RE: [Sheflug] stupid C question

[Ross Goodley <nospam [at] krisalis.co.uk>]

*(int *)user_data = 9;  ?

-----Original Message-----
From: José Luis Gómez Dans [mailto:nospam [at] sheffield.ac.uk]
Sent: Friday, November 03, 2000 02:00
To: sheflug [at] vuw.ac.nz
Subject: [Sheflug] stupid C question


Hi!
	This should be an easy one for seasoned C programmer. OK, I am
using GTK+ mixed with Fortran 95 in a project. The GTK+ bit and the
skeleton of the program is C, but I do use F95 routines to get the work
done. Due to this and the fact that the F95 compiler is _scary_ to say
the least (tends to corrupt some of the global variable space), I want
to minimize the number of global variables. So, without using global
variables (otherwise, solution would be easy!), let's start:

	In a function (interface.c), I define several local variables. I
then want to pass them on to event call-backs using
gtk_signal_connect(). As an example

gtk_signal_connect (GTK_OBJECT (interferogram_polar), "clicked",
		      GTK_SIGNAL_FUNC (on_interogram_polar_clicked),
		      		      NULL) ;
Where the NULL can be exchanged by a variable that will be passed on to
on_interferogram_polar_clicked(). The latter is defined as
void
on_select_ampl_polaristaion_changed    (GtkEditable     *editable,
                                        gpointer         user_data) ;
and the NULL (or variable) goes in user_data. 

	Let's assume that I pass an integer. I do this by having
&my_int_var instead of the NULL in the gtk_signal_connect function. I
then do some processing in the callback handler, and end up with a new
integer value that needs to update the value of my_int_var in the first
function. How can I set this value?

	The problem is that user_data is a gpointer (==void *), so I
cannot access it as I'd normally do with a pointer. This is illegal:
*user_data = 9 ; (say)

	I can set user_data to a value in memory, but as soon as the
callback function dies, the value is cleared:
user_data = &callback_int ;
so the value in the main function isn't updated.

	Graphically,
	
	main<------Update value------------|
	|                                  |
	|                                  |
	|----->callback->{change value}----|

	It must be possible to do this without a global variable!!!

	Thanks,
	José
-- 
José L Gómez Dans			PhD student
					Radar & Communications Group
					Department of Electronic Engineering
					University of Sheffield UK
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