c – 为什么在阅读和写入管道时需要关闭fds？

#include <stdio.h>
#include <unistd.h>
#include <sys/types.h>

int main(void)
{
        int     fd[2],nbytes;
        pid_t   childpid;
        char    string[] = "Hello,world!n";
        char    readbuffer[80];

        pipe(fd);

        if((childpid = fork()) == -1)
        {
                perror("fork");
                exit(1);
        }

        if(childpid == 0)
        {
                /* Child process closes up input side of pipe */
                close(fd[0]);

                /* Send "string" through the output side of pipe */
                write(fd[1],string,(strlen(string)+1));
            exit(0);
    }
    else
    {
            /* Parent process closes up output side of pipe */
            close(fd[1]);

            /* Read in a string from the pipe */
            nbytes = read(fd[0],readbuffer,sizeof(readbuffer));
            printf("Received string: %s",readbuffer);
    }

   return(0);

}

但是,如果我的一个进程需要持续写入管道,而另一个管道需要读取呢？

上面的例子似乎只适用于一次写入和一次读取.