[a1369209993]

This fails horribly if called recursively (or from a signal handler). You need something like:

  wrapped_qsort(/* array,callback */)
    {
    auto tmp = CBQ;
    CBQ = wrap(callback);
    qsort(array.ptr,array.len,cbq_callback);
    CBQ = tmp; /* pop old value from stack */
    }

[richardwhiuk]

It'll fail from a signal handler inside qsort, or inside the callback function.

It won't fail recursively - while the second call is happening, the first will be stored on the stack (see the take and replace in the callback shim function)