Function cleanup_handler

test/pthread/test_pthread_cancel.c:18–23

_Atomic bool done_cleanup = false;

void cleanup_handler(void *arg) {
  long a = (long)arg;
  emscripten_outf("Called clean-up handler with arg %ld", a);
  assert(a == 42);
  done_cleanup = true;
}

void *thread_start(void *arg) {
  pthread_cleanup_push(cleanup_handler, (void*)42);