sd_bus_message_new

Name

sd_bus_message_new_signal, sd_bus_message_new_signal_to — Create a signal message

Synopsis

#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)`;

Description¶

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).

Return Value¶

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

Errors¶

Returned errors may indicate the following problems:

-EINVAL¶

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

-ENOTCONN¶

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

-ENOMEM¶

Memory allocation failed.

Notes¶

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.

Examples¶

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,
                                "/org/freedesktop/systemd1",
                                "org.freedesktop.systemd1.Manager",
                                "UnitFilesChanged");
  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.