说明：  实现进程的管道通信。用系统调用pipe( )建立一管道，二个子进程P1和P2分别向管道各写一句话： Child 1 is sending a message! Child 2 is sending a message! 父进程从管道中读出二个来自子进程的信息并显示（要求先接收P1，后P2）。
(process of the realization of the communication channel. With system call pipe () to establish a pipeline, the process of two-P1 and P2 respectively to the pipeline was a sentence : Child 1 is sending a message! Child 2 is sending a message! Father process from the pipeline read out from the two sub-process and display information (required first to receive P1, After P2).)