[Vala] Doubt with "hasemitter"

[rastersoft <raster rastersoft com>]

Hello:

I had some odd problems with a button callback: when it is clicked and 
try to operate with "this", I receive a core dump.

I discovered something odd: the documentation in Valadoc shows the 
button callback as

    [ HasEmitter ]
    public virtual signal void *clicked* ()

I created the callback like that, and it failed with the core dump. But 
if I define the callback as:

    public virtual signal void *clicked* (Gtk.Widget emitter)

it works fine.

So, what does mean the "hasemitter" attribute, what has to do with the 
callbacks with the "Gtk.Widget" parameter removed, and how can I know 
the true signature for these signals?

Thanks.