sd_bus_set_watch_bind - Man Page

#include <systemd/sd-bus​.h>

int sd_bus_set_watch_bind(sd_bus *bus, int b);

int sd_bus_get_watch_bind(sd_bus *bus);

sd_bus_set_watch_bind() may be used to enable or disable client-side watching of server socket binding for a bus connection object​. If b is true, the feature is enabled, otherwise disabled (which is the default)​. When enabled, and the selected bus address refers to an AF_UNIX socket in the file system which does not exist while the connection attempt is made, an inotify(7) watch is installed on it, waiting for the socket to appear​. As soon as the socket appears the connection is made​. This functionality is useful in particular in early-boot programs that need to run before the system bus is available, but want to connect to it the instant it may be connected to​.

sd_bus_get_watch_bind() may be used to query the current setting of this feature​. It returns zero when the feature is disabled, and positive if enabled​.

Note that no timeout is applied while we wait for the socket to appear​. This means that any synchronous remote operation (such as sd_bus_call(3), sd_bus_add_match(3) or sd_bus_request_name(3)), that is used on a connection with this feature enabled that has not been established yet, might block forever if the socket is never created​. However, asynchronous remote operations (such as sd_bus_send(3), sd_bus_call_async(3), sd_bus_add_match_async(3)) do not block in this case, and safely enqueue the requested operations to be dispatched the instant the connection is set up​.

Use sd_bus_is_ready(3) to determine whether the connection is fully established, i​.e​. whether the peer socket has been bound, connected to and authenticated​. Use sd_bus_set_connected_signal(3) to be notified when the connection is fully established​.

On success, these functions return 0 or a positive 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​.

All functions listed here are thread-agnostic and only a single thread may operate on a given object at any given time​. Different threads may access the same object at different times​. Multiple independent objects may be used from different threads in parallel​.

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. ​Create a simple system service that publishes a property on the system bus and can reconnect when D-Bus disconnects and reconnects

/* SPDX-License-Identifier: MIT-0 */

/* A D-Bus service that automatically reconnects when the system bus is
 * restarted​.
 *
 * Compile with 'cc sd_bus_service_reconnect​.c $(pkg-config --libs --cflags libsystemd)'
 *
 * To allow the program to take ownership of the name 'org​.freedesktop​.ReconnectExample',
 * add the following as /etc/dbus-1/system​.d/org​.freedesktop​.ReconnectExample​.conf
 * and then reload the broker with 'systemctl reload dbus':

<?xml version="1​.0"?> <!--*-nxml-*-->
<!DOCTYPE busconfig PUBLIC "-//freedesktop//DTD D-BUS Bus Configuration 1​.0//EN"
  "http://www​.freedesktop​.org/standards/dbus/1​.0/busconfig​.dtd">
<busconfig>
  <policy user="root">
    <allow own="org​.freedesktop​.ReconnectExample"/>
    <allow send_destination="org​.freedesktop​.ReconnectExample"/>
    <allow receive_sender="org​.freedesktop​.ReconnectExample"/>
  </policy>

  <policy context="default">
    <allow send_destination="org​.freedesktop​.ReconnectExample"/>
    <allow receive_sender="org​.freedesktop​.ReconnectExample"/>
  </policy>
</busconfig>

 *
 * To get the property via busctl:
 *
 * $ busctl --user get-property org​.freedesktop​.ReconnectExample \
 *                              /org/freedesktop/ReconnectExample \
 *                              org​.freedesktop​.ReconnectExample \
 *                              Example
 * s "example"
 */

#include <errno​.h>
#include <stdio​.h>
#include <stdlib​.h>
#include <systemd/sd-bus​.h>

#define _cleanup_(f) __attribute__((cleanup(f)))

static int log_error(int r, const char *str) {
  fprintf(stderr, "%s failed: %s\n", str, strerror(-r));
  return r;
}

typedef struct object {
  const char *example;
  sd_bus **bus;
  sd_event **event;
} object;

static int property_get(
                sd_bus *bus,
                const char *path,
                const char *interface,
                const char *property,
                sd_bus_message *reply,
                void *userdata,
                sd_bus_error *error) {

  object *o = userdata;

  if (strcmp(property, "Example") == 0)
    return sd_bus_message_append(reply, "s", o->example);

  return sd_bus_error_setf(error,
                           SD_BUS_ERROR_UNKNOWN_PROPERTY,
                           "Unknown property '%s'",
                           property);
}

