操作系统实验模拟生产者消费者问题
PROGRAM NAME: PRODUCER_CONSUMER */
/* This program simulates two processes, producer which */
/* continues to produce message and put it into a buffer */
/* [implemented by PIPE], and consumer which continues to get */
/* message from the buffer and use it. */
/* The program also demonstrates the synchronism between */
/* processes and uses of PIPE.