在c中使用条件变量

Question

我有一个用c++编写的多线程程序，它有3个线程、主线程、生产者线程和使用者线程。代码使用条件变量来处理信令和处理发出的信号。问题是，当我运行程序时，似乎有些发出的信号丢失了，没有被任何处理程序捕获。有人能给我解释一下这些输出吗？

全球变量：

static pthread_mutex_t mtx = PTHREAD_MUTEX_INITIALIZER;
static pthread_cond_t cond = PTHREAD_COND_INITIALIZER;

int avail = 0;

生产者线：

static void *producer_thread(void *arg)
{   
    for (size_t i = 0; i < 10; i++)
    {
        pthread_mutex_lock(&mtx);
        avail = 1;
    //    sleep(2);

        
        pthread_mutex_unlock(&mtx);

        pthread_cond_broadcast(&cond);

        printf("sent signal#%ld from signaling thread\n", i);
    }
    
    pthread_exit(NULL);
}

消费者线索：

static void *consumer_thread(void *arg)
{
    while (1)
    {
        pthread_mutex_lock(&mtx);
        
        while(0 == avail)
        {
            pthread_cond_wait(&cond, &mtx);
        }

        printf("received signal from consuming_thread\n");
        avail = 0;
        pthread_mutex_unlock(&mtx);
    }
}

主螺纹：

int main(int argc, char *argv[])
{
    pthread_t tid1, tid2;
    int rc;

    rc = pthread_create(&tid1, NULL, consumer_thread, NULL);
    if(rc){
        printf("error in pthread_create form consuming_thread\n");
        exit(-1);
    }

    rc = pthread_create(&tid2, NULL, producer_thread, NULL);
    if(rc){
        printf("error in pthread_create form signalling thread\n");
        exit(-1);
    }

    pthread_exit(NULL);
}

样本输出：

sent signal#0 from signaling thread
received signal from consuming_thread
sent signal#1 from signaling thread
sent signal#2 from signaling thread
sent signal#3 from signaling thread
sent signal#4 from signaling thread
sent signal#5 from signaling thread
sent signal#6 from signaling thread
sent signal#7 from signaling thread
sent signal#8 from signaling thread
received signal from consuming_thread
sent signal#9 from signaling thread
received signal from consuming_thread

Answer 1

一个问题是，在将avail设置为1之前，生产者线程不会验证它是否为零。当条件广播时，这并不能保证等待的使用者线程立即被调度，甚至是第一个。当它被调度时，它仍然必须首先锁定它在开始等待时放弃的互斥锁。在这种情况发生之前，生产者的循环完全有可能运行几次。

如果您想在生产之后强制消费，您应该让生产者检查是否已经使用了avail-able东西，例如编写如下循环：

for (size_t i = 0; i < 10; i++)
{    
    int proceed = 0;
    while (!proceed) {
        pthread_mutex_lock(&mtx);
        if (avail == 0) {
            avail = 1;
            proceed = 1;
        }
        pthread_mutex_unlock(&mtx);
    }
    pthread_cond_broadcast(&cond);
    printf("sent signal#%ld from signaling thread\n", i);
}