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sd_bus_message_new_signal, sd_bus_message_new_signal_to - 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);

int sd_bus_message_new_signal_to(sd_bus *bus, sd_bus_message **m, const char *destination, 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.

sd_bus_message_new_signal_to() is a shorthand for creating a new bus message to a specific destination. It's behavior is similar to calling sd_bus_message_new_signal() followed by calling sd_bus_message_set_destination(3).


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.


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. Send a simple signal

/* SPDX-License-Identifier: MIT-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), sd_bus_message_set_destination(3)

systemd 256~rc1