I seem to be missing something: the cod you posted contains an
unconditional
buf = _malloc_r(rptr, BUFSIZ);
which as I have understood is a replacement for
unsigned char buf[BUFSIZ];
Now, if the function is never called, the buffer is never allocated in
both cases. On the other hand, if the function gets called, in both
cases it will reserve 1024 bytes, either on the heap or the stack. I
don't see how your patch would entirely prevent the reservation of
1024 bytes in the case of unbuffered streams.