int
main(int argc, char **argv)
{
    int        fd[2], n;
    char    c;
    pid_t    childpid;

    Pipe(fd);        /* assumes a full-duplex pipe (e.g., svr4) */
    if ( (childpid = Fork()) == 0) {        /* child */
        sleep(3);
        if ( (n = Read(fd[0], &c, 1)) != 1){
        printf("child: read returned %d", n);
        exit(-1);
        }
            
        printf("child read %c\n", c);
        Write(fd[0], "c", 1); //子进程为什么没有退出？？
        exit(0);
    }
        /* 4parent */
    Write(fd[1], "p", 1);
    if ( (n = Read(fd[1], &c, 1)) != 1){
        printf("parent: read returned %d", n);
        exit(-1);
    }
        
    printf("parent read %c\n", c);
    exit(0);
}