sd_bus_message_new_signal - Create a signal message

#include <systemd/sd-bus.h>

int sd_bus_message_new_signal(sd_bus *bus, sd_bus_message **m, const char *path, const char *interface, const char *member);

The sd_bus_message_new_signal() function creates a new bus message object that encapsulates a D-Bus signal, and returns it in the m output parameter. The signal will be sent to path path, on the interface interface, member member. When this message is sent, no reply is expected. See sd_bus_message_new_method_call(1) for a short description of the meaning of the path, interface, and member parameters.

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

Returned errors may indicate the following problems:


The output parameter m is NULL.

The path parameter is not a valid D-Bus path ("/an/object/path"), the interface parameter is not a valid D-Bus interface name (""), or the member parameter is not a valid D-Bus member ("Name").


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


Memory allocation failed.

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

Example 1. Send a simple signal

/* SPDX-License-Identifier: CC0-1.0 */
#include <systemd/sd-bus.h>
#define _cleanup_(f) __attribute__((cleanup(f)))
int send_unit_files_changed(sd_bus *bus) {

_cleanup_(sd_bus_message_unrefp) sd_bus_message *message = NULL;
int r;
r = sd_bus_message_new_signal(bus, &message,
if (r < 0)
return r;
return sd_bus_send(bus, message, NULL); }

This function in systemd sources is used to emit the "UnitFilesChanged" signal when the unit files have been changed.

systemd(1), sd-bus(3), sd_bus_emit_signal(3)

systemd 251