c - pthread_cond_broadcast 不起作用？

Question

我写了这个程序:

pthread_cond_t placeFootCondition;

pthread_mutex_t rf,lf;

void* rss(void* in){
    while(1){
        pthread_mutex_lock(&rf);
        printf ( "rss: waiting for condition\n" );
        pthread_cond_wait(&placeFootCondition,&rf);
        printf ( "                  Right Single Support \n" );
        sleep(1);
        pthread_mutex_unlock(&rf);

    }
}

void* lss(void* in){
    while(1){
        pthread_mutex_lock(&lf);
        printf ( "lss: waiting for condition\n" );
        pthread_cond_wait(&placeFootCondition,&lf);
        printf ( "Left Single Support \n" );
        sleep(1);
        pthread_mutex_unlock(&lf);

    } 
}


int main(){
    int rc;
    pthread_mutex_init(&rf,NULL);
    pthread_mutex_init(&lf,NULL);
    pthread_cond_init(&placeFootCondition,NULL);
    pthread_create(&t1,NULL,rss,NULL);
    pthread_create(&t2,NULL,lss,NULL);
    sleep(1);
    rc=pthread_cond_broadcast(&placeFootCondition);
    if(rc!=0) printf ( "rc=%i\n",rc );
    sleep(5);
}

但是程序的输出是

rss: waiting for condition
lss: waiting for condition
              Right Single Support 
rss: waiting for condition

pthread_cond_broadcast(&placeFootCondition) 不应该唤醒所有线程？？？

Answer 1

这是你正在做的:

pthread_cond_wait(&placeFootCondition,&rf); /* First thread uses rf mutex. */

pthread_cond_wait(&placeFootCondition,&lf); /* Second thread uses lf mutex. */

这是the standard说到使用不同的互斥量:

When a thread waits on a condition variable, having specified a particular mutex to either the pthread_cond_timedwait() or the pthread_cond_wait() operation, a dynamic binding is formed between that mutex and condition variable that remains in effect as long as at least one thread is blocked on the condition variable. During this time, the effect of an attempt by any thread to wait on that condition variable using a different mutex is undefined.

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最重要的是，在等待条件变量时，您应该在每个线程中使用相同的互斥体。