c - pthread_cond_broadcast 不向线程发送信号

Question

我真的很难理解如何以允许线程同时运行的方式锁定和解锁互斥体。现在我试图让每个线程都做一些事情，然后等到所有线程都准备好，然后重复它。这应该反复发生，所以我把它都放在一个条件循环中。我的问题是，似乎我的广播从未被任何线程接收到，因为等待信号的每个线程都在永远等待，而发送信号的线程仍在继续。

我简化了我的代码，使其尽可能简单，同时仍可编译和运行:

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

#define num_threads 4

int repeat = 1;
int timesLooped = 0;
int waitingThreads = 0;
int threadNo = -1;
pthread_mutex_t mutex1;
pthread_cond_t condition;


//thread body
void* threadBody(void* arg){

   
   //sensitive info: requires lock
   if(pthread_mutex_lock(&mutex1)){ printf("error1\n"); }
     threadNo++;
     int threadId = threadNo;
     //other info is set here as well in my code
   if(pthread_mutex_unlock(&mutex1)){ printf("error2\n"); }

   //main loop in the thread body
   while(repeat == 1){

      if(pthread_mutex_lock(&mutex1)){ printf("error3\n"); }

      //wait until all threads are ready
      while(waitingThreads < num_threads - 1){

         printf(" %d! ", threadId);
         waitingThreads++;
         pthread_cond_wait(&condition, &mutex1);

         printf(" %d ", threadId);
         if(pthread_mutex_unlock(&mutex1)){ printf("error4\n"); }

      }
  
      //last thread will broadcast
      if(waitingThreads == num_threads - 1){

         printf("\n\nthread %d was last! broadcasting to let everyone proceed...", threadId);
         timesLooped++;
         waitingThreads = 0;
        if(timesLooped == 3){
            repeat = 0;
         }
sleep(1);
         pthread_cond_broadcast(&condition);
         if(pthread_mutex_unlock(&mutex1)){ printf("error5\n"); }
         

 


      }



   }
printf("\n\nexiting thread %d\n", threadId);
   pthread_exit((void*) arg);
}





int main(int argc, char** argv){

   pthread_t threads[num_threads];
   void* retval;

   //create threads
   for(long i = 0; i < num_threads; i++){
      pthread_create(&threads[i], NULL, threadBody, (void*) i);
   }

   //join threads
   for(long j = 0; j < num_threads; j++){
      pthread_join(threads[j], &retval);
   }


printf("\n\nDONE\n");
}

这给了我一个输出:

thread 3 was last! broadcasting to let everyone proceed...

thread 2 was last! broadcasting to let everyone proceed...

thread 1 was last! broadcasting to let everyone proceed...

exiting thread 1 (deadlock, other threads never exit)

Answer 1

你的程序中至少有两个错误。

1. 您有一场数据竞赛(参见 blog post)。

   当线程 0 到 num_threads - 1 访问 waitingThreads 时 if (waitingThreads == num_threads - 1) ...，他们在锁外这样做(即他们比赛)。

2. 您不允许除最后一个以外的线程运行(这是您的主要问题)。

   pthread_cond_wait 在两个条件都已发出信号时返回，并且可以重新获取互斥体。

   但是您的最后一个线程在释放它时立即重新获取互斥量，因此其他线程无法继续。

   我希望如果您在最后一个线程中的 pthread_mutex_unlock 之后添加 sleep 或 usleep，您的程序将按照您的预期开始工作.