如果不是所有的字符都能够在函数调用时排队，则最终的返回值是发送的字符数。
If you call the above code repeatedly, this count may reach the backout threshold configured on the destination, at which point the messages will be re-queued and no longer available for receipt.如果重复调用上述代码，此计数可能会达到目的地上配置的搁置阈值，此时这些消息就将重新进行排队，暂时不再能进行接收。
If multiple threads attempt to lock a semaphore after the initial call above, they block and their requests are queued until the pthread_mutex_unlock call is performed.如果多个线程在完成上面的初始调用后尝试锁定信号量，则这些线程会被阻塞，它们的请求进入队列，直到执行pthread _ mutex_unlock调用。
