本文主要是介绍线程屏障（基于linuxthreads-2.3），希望对大家解决编程问题提供一定的参考价值，需要的开发者们随着小编来一起学习吧！
线程屏障是线程同步的一个方式。线程执行完一个操作后，可能需要等待其他线程也完成某个动作，这时候，当前该线程就会被挂起，直到其他线程也完成了某个操作，最后所有线程被唤醒。屏障主要有三个函数。
int
pthread_barrier_wait(pthread_barrier_t *barrier)
{pthread_descr self = thread_self();pthread_descr temp_wake_queue, th;int result = 0;__pthread_lock(&barrier->__ba_lock, self);/* If the required number of threads have achieved rendezvous... */// pthread_barrier_wait被调用的次数达到阈值，__ba_present + 1 == __ba_required if (barrier->__ba_present >= barrier->__ba_required - 1){/* ... then this last caller shall be the serial thread */result = PTHREAD_BARRIER_SERIAL_THREAD;/* Copy and clear wait queue and reset barrier. */// 被阻塞的线程队列temp_wake_queue = barrier->__ba_waiting;// 重置字段barrier->__ba_waiting = NULL;barrier->__ba_present = 0;}else{result = 0;// 执行pthread_barrier_wait一次，加一barrier->__ba_present++;// 插入等待队列enqueue(&barrier->__ba_waiting, self);}__pthread_unlock(&barrier->__ba_lock);// 调用pthread_barrier_wait的次数还不够if (result == 0){/* Non-serial threads have to suspend */// 挂起当前线程suspend(self);/* We don't bother dealing with cancellation because the POSIXspec for barriers doesn't mention that pthread_barrier_waitis a cancellation point. */}else{/* Serial thread wakes up all others. */// 唤醒其他的线程while ((th = dequeue(&temp_wake_queue)) != NULL)restart(th);}return result;
}int
pthread_barrier_init(pthread_barrier_t *barrier,const pthread_barrierattr_t *attr,unsigned int count)
{if (count == 0)return EINVAL;__pthread_init_lock(&barrier->__ba_lock);// 需要执行pthread_barrier_wait的次数barrier->__ba_required = count;// 已经调用pthread_barrier_wait的次数barrier->__ba_present = 0;// 调用pthread_barrier_wait被阻塞的线程队列barrier->__ba_waiting = NULL;return 0;
}int
pthread_barrier_destroy(pthread_barrier_t *barrier)
{// 有线程在等待if (barrier->__ba_waiting != NULL) return EBUSY;return 0;
}
从代码里我们知道，屏障的本质就是计数，还没有达到某个数的时候，当前线程就被阻塞，等到最后一个线程执行pthread_barrier_wait函数并且得到了某个数的时候，全部线程被唤醒。
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