一. pthread_exit() 和 return的使用

The pthread_join() primitive, shown on line 24, is analogous to the fork-join wait() primitive. It blocks until the thread specified by the tid variable completes execution, either by invoking pthread_exit() or by returning from the thread’s top-level function.

The thread’s exit value will be stored through the pointer passed as the second argument to pthread_join(). The thread’s exit value is either the value passed to pthread_exit() or the value returned by the thread’s top-level function, depending on how the thread in question exits.

二.如果在pthread环境中要让多个线程在同一时刻同时获取同一把锁，如何实现？

What if I want several threads to hold the same lock at the same time?

The first thing you should do is to ask yourself why you would want to do
such a thing. If the answer is “because I have a lot of data that is read by many
threads, and only occasionally updated”, then POSIX reader-writer locks
might be what you are looking for. These are introduced in Section 4.2.4.
Another way to get the effect of multiple threads holding the same lock
is for one thread to acquire the lock, and then use pthread_create() to
create the other threads. The question of why this would ever be a good idea
is left to the reader.

三.poll的用法

poll(NULL, 0 , 毫秒数)；        //这种用法其实就是为了将当前线程block传入的毫秒数