Migrating to GDBus

Conceptual differences

The central concepts of D-Bus are modelled in a very similar way in dbus-glib and GDBus. Both have objects representing connections, proxies and method invocations. But there are some important differences:

dbus-glib uses the libdbus reference implementation, GDBus doesn’t. Instead, it relies on GIO streams as transport layer, and has its own implementation for the D-Bus connection setup and authentication. Apart from using streams as transport, avoiding libdbus also lets GDBus avoid some thorny multithreading issues.
dbus-glib uses the GObject type system for method arguments and return values, including a homegrown container specialization mechanism. GDBus relies on the GVariant type system which is explicitly designed to match D-Bus types.
dbus-glib models only D-Bus interfaces and does not provide any types for objects. GDBus models both D-Bus interfaces (via the GDBusInterface, GDBusProxy and GDBusInterfaceSkeleton types) and objects (via the GDBusObject, GDBusObjectSkeleton and GDBusObjectProxy types).
GDBus includes native support for the org.freedesktop.DBus.Properties (via the GDBusProxy type) and org.freedesktop.DBus.ObjectManager D-Bus interfaces, dbus-glib doesn’t.
The typical way to export an object in dbus-glib involves generating glue code from XML introspection data using dbus-binding-tool. GDBus provides a similar tool called gdbus-codegen that can also generate Docbook D-Bus interface documentation.
dbus-glib doesn’t provide any convenience API for owning and watching bus names, GDBus provides the g_bus_own_name() and g_bus_watch_name() family of convenience functions.
GDBus provides API to parse, generate and work with Introspection XML, dbus-glib doesn’t.
GTestDBus provides API to create isolated unit tests

API comparison

dbus-glib	GDBus
`DBusGConnection`	`GDBusConnection`
`DBusGProxy`	`GDBusProxy`, `GDBusInterface` - also see `GDBusObjectProxy`
`DBusGObject`	`GDBusInterfaceSkeleton`, `GDBusInterface` - also see `GDBusObjectSkeleton`
`DBusGMethodInvocation`	`GDBusMethodInvocation`
`dbus_g_bus_get()`	`g_bus_get_sync()`, also see `g_bus_get()`
`dbus_g_proxy_new_for_name()`	`g_dbus_proxy_new_sync()` and `g_dbus_proxy_new_for_bus_sync()`, also see `g_dbus_proxy_new()`
`dbus_g_proxy_add_signal()`	not needed, use the generic “g-signal”
`dbus_g_proxy_connect_signal()`	use `g_signal_connect()` with “g-signal”
`dbus_g_connection_register_g_object()`	`g_dbus_connection_register_object()` - also see `g_dbus_object_manager_server_export()`
`dbus_g_connection_unregister_g_object()`	`g_dbus_connection_unregister_object()` - also see `g_dbus_object_manager_server_unexport()`
`dbus_g_object_type_install_info()`	introspection data is installed while registering an object, see `g_dbus_connection_register_object()`
`dbus_g_proxy_begin_call()`	`g_dbus_proxy_call()`
`dbus_g_proxy_end_call()`	`g_dbus_proxy_call_finish()`
`dbus_g_proxy_call()`	`g_dbus_proxy_call_sync()`
`dbus_g_error_domain_register()`	`g_dbus_error_register_error_domain()`
`dbus_g_error_has_name()`	no direct equivalent, see `g_dbus_error_get_remote_error()`
`dbus_g_method_return()`	`g_dbus_method_invocation_return_value()`
`dbus_g_method_return_error()`	`g_dbus_method_invocation_return_error()` and variants
`dbus_g_method_get_sender()`	`g_dbus_method_invocation_get_sender()`

Owning bus names

Using dbus-glib, you typically call RequestName manually to own a name, like in the following excerpt:

error = NULL;
res = dbus_g_proxy_call (system_bus_proxy,
                         "RequestName",
                         &error,
                         G_TYPE_STRING, NAME_TO_CLAIM,
                         G_TYPE_UINT,   DBUS_NAME_FLAG_ALLOW_REPLACEMENT,
                         G_TYPE_INVALID,
                         G_TYPE_UINT,   &result,
                         G_TYPE_INVALID);
if (!res)
  {
    if (error != NULL)
      {
        g_warning ("Failed to acquire %s: %s",
                   NAME_TO_CLAIM, error->message);
        g_error_free (error);
      }
    else
      {
        g_warning ("Failed to acquire %s", NAME_TO_CLAIM);
      }
    goto out;
  }

