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 fa