此程序在运行的时候,产生一个子进程.此子进程每隔1秒钟,向父进程发送一个SIGUSR1信号,父进程收到信号后,打印当前系统时间.当经过10秒钟后,子进程结束运行,并发送SIGUSR2信号,通知父进程 父进程收到SIGUSR2信号后,退出.-this program in running time, have a child process. This process for every one seconds, father process to send a signal SIGUSR1 father, process signals, Print the current system time. When, after 10 seconds later, the child process ends, and send SIGUSR2 signal the father of the father of the process of notification process signals received SIGUSR2 withdrawal.