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，生产者线程和消费者线程。该代码使用条件变量来处理信号和处理发出的信号。问题是当我运行程序似乎有些发出的信号丢失并且没有被任何处理程序捕获。有人可以向我解释这些输出吗？

全局变量：

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

int avail = 0;

生产者线程：

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);
}

消费者线程：

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);
    }
}

主线程：

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);
}

示例输出：

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

一个问题是您的生产者线程在将 avail 设置为 1 之前不验证它是否为零。当条件广播时，不能保证等待的消费者线程立即甚至首先得到调度。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量。生产者的循环完全有可能在此之前运行几次。

如果你想在生产后强制消费，你应该让生产者检查 avail-able 的东西是否已经被消费，对于 example 通过写你的循环如下：

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);
}

在c中使用条件变量

using condition variable in c

c

multithreading

condition-variable