Why is `pthread_testcancel` called multiple times in the `nanosleep` implementation of winpthreads?-CodePudding

The nanosleep implementation of winpthreads, which is a port of POSIX threads to Windows, calls pthread_testcancel multiple times in its implementation.

nanosleep in [nanosleep.c] calls pthread_delay_np_ms which is not exported and does the actual sleeping. This is the code of pthread_delay_np_ms in [thread.c].

int
pthread_delay_np_ms (DWORD to)
{
  struct _pthread_v *s = __pthread_self_lite ();

  if (!to)
    {
      pthread_testcancel ();
      Sleep (0);
      pthread_testcancel ();
      return 0;
    }
  pthread_testcancel ();
  if (s->evStart)
    WaitForSingleObject (s->evStart, to);
  else
    Sleep (to);
  pthread_testcancel ();
  return 0;
}

You can see pthread_testcancel is called multiple times for some reason.

What is the reason for the additional code while I think this could be as simple as follows?

int
pthread_delay_np_ms (DWORD to)
{
  Sleep (to);
  return 0;
}

What does pthread_testcancel achieve? Also, a side question is why is WaitForSingleObject preferred over Sleep when a dummy event handle is available?

CodePudding user response：

Another thread can call pthread_cancel in which case WaitForSingleObject will return immediately and the thread will be killed while your simple Sleep would keep a now pointless thread around for an unknown amount of time.

Why they are calling pthread_testcancel before Sleep(0) and if (s->evStart), I don't know. Maybe you should ask the authors directly if you care that much about it. I assume they are there for a reason...