用系统调用叉（）来创建子进程。运行时，系统一。

代码说明：

使用系统调用fork()创建两个子进程。当此程序运行时，在系统中有一个父进程和两个子进程活动。让每一个进程在屏幕上现实一个字符：父进程显示字符’a’；子进程分别显示字符’b’和’c’。试观察记录屏幕上的显示结果并分析原因。-use system calls fork () to create two child process. When running, the system is a process and the father of two sub-process activities. Each process in a realistic one screen characters : the father of the process show the character "a"; Child showed the characters" b "and" c ". Let observation records on the screen to show the results and analysis of the reasons.