调试程序发现起了一个子线程后，主线程上的sleep不生效了，看到这才明白...
— Function: unsigned int sleep (unsigned int seconds)
The
sleepfunction waits for seconds or until a signal is delivered, whichever happens first.If
sleepfunction returns because the requested interval is over, it returns a value of zero. If it returns because of delivery of a signal, its return value is the remaining time in the sleep interval.
参考：http://www.gnu.org/software/libc/manual/html_node/Sleeping.html