if (result != DBUS_REQUEST_NAME_REPLY_PRIMARY_OWNER)
  {
    if (error != NULL)
      {
        g_warning ("Failed to acquire %s: %s",
                   NAME_TO_CLAIM, error->message);
        g_error_free (error);
      }
    else
      {
        g_warning ("Failed to acquire %s", NAME_TO_CLAIM);
      }
    exit (1);
  }

dbus_g_proxy_add_signal (system_bus_proxy, "NameLost",
                         G_TYPE_STRING, G_TYPE_INVALID);
dbus_g_proxy_connect_signal (system_bus_proxy, "NameLost",
                             G_CALLBACK (on_name_lost), NULL, NULL);

/* further setup ... */

While you can do things this way with GDBus too, using g_dbus_proxy_call_sync(), it is much nicer to use the high-level API for this:

static void
on_name_acquired (GDBusConnection *connection,
                  const gchar     *name,
                  gpointer         user_data)
{
  /* further setup ... */
}

/* ... */

  owner_id = g_bus_own_name (G_BUS_TYPE_SYSTEM,
                             NAME_TO_CLAIM,
                             G_BUS_NAME_OWNER_FLAGS_ALLOW_REPLACEMENT,
                             on_bus_acquired,
                             on_name_acquired,
                             on_name_lost,
                             NULL,
                             NULL);

  g_main_loop_run (loop);

  g_bus_unown_name (owner_id);

Note that g_bus_own_name() works asynchronously and requires you to enter your mainloop to await the on_name_acquired() callback. Also note that in order to avoid race conditions (e.g. when your service is activated by a method call), you have to export your manager object before acquiring the name. The on_bus_acquired() callback is the right place to do such preparations.

Creating proxies for well-known names

dbus-glib lets you create proxy objects for well-known names, like the following example:

proxy = dbus_g_proxy_new_for_name (system_bus_connection,
                                   "org.freedesktop.Accounts",
                                   "/org/freedesktop/Accounts",
                                   "org.freedesktop.Accounts");

For a DBusGProxy constructed like this, method calls will be sent to the current owner of the name, and that owner can change over time.

The same can be achieved with GDBusProxy:

error = NULL;
proxy = g_dbus_proxy_new_for_bus_sync (G_BUS_TYPE_SYSTEM,
                                       G_DBUS_PROXY_FLAGS_NONE,
                                       NULL, /* GDBusInterfaceInfo */
                                       "org.freedesktop.Accounts",
                                       "/org/freedesktop/Accounts",
                                       "org.freedesktop.Accounts",
                                       NULL, /* GCancellable */
                                       &error);

For an added layer of safety, you can specify what D-Bus interface the proxy is expected to conform to by using the GDBusInterfaceInfo type. Additionally, GDBusProxy loads, caches and tracks changes to the D-Bus properties on the remote object. It also sets up match rules so D-Bus signals from the remote object are delivered locally.

The GDBusProxy type normally isn’t used directly - instead proxies subclassing GDBusProxy generated by gdbus-codegen is used, see the section called “Using gdbus-codegen”.

Generating code and docs

Using gdbus-codegen

dbus-glib comes with dbus-binding-tool, which can produce somewhat nice client- and server-side wrappers for a D-Bus interface. With GDBus, gdbus-codegen is used and like its counterpart, it also takes D-Bus Introspection XML as input:

Example D-Bus Introspection XML

<node>
  <!-- org.gtk.GDBus.Example.ObjectManager.Animal:
       @short_description: Example docs generated by gdbus-codegen
       @since: 2.30

       This D-Bus interface is used to describe a simple animal.
    -->
  <interface name="org.gtk.GDBus.Example.ObjectManager.Animal">
    <!-- Mood: The mood of the animal.
         @since: 2.30

         Known values for this property include
         <literal>Happy</literal> and <literal>Sad</literal>. Use the
         org.gtk.GDBus.Example.ObjectManager.Animal.Poke() method to
         change this property.

