2010_01_06_cond.md

[pthread] 同步机制(3) -- cond

cond, condition (条件量)，某个线程拿到 mutex，但发现某些条件没达到（比如队列中无任务），则需要 block，直到条件达到位置，此时 cond 就有作用了。

• pthread_signal, 只唤醒所有等待 cond 线程中的一个。
• pthread_broadcast, 唤醒所有等待 cond 的线程。其实还是一个个唤醒，一个被唤醒，释放 mutex，然后另一个被唤醒。

cond 典型的用法如下：

pthread_mutex_lock(&m);
while ( cond is not ok )
{
    pthread_cond_wait(&cond, &m);
}
... do sth.
pthread_mutex_unlock(&m);

为何这里需要一个 while 呢？因为 pthread_broadcast 会唤醒所有等待 cond 的线程，比如队列中只有一个任务，而唤醒了三个线程，则只有一个线程能拿到任务，而另两个需要继续 wait on cond。

cond 也有 process-shared 的属性，意义不大，忽略了。而 clock 属性是实时系统(realtime)才用的，略过不表。

#include <stdio.h>
#include <unistd.h>
#include <time.h>
#include <pthread.h>

int global_val = 0;
pthread_mutex_t global_mutex;
pthread_cond_t  global_cond;

void *mythr(void *arg)
{
    printf("thr #%d, before lock, time = %d\n", (int)arg, time(NULL));
    pthread_mutex_lock(&global_mutex);
    while ( global_val > 0 )
    {
        printf("thr #%d, before cond wait, time = %d\n", (int)arg, time(NULL));
        pthread_cond_wait(&global_cond, &global_mutex);
    }
    global_val = 1;
    printf("thr #%d, cond, time = %d\n", (int)arg, time(NULL));
    pthread_mutex_unlock(&global_mutex);
    return (void *)0;
}

int main()
{
    pthread_t pid1, pid2, pid3, pid4;

    pthread_cond_init(&global_cond, NULL);
    pthread_mutex_init(&global_mutex, NULL);

    pthread_create(&pid1, NULL, mythr, (void *)1);
    pthread_create(&pid2, NULL, mythr, (void *)2);
    pthread_create(&pid3, NULL, mythr, (void *)3);
    pthread_create(&pid4, NULL, mythr, (void *)4);

    sleep(5);
    global_val = 0;
    pthread_cond_broadcast(&global_cond);

    sleep(5);
    global_val = 0;
    pthread_cond_signal(&global_cond);

    sleep(5);
    global_val = 0;
    pthread_cond_signal(&global_cond);

    pthread_join(pid1, NULL);
    pthread_join(pid2, NULL);
    pthread_join(pid3, NULL);
    pthread_join(pid4, NULL);

    return 0;
}

$ ./a.out 
thr #1, before lock, time = 1262783837
thr #3, before lock, time = 1262783837
thr #4, before lock, time = 1262783837
thr #2, before lock, time = 1262783837
thr #3, cond, time = 1262783837
thr #4, before cond wait, time = 1262783837
thr #2, before cond wait, time = 1262783837
thr #1, before cond wait, time = 1262783837

// broadcast, all wakeup
thr #4, cond, time = 1262783842
thr #2, before cond wait, time = 1262783842
thr #1, before cond wait, time = 1262783842

// signal, single wakeup
thr #2, cond, time = 1262783847

thr #1, cond, time = 1262783852