/* https://www​.freedesktop​.org/software/systemd/man/sd_bus_add_object​.html */
static const sd_bus_vtable vtable[] = {
  SD_BUS_VTABLE_START(0),
  SD_BUS_PROPERTY(
    "Example", "s",
    property_get,
    0,
    SD_BUS_VTABLE_PROPERTY_CONST),
  SD_BUS_VTABLE_END
};

static int setup(object *o);

static int on_disconnect(sd_bus_message *message, void *userdata, sd_bus_error *ret_error) {
  int r;

  r = setup((object *)userdata);
  if (r < 0) {
    object *o = userdata;
    r = sd_event_exit(*o->event, r);
    if (r < 0)
      return log_error(r, "sd_event_exit()");
  }

  return 1;
}

/* Ensure the event loop exits with a clear error if acquiring the well-known
 * service name fails */
static int request_name_callback(sd_bus_message *m, void *userdata, sd_bus_error *ret_error) {
  int r;

  if (!sd_bus_message_is_method_error(m, NULL))
    return 1;

  const sd_bus_error *error = sd_bus_message_get_error(m);

  if (sd_bus_error_has_names(error, SD_BUS_ERROR_TIMEOUT, SD_BUS_ERROR_NO_REPLY))
    return 1; /* The bus is not available, try again later */

  fprintf(stderr, "Failed to request name: %s\n", error->message);
  object *o = userdata;
  r = sd_event_exit(*o->event, -sd_bus_error_get_errno(error));
  if (r < 0)
    return log_error(r, "sd_event_exit()");

  return 1;
}

static int setup(object *o) {
  int r;

  /* If we are reconnecting, then the bus object needs to be closed, detached
   * from the event loop and recreated​.
   * https://www​.freedesktop​.org/software/systemd/man/sd_bus_detach_event​.html
   * https://www​.freedesktop​.org/software/systemd/man/sd_bus_close_unref​.html
   */
  if (*o->bus) {
    r = sd_bus_detach_event(*o->bus);
    if (r < 0)
      return log_error(r, "sd_bus_detach_event()");
    *o->bus = sd_bus_close_unref(*o->bus);
  }

  /* Set up a new bus object for the system bus, configure it to wait for D-Bus
   * to be available instead of failing if it is not, and start it​. All the
   * following operations are asynchronous and will not block waiting for D-Bus
   * to be available​.
   * https://www​.freedesktop​.org/software/systemd/man/sd_bus_new​.html
   * https://www​.freedesktop​.org/software/systemd/man/sd_bus_set_address​.html
   * https://www​.freedesktop​.org/software/systemd/man/sd_bus_set_bus_client​.html
   * https://www​.freedesktop​.org/software/systemd/man/sd_bus_negotiate_creds​.html
   * https://www​.freedesktop​.org/software/systemd/man/sd_bus_set_watch_bind​.html
   * https://www​.freedesktop​.org/software/systemd/man/sd_bus_set_connected_signal​.html
   * https://www​.freedesktop​.org/software/systemd/man/sd_bus_start​.html
   */
  r = sd_bus_new(o->bus);
  if (r < 0)
    return log_error(r, "sd_bus_new()");
  r = sd_bus_set_address(*o->bus, "unix:path=/run/dbus/system_bus_socket");
  if (r < 0)
    return log_error(r, "sd_bus_set_address()");
  r = sd_bus_set_bus_client(*o->bus, 1);
  if (r < 0)
    return log_error(r, "sd_bus_set_bus_client()");
  r = sd_bus_negotiate_creds(*o->bus, 1, SD_BUS_CREDS_UID|SD_BUS_CREDS_EUID|SD_BUS_CREDS_EFFECTIVE_CAPS);
  if (r < 0)
    return log_error(r, "sd_bus_negotiate_creds()");
  r = sd_bus_set_watch_bind(*o->bus, 1);
  if (r < 0)
    return log_error(r, "sd_bus_set_watch_bind()");
  r = sd_bus_start(*o->bus);
  if (r < 0)
    return log_error(r, "sd_bus_start()");