         This property influences how often the animal jumps up and
         down, see the
         #org.gtk.GDBus.Example.ObjectManager.Animal::Jumped signal
         for more details.
    -->
    <property name="Mood" type="s" access="read"/>

    <!--
        Poke:
        @make_sad: Whether to make the animal sad.
        @make_happy: Whether to make the animal happy.
        @since: 2.30

        Method used to changing the mood of the animal. See also the
        #org.gtk.GDBus.Example.ObjectManager.Animal:Mood property.
      -->
    <method name="Poke">
      <arg direction="in" type="b" name="make_sad"/>
      <arg direction="in" type="b" name="make_happy"/>
    </method>

    <!--
        Jumped:
        @height: Height, in meters, that the animal jumped.
        @since: 2.30

        Emitted when the animal decides to jump.
      -->
    <signal name="Jumped">
      <arg type="d" name="height"/>
    </signal>

    <!--
        Foo:
        Property with no <quote>since</quote> annotation (should inherit the 2.30 from its containing interface).
      -->
    <property name="Foo" type="s" access="read"/>

    <!--
        Bar:
        @since: 2.36
        Property with a later <quote>since</quote> annotation.
      -->
    <property name="Bar" type="s" access="read"/>
  </interface>

  <!-- org.gtk.GDBus.Example.ObjectManager.Cat:
       @short_description: More example docs generated by gdbus-codegen

       This D-Bus interface is used to describe a cat. Right now there
       are no properties, methods or signals associated with this
       interface so it is essentially a <ulink
       url="http://en.wikipedia.org/wiki/Marker_interface_pattern">Marker
       Interface</ulink>.

       Note that D-Bus objects implementing this interface also
       implement the #org.gtk.GDBus.Example.ObjectManager.Animal
       interface.
    -->
  <interface name="org.gtk.GDBus.Example.ObjectManager.Cat">
  </interface>
</node>

If this XML is processed like this

gdbus-codegen --interface-prefix org.gtk.GDBus.Example.ObjectManager. \
              --generate-c-code generated-code                        \
              --c-namespace Example                       \
              --c-generate-object-manager                 \
              --generate-docbook generated-docs                       \
              gdbus-example-objectmanager.xml

then two files generated-code.h and generated-code.c are generated. Additionally, two XML files generated-docs-org.gtk.GDBus.Example.ObjectManager.Animal and generated-docs-org.gtk.GDBus.Example.ObjectManager.Cat with Docbook XML are generated.

While the contents of generated-code.h and generated-code.c are best described by the gdbus-codegen manual page, here’s a brief example of how this generated code can be used:

#include "gdbus-object-manager-example/objectmanager-gen.h"

/* ---------------------------------------------------------------------------------------------------- */

static GDBusObjectManagerServer *manager = NULL;

static gboolean
on_animal_poke (ExampleAnimal          *animal,
                GDBusMethodInvocation  *invocation,
                gboolean                make_sad,
                gboolean                make_happy,
                gpointer                user_data)
{
  if ((make_sad && make_happy) || (!make_sad && !make_happy))
    {
      g_dbus_method_invocation_return_dbus_error (invocation,
                                                  "org.gtk.GDBus.Examples.ObjectManager.Error.Failed",
                                                  "Exactly one of make_sad or make_happy must be TRUE");
      goto out;
    }

  if (make_sad)
    {
      if (g_strcmp0 (example_animal_get_mood (animal), "Sad") == 0)
        {
          g_dbus_method_invocation_return_dbus_error (invocation,
                                                      "org.gtk.GDBus.Examples.ObjectManager.Error.SadAnimalIsSad",
                                                      "Sad animal is already sad");
          goto out;
        }

      example_animal_set_mood (animal, "Sad");
      example_animal_complete_poke (animal, invocation);
      goto out;
    }

  if (make_happy)
    {
      if (g_strcmp0 (example_animal_get_mood (animal), "Happy") == 0)
        {
          g_dbus_method_invocation_return_dbus_error (invocation,
                                                      "org.gtk.GDBus.Examples.ObjectManager.Error.HappyAnimalIsHappy",
                                                      "Happy animal is already happy");
          goto out;
        }

      example_animal_set_mood (animal, "Happy");
      example_animal_complete_poke (animal, invocation);
      goto out;
    }

  g_assert_not_reached ();

 out:
  return G_DBUS_METHOD_INVOCATION_HANDLED;
}


static void
on_bus_acquired (GDBusConnection *connection,
                 const gchar     *name,
                 gpointer         user_data)
{
  ExampleObjectSkeleton *object;
  guint n;

  g_print ("Acquired a message bus connection\n");

