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From: bjaspan@athena.mit.edu (Barr3y Jaspan)
Newsgroups: comp.windows.x
Subject: Re: Multiple Parameters to a Callback Function
Keywords: XtAddCallback ClientData Mutiple X11R4
Message-ID: <1990Mar12.180144.8218@athena.mit.edu>
Date: 12 Mar 90 18:01:44 GMT
References: <186@ria.ccs.uwo.ca>
Sender: news@athena.mit.edu (News system)
Reply-To: bjaspan@athena.mit.edu (Barr3y Jaspan)
Organization: Massachusetts Institute of Technology
Lines: 38

In article <186@ria.ccs.uwo.ca>, brown@csd.uwo.ca (Mike Brown) writes:
> 
>   We would like to pass multiple parameters to a function that has been 
>   registered with XtAddCallback.  Normally you can only pass one param
>   which then shows up as client_data.
> 
>   We've tried passing a pointer to an array of parameters (cast to
XtPointer),
>   but that failed - the contents are scrambled.

I sounds like you are doing something wrong.  I've passed pointers to allocated
structures as client_data to a callback before, and it worked perfectly --
after I very carefully made sure I was dereferencing everything in exactly
the right way.

Something like this should work:

{
    random_type *data;

    data = (random_type *) malloc(n*sizeof(random_type));
    magically_fill_in_array(data);

    XtAddCallback(widget, XtNcallback, cb_function, data);
}

void cb_function(w, call_data, client_data)
   Widget w;
   caddr_t *call_data, *client_data;
{
   print_random_type(((random_type *) client_data)[0]);
}

Note that I just typed this off the top of my head.  The point is that you
can pass ANY 4-byte value as client_data, even the address of an array.

Barry Jaspan, MIT-Project Athena
bjaspan@athena.mit.edu