GSource callback

[Krzysztof <kj limes com pl>]

The documentation for Glib function g_source_set_callback () tells this:
"The exact type of func depends on the type of source; ie. you should not count on func being called with data as its first parameter."
What does it exactly mean? The callback function has the type:

gboolean (*GSourceFunc) (gpointer user_data);

So what should I expect as a parameter when glib calls my callback function with a gpointer data. How can I find my data?

-- 
Regards
Krzysztof J.