编制一段程序，使用系统调用fork（）创建三个子进程，在各个子进程中再使用系统调用getpid（）和getppid（）获取当前进程和父进程的进程标识号并输出。-The preparation of a program, use the system call fork () to create three sub-processes, the various sub-processes re-use system call getpid () and getppid () Get the current process and the process ID parent process and output.