  /* Create a new org.freedesktop.DBus.ObjectManager rooted at /example/Animals */
  manager = g_dbus_object_manager_server_new ("/example/Animals");

  for (n = 0; n < 10; n++)
    {
      gchar *s;
      ExampleAnimal *animal;

      /* Create a new D-Bus object at the path /example/Animals/N where N is 000..009 */
      s = g_strdup_printf ("/example/Animals/%03d", n);
      object = example_object_skeleton_new (s);
      g_free (s);

      /* Make the newly created object export the interface
       * org.gtk.GDBus.Example.ObjectManager.Animal (note
       * that @object takes its own reference to @animal).
       */
      animal = example_animal_skeleton_new ();
      example_animal_set_mood (animal, "Happy");
      example_object_skeleton_set_animal (object, animal);
      g_object_unref (animal);

      /* Cats are odd animals - so some of our objects implement the
       * org.gtk.GDBus.Example.ObjectManager.Cat interface in addition
       * to the .Animal interface
       */
      if (n % 2 == 1)
        {
          ExampleCat *cat;
          cat = example_cat_skeleton_new ();
          example_object_skeleton_set_cat (object, cat);
          g_object_unref (cat);
        }

      /* Handle Poke() D-Bus method invocations on the .Animal interface */
      g_signal_connect (animal,
                        "handle-poke",
                        G_CALLBACK (on_animal_poke),
                        NULL); /* user_data */

      /* Export the object (@manager takes its own reference to @object) */
      g_dbus_object_manager_server_export (manager, G_DBUS_OBJECT_SKELETON (object));
      g_object_unref (object);
    }

  /* Export all objects */
  g_dbus_object_manager_server_set_connection (manager, connection);
}

static void
on_name_acquired (GDBusConnection *connection,
                  const gchar     *name,
                  gpointer         user_data)
{
  g_print ("Acquired the name %s\n", name);
}

static void
on_name_lost (GDBusConnection *connection,
              const gchar     *name,
              gpointer         user_data)
{
  g_print ("Lost the name %s\n", name);
}


gint
main (gint argc, gchar *argv[])
{
  GMainLoop *loop;
  guint id;

  loop = g_main_loop_new (NULL, FALSE);

  id = g_bus_own_name (G_BUS_TYPE_SESSION,
                       "org.gtk.GDBus.Examples.ObjectManager",
                       G_BUS_NAME_OWNER_FLAGS_ALLOW_REPLACEMENT |
                       G_BUS_NAME_OWNER_FLAGS_REPLACE,
                       on_bus_acquired,
                       on_name_acquired,
                       on_name_lost,
                       loop,
                       NULL);

  g_main_loop_run (loop);

  g_bus_unown_name (id);
  g_main_loop_unref (loop);

  return 0;
}

This, on the other hand, is a client-side application using generated code:

#include "gdbus-object-manager-example/objectmanager-gen.h"

/* ---------------------------------------------------------------------------------------------------- */

static void
print_objects (GDBusObjectManager *manager)
{
  GList *objects;
  GList *l;

  g_print ("Object manager at %s\n", g_dbus_object_manager_get_object_path (manager));
  objects = g_dbus_object_manager_get_objects (manager);
  for (l = objects; l != NULL; l = l->next)
    {
      ExampleObject *object = EXAMPLE_OBJECT (l->data);
      GList *interfaces;
      GList *ll;
      g_print (" - Object at %s\n", g_dbus_object_get_object_path (G_DBUS_OBJECT (object)));

      interfaces = g_dbus_object_get_interfaces (G_DBUS_OBJECT (object));
      for (ll = interfaces; ll != NULL; ll = ll->next)
        {
          GDBusInterface *interface = G_DBUS_INTERFACE (ll->data);
          g_print ("   - Interface %s\n", g_dbus_interface_get_info (interface)->name);

