gupnp_service_proxy_send_action (proxy,
                                   "AddPortMapping", &error,
                                   /* In arguments */
                                   "NewRemoteHost", G_TYPE_STRING, "",
                                   "NewExternalPort", G_TYPE_UINT, external_port,
                                   "NewProtocol", G_TYPE_STRING, "TCP",
                                   "NewInternalPort", G_TYPE_UINT, internal_port,
                                   "NewInternalClient", G_TYPE_STRING, internal_host,
                                   "NewEnabled", G_TYPE_BOOLEAN, TRUE,
                                   "NewPortMappingDescription", G_TYPE_STRING, desc,
                                   "NewLeaseDuration", G_TYPE_UINT, 0,
                                   NULL,
                                   /* Out arguments */
                                   NULL);

Now, that really is quite tiresome. It basically means that you need to have the service reference to hand when coding, because you need to know the name and type of each argument. Luckily for DBus part of dbus-glib is a binding tool which can create type-safe wrappers so that making method calls is much easier. Wouldn't it be nice if there was something similar for GUPnP, which generated inline functions with prototypes like this:

static inline gboolean
AddPortMapping (GUPnPServiceProxy *proxy,
                char * in_NewRemoteHost,
                unsigned int in_NewExternalPort,
                char * in_NewProtocol,
                unsigned int in_NewInternalPort,
                char * in_NewInternalClient,
                gboolean in_NewEnabled,
                char * in_NewPortMappingDescription,
                unsigned int in_NewLeaseDuration,
                GError **error);

Well, now there is. I've put the initial code here but will be moving this into GUPnP itself shortly. The next task is to add asynchronous wrappers just as in dbus-glib, but that shouldn't be too hard.