Question

我对C语言中的系统编程非常陌生，我使用pthreads创建两个线程，增加计数器并显示值。如果计数器值为偶数，则线程1递增，如果计数器值为奇数，则线程2递增。

pthread_mutex_t count_mutex     = PTHREAD_MUTEX_INITIALIZER;
pthread_cond_t  condition_var   = PTHREAD_COND_INITIALIZER;

void *functionCount1();
void *functionCount2();
int  count = 0;
#define COUNT_DONE  2

int main()
{
   pthread_t thread1, thread2;

   pthread_create( &thread1, NULL, &functionCount1, NULL);
   pthread_create( &thread2, NULL, &functionCount2, NULL);

   pthread_join( thread1, NULL);
   pthread_join( thread2, NULL);

   printf("Final count: %d\n",count);

   exit(0);
}


void *functionCount1()
{
    for(;;)
    {
        pthread_mutex_lock( &count_mutex );

        pthread_cond_wait( &condition_var, &count_mutex );
        count++;
        printf("Counter value functionEven: %d\n",count);

        pthread_mutex_unlock( &count_mutex );

        if(count >= COUNT_DONE) {
            printf("Thread 1: Exit condition reached.\n");
            return(NULL); 

        }
    }
}

void *functionCount2()
{
    for(;;)
    {
        pthread_mutex_lock( &count_mutex );

        if( count % 2 == 0)
        {
            pthread_cond_signal( &condition_var );
        }
        else
        {
            count++;
            printf("Counter value functionOdd: %d.\n",count); 
        }

        pthread_mutex_unlock( &count_mutex );

        if(count >= COUNT_DONE) {
            printf("Thread 2: Exit condition reached.\n");
            return(NULL); 
        }
    }

}

从我可以观察到的问题是，线程2首先到达它的存在状态并且它使线程1挂起。

Counter value functionEven: 1
Counter value functionOdd: 2.
Thread 2: Exit condition reached.
(This is where it hangs, I have to press Ctrl-C to end)^C

当线程2到达其退出状态时，我尝试了另一个锁，只有当线程1存在条件被调用时才解锁它。但这似乎没有做任何事情。

pthread_mutex_t mutex2 = PTHREAD_MUTEX_INITIALIZER;

if(count >= COUNT_DONE) {
    printf("Thread 1: Exit condition reached.\n");
    pthread_mutex_unlock( &mutex2 );
    return(NULL); 
}

if(count >= COUNT_DONE) {
    pthread_mutex_lock( &mutex2 );
    printf("Thread 2: Exit condition reached.\n");
    return(NULL); 
}

适当计数的最终解决方案。

下面的答案帮助了很多，它解决了主要问题。

pthread_cond_wait( &condition_var, &count_mutex );
if(count >= COUNT_DONE) {
    return(NULL); 
}
count++;
printf("Counter value functionEven: %d\n",count);

Answer 1

在退出functionCount2之前，您必须按如下方式发出条件变量的信号;

    if(count >= COUNT_DONE) {
        printf("Thread 2: Exit condition reached.\n");
        pthread_cond_signal( &condition_var );
        return(NULL);
    }

C Pthread：可能的死锁，线程1在线程2退出时挂起

1 个答案: