Question

When defining a callback proc in Xt (for example XtTimerCallbackProc), client_data is specified as an XtPointer. Is it safe to pass an int via client_data, rather than an actual pointer, and cast it back inside the procedure?

For example:

void foo(void) {
   ...
   int data = 1;
   XtAppAddTimeout(app_context, 1000, timer_cb, data);
   ...
}

void timer_cb(XtPointer client_data, XtIntervalId *timer)
{
   int my_data = (int) client_data;
   ...
}

Answer 1

It should be safe, but you should use an integer type that have the same size as a pointer.

The standard C99 type uintptr_t can be used for this purpose.

As per comments point out, int->ptr->int is not guaranteed by the use of this type. It should work more than often, but it is not as per specifications.

The alternative is to pass a pointer. If your app is single threaded, you might just do

static int data; and passs &data. If your app needs to be reentrant, then malloc an int.

As a matter of fact, I don’t think the standard support int->ptr->int in any way.