c - 这个 pthread_cond_wait() 示例如何工作？

Question

我从这个网站得到了下面的代码:

https://computing.llnl.gov/tutorials/pthreads/#Abstract

This simple example code demonstrates the use of several Pthread condition variable routines. The main routine creates three threads. Two of the threads perform work and update a "count" variable. The third thread waits until the count variable reaches a specified value.

我的问题是 - 下面的代码如何确保两个工作线程之一在观察线程锁定互斥锁之前不锁定互斥锁？如果发生这种情况，观察者线程将被锁定并且 pthread_cond_wait(&count_threshold_cv, &count_mutex) 永远不会被调用？

我假设 pthread_create() 实际上也开始了线程。这是可行的唯一原因，因为观察者线程的 pthread_create() 在两个工作线程的 pthread_create() 之前开始？!当然这不是铸铁的，调度可能会导致工作线程在观察者线程之前开始？甚至编译器也可能重新排序这些代码行？

#include <pthread.h>
#include <stdio.h>
#include <stdlib.h>

#define NUM_THREADS  3
#define TCOUNT 10
#define COUNT_LIMIT 12

int     count = 0;
int     thread_ids[3] = {0,1,2};
pthread_mutex_t count_mutex;
pthread_cond_t count_threshold_cv;

void *inc_count(void *t) 
{
  int i;
  long my_id = (long)t;

  for (i=0; i<TCOUNT; i++) {
    pthread_mutex_lock(&count_mutex);
    count++;

    /* 
    Check the value of count and signal waiting thread when condition is
    reached.  Note that this occurs while mutex is locked. 
    */
    if (count == COUNT_LIMIT) {
      pthread_cond_signal(&count_threshold_cv);
      printf("inc_count(): thread %ld, count = %d  Threshold reached.\n", 
             my_id, count);
      }
    printf("inc_count(): thread %ld, count = %d, unlocking mutex\n", 
       my_id, count);
    pthread_mutex_unlock(&count_mutex);

    /* Do some "work" so threads can alternate on mutex lock */
    sleep(1);
    }
  pthread_exit(NULL);
}

void *watch_count(void *t) 
{
  long my_id = (long)t;

  printf("Starting watch_count(): thread %ld\n", my_id);

  /*
  Lock mutex and wait for signal.  Note that the pthread_cond_wait 
  routine will automatically and atomically unlock mutex while it waits. 
  Also, note that if COUNT_LIMIT is reached before this routine is run by
  the waiting thread, the loop will be skipped to prevent pthread_cond_wait
  from never returning. 
  */
  pthread_mutex_lock(&count_mutex);
  while (count<COUNT_LIMIT) {
    pthread_cond_wait(&count_threshold_cv, &count_mutex);
    printf("watch_count(): thread %ld Condition signal received.\n", my_id);
    count += 125;
    printf("watch_count(): thread %ld count now = %d.\n", my_id, count);
    }
  pthread_mutex_unlock(&count_mutex);
  pthread_exit(NULL);
}

int main (int argc, char *argv[])
{
  int i, rc;
  long t1=1, t2=2, t3=3;
  pthread_t threads[3];
  pthread_attr_t attr;

  /* Initialize mutex and condition variable objects */
  pthread_mutex_init(&count_mutex, NULL);
  pthread_cond_init (&count_threshold_cv, NULL);

  /* For portability, explicitly create threads in a joinable state */
  pthread_attr_init(&attr);
  pthread_attr_setdetachstate(&attr, PTHREAD_CREATE_JOINABLE);
  pthread_create(&threads[0], &attr, watch_count, (void *)t1);
  pthread_create(&threads[1], &attr, inc_count, (void *)t2);
  pthread_create(&threads[2], &attr, inc_count, (void *)t3);

  /* Wait for all threads to complete */
  for (i=0; i<NUM_THREADS; i++) {
    pthread_join(threads[i], NULL);
  }
  printf ("Main(): Waited on %d  threads. Done.\n", NUM_THREADS);

  /* Clean up and exit */
  pthread_attr_destroy(&attr);
  pthread_mutex_destroy(&count_mutex);
  pthread_cond_destroy(&count_threshold_cv);
  pthread_exit(NULL);

}

Answer 1

My question is- how does the below code ensure that one of the two worker threads doesn't lock on the >mutex before the watcher thread locks on it?

代码不需要确保这一点。它不依赖于调用 pthread_cond_wait() 的观察线程。

观察线程检查count<COUNT_LIMIT ，这是线程关心的实际情况 - 或者相反，当 count >= COUNT_LIMIT 时- 观察线程知道其他线程已完成。

pthread_cond_wait() 中使用的 pthread 条件变量只是在线程未完成的情况下才需要，因此观察线程可以进入休眠状态并唤醒以检查它关心的条件。

也就是说，这个例子看起来有点傻，不太清楚观察者线程想要通过 count += 125; 实现什么。