c - 如果我们将 sigev_notify 用作 SIGEV_THREAD，为什么 timer_create 会生成线程

Question

根据 timer_create 函数，如果我们将 SIGEV_THREAD 用作 sigev_notify，那么每次到期都会创建新线程。但是为什么 timer_create 函数会生成新线程，在我的例子下面，

int main(){

  printf ("My process ID %d", getpid ());

  int status = 0;

  timer_t timer_id;

  memset (&timer_id, 0, sizeof (timer_t));

  int j = 10;

  struct itimerspec ts;

  struct sigevent se;

  se.sigev_notify = SIGEV_THREAD;

  se.sigev_value.sival_int = j;

  se.sigev_notify_function = timer_thread;

  ts.it_value.tv_sec = 3;

  ts.it_interval.tv_sec =0;

  status = timer_create (CLOCK_REALTIME, &se, &timer_id);

  printf ("timer_id is %ld\n",(long int)timer_id);

  assert (!status && "Create timer");

  status = timer_settime (timer_id, 0, &ts, 0);

  assert (!status && "Set timer");

return 0;

}

在此特定进程 ID 中运行的线程数是两个，

PID SPID TTY 时间命令

9945 9945 点/20 00:00:18 timer_create

9945 9946 点/20 00:00:00 timer_create

如果我们使用 SIGEV_SIGNAL ，timer_create 不会创建线程。

请有人告诉我为什么在 timer_create 函数期间创建线程(如果我们使用 SIGEV_THREAD)......？？

Answer 1

它需要一个辅助线程来满足SIGEV_THREAD 要求来调用另一个线程的回调，man sigevent :

SIGEV_THREAD

Notify the process by invoking sigev_notify_function "as if" it were the start function of a new thread. (Among the implementation possibilities here are that each timer notification could result in the creation of a new thread, or that a single thread is created to receive all notifications.) The function is invoked with sigev_value as its sole argument. If sigev_notify_attributes is not NULL, it should point to a pthread_attr_t structure that defines attributes for the new thread (see pthread_attr_init(3)).

如果您查看 timer_create 的 glibc 实现您可能会注意到它创建了一个线程来处理所有带有 SIGEV_THREAD 选项的计时器:

/* Create the helper thread.  */
pthread_once (&__helper_once, __start_helper_thread);