调用pthread_cond_broadcast后为什么不调用pthread_cond_wait？

Question

The title basically speaks for itself.After sleeping for 5 sec and unlocking the mutex,it finally return from pthread_cond_wait. It seems like pthread_cond_wait knows that it can`t get the mutex,so it waits,after unlocking the mutex, pthread_cond_wait gets the mutex,then goes on.. is that so?

   #include <pthread.h>



pthread_mutex_t alarm_mutex = PTHREAD_MUTEX_INITIALIZER;
pthread_cond_t alarm_cond = PTHREAD_COND_INITIALIZER;
int s_i = 0;




void* alarm_thread ( void* arg )
{
    pthread_mutex_lock ( &alarm_mutex );

    while ( 1 )
    {
        while ( s_i == 1 )
        {
            pthread_cond_wait ( &alarm_cond, &alarm_mutex );
            printf("I am here");
        }
    }
}

int main ( int argc, char* argv[] )
{
    pthread_t thread;
    pthread_create ( &thread, NULL, alarm_thread, NULL );
    pthread_mutex_lock ( &alarm_mutex );
    s_i = 1;
    pthread_cond_broadcast ( &alarm_cond );
    sleep ( 5 );
    pthread_mutex_unlock ( &alarm_mutex );
}

Answer 1

The pthread_cond_wait function releases the mutex while it waits and then returns with the mutex acquired again. If another thread holds the mutex, then that thread can't resume execution until the mutex is released. If this causes a problem for you, you're using condition variables incorrectly.

The mutex needs to be released while the thread is waiting, otherwise other threads couldn't acquire it. The mutex needs to be re-acquired before returning -- if for no other reason, to allow the thread to check if it needs to call pthread_cond_wait again.