每日疑问，多线程场景，下面的代码是否可以保证只执行一次

看着这个代码的目的是只注册一次sighandler，但是这个判定是否可以保证一次？但是根据代码看，其实不会有关键区的读写，所以即使有同步问题，也不会产生什么特别的影响。所以代码没有问题，但是这是一个坑。

c 复制代码

int
__pthread_cancel (pthread_t th)
{
  volatile struct pthread *pd = (volatile struct pthread *) th;

  if (pd->tid == 0)
    /* The thread has already exited on the kernel side.  Its outcome
       (regular exit, other cancelation) has already been
       determined.  */
    return 0;

  static int init_sigcancel = 0;
  if (atomic_load_relaxed (&init_sigcancel) == 0)
    {
      struct sigaction sa;
      sa.sa_sigaction = sigcancel_handler;
      /* The signal handle should be non-interruptible to avoid the risk of
	 spurious EINTR caused by SIGCANCEL sent to process or if
	 pthread_cancel() is called while cancellation is disabled in the
	 target thread.  */
      sa.sa_flags = SA_SIGINFO | SA_RESTART;
      __sigemptyset (&sa.sa_mask);
      __libc_sigaction (SIGCANCEL, &sa, NULL);
      atomic_store_relaxed (&init_sigcancel, 1);
    }