SD_BUS_SET_WATCH_BIND(3)

NAME
sd_bus_set_watch_bind, sd_bus_get_watch_bind - Control socket binding watching on bus connections

SYNOPSIS

#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);

DESCRIPTION

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 the 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.

RETURN VALUE

On success, these functions return 0 or a positive integer. On failure, they return a negative errno-style error code.

  Errors

Returned errors may indicate the following problems:

-ECHILD
    The bus connection has been created in a different process, library or module instance.

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.

EXAMPLE

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.

HISTORY

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

SEE ALSO

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)