          /* Note that @interface is really a GDBusProxy instance - and additionally also
           * an ExampleAnimal or ExampleCat instance - either of these can be used to
           * invoke methods on the remote object. For example, the generated function
           *
           *  void example_animal_call_poke_sync (ExampleAnimal  *proxy,
           *                                      gboolean        make_sad,
           *                                      gboolean        make_happy,
           *                                      GCancellable   *cancellable,
           *                                      GError        **error);
           *
           * can be used to call the Poke() D-Bus method on the .Animal interface.
           * Additionally, the generated function
           *
           *  const gchar *example_animal_get_mood (ExampleAnimal *object);
           *
           * can be used to get the value of the :Mood property.
           */
        }
      g_list_free_full (interfaces, g_object_unref);
    }
  g_list_free_full (objects, g_object_unref);
}

static void
on_object_added (GDBusObjectManager *manager,
                 GDBusObject        *object,
                 gpointer            user_data)
{
  gchar *owner;
  owner = g_dbus_object_manager_client_get_name_owner (G_DBUS_OBJECT_MANAGER_CLIENT (manager));
  g_print ("Added object at %s (owner %s)\n", g_dbus_object_get_object_path (object), owner);
  g_free (owner);
}

static void
on_object_removed (GDBusObjectManager *manager,
                   GDBusObject        *object,
                   gpointer            user_data)
{
  gchar *owner;
  owner = g_dbus_object_manager_client_get_name_owner (G_DBUS_OBJECT_MANAGER_CLIENT (manager));
  g_print ("Removed object at %s (owner %s)\n", g_dbus_object_get_object_path (object), owner);
  g_free (owner);
}

static void
on_notify_name_owner (GObject    *object,
                      GParamSpec *pspec,
                      gpointer    user_data)
{
  GDBusObjectManagerClient *manager = G_DBUS_OBJECT_MANAGER_CLIENT (object);
  gchar *name_owner;

  name_owner = g_dbus_object_manager_client_get_name_owner (manager);
  g_print ("name-owner: %s\n", name_owner);
  g_free (name_owner);
}

static void
on_interface_proxy_properties_changed (GDBusObjectManagerClient *manager,
                                       GDBusObjectProxy         *object_proxy,
                                       GDBusProxy               *interface_proxy,
                                       GVariant                 *changed_properties,
                                       const gchar *const       *invalidated_properties,
                                       gpointer                  user_data)
{
  GVariantIter iter;
  const gchar *key;
  GVariant *value;
  gchar *s;

  g_print ("Properties Changed on %s:\n", g_dbus_object_get_object_path (G_DBUS_OBJECT (object_proxy)));
  g_variant_iter_init (&iter, changed_properties);
  while (g_variant_iter_next (&iter, "{&sv}", &key, &value))
    {
      s = g_variant_print (value, TRUE);
      g_print ("  %s -> %s\n", key, s);
      g_variant_unref (value);
      g_free (s);
    }
}

gint
main (gint argc, gchar *argv[])
{
  GDBusObjectManager *manager;
  GMainLoop *loop;
  GError *error;
  gchar *name_owner;

  manager = NULL;
  loop = NULL;

  loop = g_main_loop_new (NULL, FALSE);

  error = NULL;
  manager = example_object_manager_client_new_for_bus_sync (G_BUS_TYPE_SESSION,
                                                            G_DBUS_OBJECT_MANAGER_CLIENT_FLAGS_NONE,
                                                            "org.gtk.GDBus.Examples.ObjectManager",
                                                            "/example/Animals",
                                                            NULL, /* GCancellable */
                                                            &error);
  if (manager == NULL)
    {
      g_printerr ("Error getting object manager client: %s", error->message);
      g_error_free (error);
      goto out;
    }

  name_owner = g_dbus_object_manager_client_get_name_owner (G_DBUS_OBJECT_MANAGER_CLIENT (manager));
  g_print ("name-owner: %s\n", name_owner);
  g_free (name_owner);

  print_objects (manager);

  g_signal_connect (manager,
                    "notify::name-owner",
                    G_CALLBACK (on_notify_name_owner),
                    NULL);
  g_signal_connect (manager,
                    "object-added",
                    G_CALLBACK (on_object_added),
                    NULL);
  g_signal_connect (manager,
                    "object-removed",
                    G_CALLBACK (on_object_removed),
                    NULL);
  g_signal_connect (manager,
                    "interface-proxy-properties-changed",
                    G_CALLBACK (on_interface_proxy_properties_changed),
                    NULL);

  g_main_loop_run (loop);

 out:
  if (manager != NULL)
    g_object_unref (manager);
  if (loop != NULL)
    g_main_loop_unref (loop);

  return 0;
}