c++ - 一个pthread程序在收到条件信号后无法跳出循环

Question

这是一个pthread代码，即使我设置了一个条件变量来等待信号，它也不能跳出while循环。

感谢任何帮助。

  #include <pthread.h>
  #include <iostream>
  using namespace std; 

  pthread_mutex_t myMutex = PTHREAD_MUTEX_INITIALIZER;

  int counter = 0; int WATCHCOUNT = 12 ;int totalCount = 10 ;
  int threadsID[3] = {0,1,2};
  pthread_cond_t cout_theashold_cv = PTHREAD_COND_INITIALIZER ; 

  void* watchCount(void *a) ;

  void* increaseCount(void *a)
 {
for (int i =0; i < totalCount ; ++i)
{
    pthread_mutex_lock(&myMutex);
    ++counter ;         

    cout << "I am thread " << *((int *)a) << " I increase count to " << counter << endl ;
    if (counter == WATCHCOUNT)
    {
        cout << "I am thread " << *((int *)a) << " I am before send signal with counter as " << counter << endl ;
        pthread_cond_signal(&cout_theashold_cv);
    }
    pthread_mutex_unlock(&myMutex);
  }
 } 

   int main()
   {
  pthread_t threads[3];
  pthread_create(&threads[0], NULL, increaseCount, (void *) &(threadsID[0]));
  pthread_create(&threads[1], NULL, increaseCount, (void *) &(threadsID[1]));   
  pthread_create(&threads[2], NULL, watchCount, (void *) &(threadsID[2]));
  for (int i = 0 ; i < 3 ; ++i)
  {
      pthread_join(threads[i], NULL);
  }
   return 0 ; 

 }
  // wait for cond var 
  void* watchCount(void *a)
  {
        pthread_mutex_lock(&myMutex);
        cout << "I am thread " << *((int *)a) << " I am watching count " << counter << endl ;

   //while (counter <= WATCHCOUNT)
   while (counter <= INT_MAX)
   {
        cout << "I am thread " << *((int *)a) << " before watch count current count is " << counter << endl ;
    pthread_cond_wait(&cout_theashold_cv, &myMutex);

    break ;

    cout << "I am thread " << *((int *)a) << " after watch count current count is " << counter << endl ;

}

cout << "I am thread " << *((int *)a) << " after loop and watch count current count is " << counter << endl ;

pthread_mutex_unlock(&myMutex);
}

Answer 1

线程 1 和 2 可能在线程 3 甚至开始等待条件变量之前就已完成。

这意味着它永远不会收到信号。

首先启动线程 3。在尝试启动 1 和 2 之前，确保它已经达到并正在等待条件。

你真正想要的是 counted semaphore (类似于条件变量，但它会记录 signal() 和 wait() 调用的次数)。如果它的信号多于等待，则线程不会停止。