c++ gdbus - how to fetch the property of interface using gdbus library?

Question

I would like to know the approach/code snippet to fetch the property from a dbus interface using c++ code snippet.

I have tried the following approaches with error.

Approach#1 using g_dbus_proxy_get_cached_property but it is always returning null

ifproxy = g_dbus_proxy_new_for_bus_sync (G_BUS_TYPE_SYSTEM,
                                       flags,
                                       NULL,
                                       "org.freedesktop.NetworkManager",
                                       "org/freedesktop/NetworkManager/Device/0",
                                       "org.freedesktop.NetworkManager.Device",
                                       NULL, 
                                       &error);
ret = g_dbus_proxy_get_cached_property(ifproxy, "State")

Approach#2 g_dbus_proxy_call_sync - this one says "org.freedesktop.networkmanager" isn't exported (or may not exist), can't access property "Interface"

ifproxy = g_dbus_proxy_new_for_bus_sync (G_BUS_TYPE_SYSTEM,
                                             G_DBUS_PROXY_FLAGS_NONE,
                                             NULL,
                                             "org.freedesktop.NetworkManager",
                                            "org/freedesktop/NetworkManager/Device/0",
                                             "org.freedesktop.DBus.Properties",
                                             NULL, NULL);
    g_assert (ifproxy);

    /* Get the object path of the Connection details */
    ret = g_dbus_proxy_call_sync (ifproxy,
                                  "Get",
                                  g_variant_new ("(ss)",
                                                "org/freedesktop/NetworkManager/Device/0",
                                                 "Interface"),
                                  G_DBUS_CALL_FLAGS_NONE, -1,
                                  NULL, &error);
    if (!ret) {
        g_dbus_error_strip_remote_error (error);
        g_warning ("Failed to get property: %s\n",
                   error->message);
        g_error_free (error);
        return;
    }

    name = g_variant_get_string(ret, NULL);

    //g_assert(ret != NULL);

    g_variant_get (ret, "s", &name);
    g_variant_unref (ret);

Answer 1

Thanks a lot @jku.

I was able to solve the issue of encoding. Sharing the working code below for others.

static void
list_connections (GDBusProxy *proxy)
{
    int i;
    GError *error = NULL;
    GVariant *ret, *ret1;
    char **paths;
    gchar *name;
    GDBusProxy *ifproxy;
    GDBusProxyFlags flags;

    /* Call ListConnections D-Bus method */
    ret = g_dbus_proxy_call_sync (proxy,
                                  "GetDevices",
                                  NULL,
                                  G_DBUS_CALL_FLAGS_NONE, -1,
                                  NULL, &error);
    if (!ret) {
        g_dbus_error_strip_remote_error (error);
        g_print ("ListConnections failed: %s\n", error->message);
        g_error_free (error);
        return;
    }

    g_variant_get (ret, "(^ao)", &paths);
    g_variant_unref (ret);

    flags = static_cast<GDBusProxyFlags> (G_DBUS_PROXY_FLAGS_DO_NOT_LOAD_PROPERTIES | G_DBUS_PROXY_FLAGS_DO_NOT_AUTO_START);

    for (i = 0; paths[i]; i++)
    {
        g_print ("%s\n", paths[i]);
        /*ifproxy = g_dbus_proxy_new_for_bus_sync (G_BUS_TYPE_SYSTEM,
                                           flags,
                                           NULL,
                                           "org.freedesktop.NetworkManager",
                                           paths[i],//"/org/freedesktop/NetworkManager/Devices/0"
                                           "org.freedesktop.NetworkManager.Device",
                                           NULL, 
                                           &error);

        //name = g_dbus_proxy_get_interface_name(ifproxy);
        ret = g_dbus_proxy_get_cached_property(ifproxy, "FirmwareVersion");*/

        ifproxy = g_dbus_proxy_new_for_bus_sync (G_BUS_TYPE_SYSTEM,
                                                 G_DBUS_PROXY_FLAGS_NONE,
                                                 NULL,
                                                 "org.freedesktop.NetworkManager",
                                                 "/org/freedesktop/NetworkManager/Devices/0",
                                                 "org.freedesktop.DBus.Properties",
                                                 NULL, NULL);
        g_assert (ifproxy);

        ret = g_dbus_proxy_call_sync (ifproxy,
                                      "Get",
                                      g_variant_new ("(ss)",
                                                    "org.freedesktop.NetworkManager.Device",
                                                     "Interface"),
                                      G_DBUS_CALL_FLAGS_NONE, -1,
                                      NULL, &error);
        if (!ret) {
            g_dbus_error_strip_remote_error (error);
            g_warning ("Failed to get property: %s\n",
                       error->message);
            g_error_free (error);
            return;
        }

         g_print("\nType String of Variant:- %s\n", g_variant_get_type_string (ret));

        g_variant_get (ret, "(v)", &ret1);
        g_variant_unref (ret);

         g_print("\nType String of Variant:- %s\n", g_variant_get_type_string (ret1));
        g_variant_get (ret1, "s", &name);
        g_variant_unref (ret1);


        g_print ("Interface name:- %s\n", name);
    }
    g_strfreev (paths);
}

int
main (int argc, char *argv[])
{
    GDBusProxy *proxy;
    GDBusProxyFlags flags;
    GError *error = NULL;
    flags = static_cast<GDBusProxyFlags> (G_DBUS_PROXY_FLAGS_DO_NOT_LOAD_PROPERTIES | G_DBUS_PROXY_FLAGS_DO_NOT_AUTO_START);

#if !GLIB_CHECK_VERSION (2, 35, 0)
    /* Initialize GType system */
    g_type_init ();
#endif

    proxy = g_dbus_proxy_new_for_bus_sync (G_BUS_TYPE_SYSTEM,
                                           flags,
                                           NULL, /* GDBusInterfaceInfo */
                                           "org.freedesktop.NetworkManager",
                                           "/org/freedesktop/NetworkManager",
                                           "org.freedesktop.NetworkManager",
                                           NULL, /* GCancellable */
                                           &error);
    g_assert (proxy != NULL);

    /* List connections of system settings service */
    list_connections (proxy);

    g_object_unref (proxy);

    return 0;
}

Answer 2

At least the first form should work (assuming you handle errors in your real code) but there are issues with your D-Bus object paths.

First, valid object paths start with a '/' so you probably wanted /org/freedesktop/NetworkManager/Device/0... except that doesn't seem to be a path NetworkManager uses.

Looking at their API reference, it seems you may really want /org/freedesktop/NetworkManager/Devices/N (note the plural "Devices") but note that you can't really be sure what the last part of the path (N) is going to be. In proper code you should get the devices object path from org.freedesktop.NetworkManager but for debugging you might just use a tool like d-feet to find which objects paths are available and use those.