines the sd_bus_message_new_method_call(3), sd_bus_message_append(3) and sd_bus_call_async(3) functions into a single function call.

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

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-call-method.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;
  int r;

  r = sd_bus_open_system(&bus);
  if (r < 0)
    return log_error(r, "Failed to acquire bus");

  r = sd_bus_call_method(bus, DESTINATION, PATH, INTERFACE, MEMBER, &error, &reply, "u", (unsigned) getpid());
  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, call a method, 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_message_new_method_call(3), sd_bus_message_append(3), sd_bus_call(3), sd_bus_set_property(3), sd_bus_emit_signal(3)

sd-bus(3), sd_bus_call(3), sd_bus_emit_signal(3), sd_bus_interface_name_is_valid(3), sd_bus_message_new_method_call(3), sd_bus_send(3), sd_bus_set_property(3), sd_bus_slot_ref(3), systemd.directives(7), systemd.index(7).

The man pages sd_bus_call_method_async(3), sd_bus_call_method_asyncv(3) and sd_bus_call_methodv(3) are aliases of sd_bus_call_method(3).