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From: argv@turnpike.Eng.Sun.COM (Dan Heller)
Newsgroups: comp.windows.x.motif
Subject: Re: Toggle Button Woes
Message-ID: <142439@sun.Eng.Sun.COM>
Date: 13 Sep 90 23:15:22 GMT
References: <270@sps.com>
Sender: news@sun.Eng.Sun.COM
Organization: O'Reilly && Associates
Lines: 68

In article <270@sps.com> arm@sps.com (Annette Myjak) writes:
> i want to add a toggle button to a standard selection box dialog.  (the
> documentation says i can add one child to the work area of a selection 
> box dialog.)  

You don't neven need to go that far (altho it's technically correct).
I have found that simply creating a new widget with the dialog widget
as its parent works perfectly for me.  This should probably go under
more testing (lest I get some feedback fromk someone), but it seems
reasonable (my only hisitation is that the dialog is a bulletinboard
widget and therefore needs to be better educated about the new toggle
button.

Nevertheless, I use something like:
    dialog = XmCreateDialog(...);
    XtVaCreateManagedWidget("toggle", xmToggleButtonGadgetClass, dialog,
	/* resource values */
	NULL);
Notice the "parent" is the dialog shell.

Moving along (since that really wasn't your question).

> so far so good.  the toggle button is there, and the procedure for the
> XmNvalueChangedCallback is getting executed.  
> 
> now the strange stuff.  when i added the callback, i used:
> 
> XtAddCallback (toggle, XmNvalueChangedCallback, procedure, global_val);
                                                             ^^^^^^^^^^
Notice: you used a "global", so the value of the variable at the time
the function is called will be used -- not the time the *callback*
is actually performed. "I know", you might be saying.  However, since
you are saying that the client_data (2nd) parameter of the callback
routine is showing 0 (not 71), I suspect that the global variable has 
not been initialized to 71 (as you seem to think).  By passing the
value of the global rather than the address of the variable, you are
in fact passing a constant to that function.  This constant will never
change throughout the life of the application.

Rule of thumb -- *never* pass variables *as* callback_data.
Always use the -address- of the variable (global or otherwise).
In addition to not having the problem you describe above, you can
pass the address of any global value--data structure, or char or int.
The _address_ of the variable will always work as the client_data
parameter.  However, you must realize that the client_data should
be declared appropriately:

    void
    callback_func(widget, variable, call_data)
	Widget widget;
	int *variable;
	caddr_t call_data;

This is assuming your global variable is an int, of course.
You must dereference it accordingly in the routine.

Lastly -- I recognize that I might be mistaken about my analysis
of your problem.  If you can tell me that you are actually doing:

    XtAddCallback(widget, XmNvalueChangedCallback, func, 71);

and the client_data for "func" is indeed 0, then you have a problem :-)

--
dan
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O'Reilly && Associates   argv@sun.com / argv@ora.com
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