说明：  自己实现的sleep函数,通过alarm定时.由于mysleep是个子函数，对其调用者需考虑几种情况，即调用者在调用mysleep之前设置的闹钟时间未被超过和设置的闹钟时间被超过（同未设置闹钟），且要考虑调用者是否有未决信号及其是否屏蔽SIGALRM信号，因为这将影响mysleep函数的调用。
(of achieving his sleep function, through the alarm regularly. As mysleep function, is making its caller need to consider several circumstances, in that call mysleep call set up before the alarm clock time is not over and install an alarm clock time was over (the same Failure to put in alarm clock), and the call to consider whether there is outstanding and whether the signal shielding SIGALRM signal because this will affect mysleep function call.)