Re: [gtk-list] casting func.

[Joshua Richardson <jric bu edu>]

> This is part C part GTK question.
> How can bot, f(GtkWidget *, gpointer) and g(GtkWidget *, GdkEvent *,
> gpointer) be cast to GdkSignalFunc which is typedefed to look like neither
> of the above but f()? Is that even legal from the point of view of C?

>From C's point of view, a function is a pointer (to a piece of code.)
In C you can explicitly cast any pointer to any other pointer type, but
the onus is on you to make sure that such wild casting doesn't get you
into trouble.  This is one of the main reasons some people hate C.

> Also, how are GtkSignalFunc's made use of internally? Also, it lookes like
> the latter kind is expected by GTK to return and int, but I define mine as
> void. Yet, nothing crashes. So how does all this work?

Each widget "knows" about its signals, and what kind of callbacks will be
attached to them.  This was accomplished in Gtk by associating each signal
with a "marshalling" function, which is the function that actually calls
your callback, and will do so with an appropriate set of arguments. It's
your job to make sure that your function (that you always cast to the
generic GtkSignalFunc type) will handle the arguments delivered by the
marshalling function.  The event related marshalling functions check for a
return value from your callback, as explained in the tutorial.  So, if
your function isn't returning a value (as a void function won't), then the
marshalling function is getting a random value "returned".  This, of
course, may or may not cause problems, depending on context, but obviously
should be avoided. :-)

> Pavel

Hope that helps (and is correct ;-)),  --josh

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                       Joshua Richardson
Dept. Cognitive and Neural Systems / Center for Adaptive Systems
                       Boston University
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