c - 管道：关闭管道数组中的文件描述符

问题描述

我现在正在为我的系统编程课程学习 Linux 和管道那些东西，我现在很难理解在管道数组中关闭文件描述符。

 // write the code to loop over the command line arguments (remember to skip the executable name)
for (int i = 1; i < argc; i++) {
    // call pipe before we fork
    if ((pipe(pipe_fd[i-1])) == -1) {
        perror("pipe");
        exit(1);
    }
    // call fork
    int result = fork();
    if (result < 0) {   // case: a system call error
        // handle the error
        perror("fork");
        exit(1);
    } else if (result == 0) {  // case: a child process
        // child does their work here
        // child only writes to the pipe so close reading end
        if (close(pipe_fd[i-1][0]) == -1) {
            perror("close reading end from inside child");
            exit(1);
        }
        // before we forked the parent had open the reading ends to
        // all previously forked children -- so close those
        int child_no;
        for (child_no = 0; child_no < i-1; child_no++) {
            if (close(pipe_fd[child_no][0]) == -1) {
                perror("close reading ends of previously forked children");
                exit(1);
            }
        }
        int len = strlen(argv[i]);
        // write len to the pipe as an integer
        if (write(pipe_fd[i-1][1], &len, sizeof(int)) != sizeof(int)) {
            perror("write from child to pipe");
            exit(1);
        }
        // I'm done with the pipe so close it
        if (close(pipe_fd[i-1][1]) == -1) {
            perror("close pipe after writing");
            exit(1);
        }
        // exit so I don't fork my own children on next loop iteration
        exit(0);
    } else {
        // in the parent but before doing the next loop iteration
        // close the end of the pipe that I don't want open
        if (close(pipe_fd[i-1][1]) == -1) {
            perror("close writing end of pipe in parent");
            exit(1);
        }
    }
}

我将列出我现在理解的内容：

我了解父子进程需要关闭那些他们不需要使用的 fd，在这种情况下子进程正在写入父进程，所以父进程需要关闭写入端口，子进程需要关闭读取端口。
我了解文件描述符在父进程和子进程之间共享。

上面的代码是从我的演讲幻灯片中给出的，我特别对一件事感到困惑。

在循环中，我观察到一旦这个孩子被 fork 创建，每个孩子都会关闭其阅读端口，执行此操作的代码是：

else if (result == 0) {  // case: a child process
    // child does their work here
    // child only writes to the pipe so close reading end
    if (close(pipe_fd[i-1][0]) == -1) {
        perror("close reading end from inside child");
        exit(1);
    }

从我目前的理解来看，每个孩子出生后都会关闭自己的阅读端口，我认为后一个孩子创建的应该不担心关闭以前的孩子的阅读端口。

但是在我阅读了这段代码后，我的理解似乎并不正确：

        // before we forked the parent had open the reading ends to
    // all previously forked children -- so close those
    int child_no;
    for (child_no = 0; child_no < i-1; child_no++) {
        if (close(pipe_fd[child_no][0]) == -1) {
            perror("close reading ends of previously forked children");
            exit(1);
        }
    }

我不明白为什么后面的孩子要关闭以前的孩子的阅读端口，一旦这些孩子被创建，那些阅读端口不是已经关闭了吗？

谢谢你的协助。:)

标签： clinuxpipesystem-calls

c - 管道：关闭管道数组中的文件描述符

问题描述

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