pthread_exit()用于线程退出，可以指定返回值，以便其他线程通过pthread_join（）函数获取该线程的返回值
return，是函数返回，不一定是线程函数哦！ 只有线程函数return，线程才会退出
exit（）是进程退出，如果在线程函数中调用exit，那改线程的进程也就挂了,会导致该线程所在进程的其他线程也挂掉，比较严重
下面是英文的解释，更专业：
http://www.opengroup.org/onlinep … s/pthread_exit.html
The pthread_exit() function shall terminate the calling thread and make the value value_ptr available to any successful join with the terminating thread. Any cancellation cleanup handlers that have been pushed and not yet popped shall be popped in the reverse order that they were pushed and then executed. After all cancellation cleanup handlers have been executed, if the thread has any thread-specific data, appropriate destructor functions shall be called in an unspecified order. Thread termination does not release any application visible process resources, including, but not limited to, mutexes and file descriptors, nor does it perform any process-level cleanup actions, including, but not limited to, calling any atexit() routines that may exist.
http://www.opengroup.org/onlinepubs/009695399/functions/exit.html
The exit() function shall first call all functions registered by atexit(), in the reverse order of their registration, except that a function is called after any previously registered functions that had already been called at the time it was registered. Each function is called as many times as it was registered. If, during the call to any such function, a call to the longjmp() function is made that would terminate the call to the registered function, the behavior is undefined.
If a function registered by a call to atexit() fails to return, the remaining registered functions shall not be called and the rest of the exit() processing shall not be completed. If exit() is called more than once, the behavior is undefined.
The exit() function shall then flush all open streams with unwritten buffered data, close all open streams, and remove all files created by tmpfile(). Finally, control shall be terminated with the consequences described below.