/* Align to the maximum size used by the swap optimization.  */
  _Alignas (uint64_t) char tmp[QSORT_STACK_SIZE];
  size_t total_size = total_elems * size;
  char *buf;

  if (size > INDIRECT_SORT_SIZE_THRES)
    total_size = 2 * total_elems * sizeof (void *) + size;

  if (total_size < sizeof buf)
    buf = tmp;
  else
    { /* allocate buf on heap ... */ }

Here "sizeof buf" is the size of a pointer, but (obviously?) we want QSORT_STACK_SIZE here, i.e. "sizeof tmp".