说明：  进程的创建。编制一段程序，使用系统调用fork()创建两个子进程，在此程序运 行时，系统中就有一个父进程和两个子进程在活动。让每一个进程在屏幕上显示一 个字符：父进程显示字符a，两个子进程分别显示字符b和子符c。
(The process of creation. The preparation of a program that uses system calls fork () to create two sub-processes, in the operation of this procedure Line, the system there is a parent and two child process activities. So that each process is displayed on the screen of a Characters: parent display characters a, respectively, two sub-processes and sub-character display character b c. )