线程屏障是线程同步的一种方式. 线程执行操作后，可能需要等待其他线程完成操作. 这时，当前线程将被挂起，直到另一个线程也完成一个操作，最后所有线程都被唤醒. 屏障具有三个主要功能.

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 dont bother dealing with cancellation because the POSIX

spec for barriers doesnt mention that pthread_barrier_wait

is 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;

}

从代码中我们知道，障碍的本质是计数. 当尚未达到一定数量时pthread 线程挂起，当前线程被阻止. 当最后一个线程执行pthread_barrier_wait函数并获得一定数目时pthread 线程挂起，所有线程都被唤醒.

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