Number of arguments in the gcallback function, and how to find out the name of an argument type

[uwescholz]

Hi,

I want to create a menu with a group of radio menu items with the help of gtk_action_group_add_radio_actions as shown in the GnomeUIInfo migration guide.

void
gtk_action_group_add_radio_actions (GtkActionGroup *action_group,
                                    const GtkRadioActionEntry *entries,
                                    guint n_entries,
                                    gint value,
                                    GCallback on_change,
                                    gpointer user_data);

It took long to find out that the callback function takes three arguments instead of two. Up to now I only saw that a gcallback function uses two arguments. But here the user data is received as the third argument:

void on_change(GtkAction *action, GtkRadioAction *item, gpointer *user_data) {...} 

I realized this in the testmerge.c example in the gtk sources. There was no other hint, also not in the documentation.

To this I have two questions:

1. How could I have find out that the callback function has three instead of two arguments when I want to use the user data? Is there a way to find this out?
2. I actually expected the user data to be passed on the second argument position. In this case it should have been of type “GVIEWER_WINDOW”. I realized that this was not the case by checking the type with IS_GVIEWER_WINDOW (user_data). But how could I have find out that the seconde argument is of type GTK_RADIO_ACTION? Is there a method to get the name of a pointer type? If yes, how is that done?

Thank you in advance
Uwe

[fmuellner (Florian Müllner)]

@uwescholz

There was no other hint, also not in the documentation.

It is in the documentation, albeit not in the most obvious way:

The “changed” signal of the first radio action is connected to the on_change callback

The documentation of the GtkRadioAction::changed signal then gives you the signature of the handler:

void
user_function (GtkRadioAction *action,
               GtkRadioAction *current,
               gpointer        user_data)

[uwescholz]

Oh, wow. Thank you. Yea, I could have seen that, but a direct link in the documentation to this place would have been helpfull.

Any idea to my other two questions, anyone?

[fmuellner (Florian Müllner)]

@uwescholz

1. How could I have find out that the callback function has three instead of two arguments when I want to use the user data? Is there a way to find this out?

Consult the documentation.

The first and last parameter are always the instance that emits the signal and the user_data pointer that was specified when connecting the handler(*), but as you learned signals can have additional parameters.

(*) usually in that order unless the G_CONNECT_SWAPPED flag was used, in which case they switch places

@uwescholz

2. But how could I have find out that the seconde argument is of type GTK_RADIO_ACTION? Is there a method to get the name of a pointer type?

There is no method for getting the name of an arbitrary pointer type, but you can find out the types of the additional signal parameters:

  GSignalQuery query;
  guint i;

  g_type_class_ref (GTK_TYPE_RADIO_ACTION);
  g_signal_query (g_signal_lookup ("changed", GTK_TYPE_RADIO_ACTION), &query);

  g_print ("[0]: %s\n", g_type_name (query.itype));
  for (i = 0; i < query.n_params; i++)
    g_print ("[%d]: %s\n", i + 1, g_type_name (query.param_types[i]));
  g_print ("[%d]: gpointer\n", i + 1);

But then consulting the documentation is generally faster, and hopefully also tell you what those parameters mean

[uwescholz]

Thank you, Florian. That’s the explanation I was looking for.

Have a nice weekend!

[StefanSalewski]

@fmuellner

find out the types of the additional signal parameters:

That is really interesting. It may be useful for language bindings creation, its a way to find out signal parameter types without direct gobject-introspection use.