  /* Publish an interface on the bus, specifying our well-known object access
   * path and public interface name​.
   * https://www​.freedesktop​.org/software/systemd/man/sd_bus_add_object​.html
   * https://dbus​.freedesktop​.org/doc/dbus-tutorial​.html
   */
  r = sd_bus_add_object_vtable(*o->bus,
                               NULL,
                               "/org/freedesktop/ReconnectExample",
                               "org​.freedesktop​.ReconnectExample",
                               vtable,
                               o);
  if (r < 0)
    return log_error(r, "sd_bus_add_object_vtable()");
  /* By default, the service is only assigned an ephemeral name​. Also add a
   * well-known one, so that clients know whom to call​. This needs to be
   * asynchronous, as D-Bus might not be yet available​. The callback will check
   * whether the error is expected or not, in case it fails​.
   * https://www​.freedesktop​.org/software/systemd/man/sd_bus_request_name​.html
   */
  r = sd_bus_request_name_async(*o->bus,
                                NULL,
                                "org​.freedesktop​.ReconnectExample",
                                0,
                                request_name_callback,
                                o);
  if (r < 0)
    return log_error(r, "sd_bus_request_name_async()");
  /* When D-Bus is disconnected this callback will be invoked, which will set up
   * the connection again​. This needs to be asynchronous, as D-Bus might not yet
   * be available​.
   * https://www​.freedesktop​.org/software/systemd/man/sd_bus_match_signal_async​.html
   */
  r = sd_bus_match_signal_async(*o->bus,
                                NULL,
                                "org​.freedesktop​.DBus​.Local",
                                NULL,
                                "org​.freedesktop​.DBus​.Local",
                                "Disconnected",
                                on_disconnect,
                                NULL,
                                o);
  if (r < 0)
    return log_error(r, "sd_bus_match_signal_async()");
  /* Attach the bus object to the event loop so that calls and signals are
   * processed​.
   * https://www​.freedesktop​.org/software/systemd/man/sd_bus_attach_event​.html
   */
  r = sd_bus_attach_event(*o->bus, *o->event, 0);
  if (r < 0)
    return log_error(r, "sd_bus_attach_event()");

  return 0;
}

int main(int argc, char **argv) {
  /* The bus should be relinquished before the program terminates​. The cleanup
   * attribute allows us to do it nicely and cleanly whenever we exit the block​.
   */
  _cleanup_(sd_bus_flush_close_unrefp) sd_bus *bus = NULL;
  _cleanup_(sd_event_unrefp) sd_event *event = NULL;
  object o = {
    ​.example = "example",
    ​.bus = &bus,
    ​.event = &event,
  };
  int r;

  /* Create an event loop data structure, with default parameters​.
   * https://www​.freedesktop​.org/software/systemd/man/sd_event_default​.html
   */
  r = sd_event_default(&event);
  if (r < 0)
    return log_error(r, "sd_event_default()");

  /* By default, the event loop will terminate when all sources have disappeared,
   * so we have to keep it 'occupied'​. Register signal handling to do so​.
   * https://www​.freedesktop​.org/software/systemd/man/sd_event_add_signal​.html
   */
  r = sd_event_add_signal(event, NULL, SIGINT|SD_EVENT_SIGNAL_PROCMASK, NULL, NULL);
  if (r < 0)
    return log_error(r, "sd_event_add_signal(SIGINT)");

  r = sd_event_add_signal(event, NULL, SIGTERM|SD_EVENT_SIGNAL_PROCMASK, NULL, NULL);
  if (r < 0)
    return log_error(r, "sd_event_add_signal(SIGTERM)");

  r = setup(&o);
  if (r < 0)
    return EXIT_FAILURE;

  /* Enter the main loop, it will exit only on sigint/sigterm​.
   * https://www​.freedesktop​.org/software/systemd/man/sd_event_loop​.html
   */
  r = sd_event_loop(event);
  if (r < 0)
    return log_error(r, "sd_event_loop()");

  /* https://www​.freedesktop​.org/software/systemd/man/sd_bus_release_name​.html */
  r = sd_bus_release_name(bus, "org​.freedesktop​.ReconnectExample");
  if (r < 0)
    return log_error(r, "sd_bus_release_name()");

  return 0;
}

This is particularly useful for services that are configured to survive a soft-reboot, see systemd-soft-reboot.service(8) for more details​.

sd_bus_set_watch_bind() and sd_bus_get_watch_bind() were added in version 237​.

systemd(1), sd-bus(3), inotify(7), sd_bus_call(3), sd_bus_add_match(3), sd_bus_request_name(3), sd_bus_is_ready(3), sd_bus_set_connected_signal(3)