C中的这个多管道代码是否有意义？

Question

几天我就提出了一个问题.我的解决方案符合接受的答案中的建议.但是,我的一个朋友提出了以下解决方案:

请注意,代码已更新几次(查看编辑修订版)以反映下面答案中的建议.如果您打算给出新的答案,请考虑这个新代码,而不是那些有很多问题的旧代码.

#include <stdio.h>
#include <stdlib.h>
#include <fcntl.h>
#include <unistd.h>

int main(int argc, char *argv[]){
    int fd[2], i, aux, std0, std1;

    do {
        std0 = dup(0); // backup stdin
        std1 = dup(1); // backup stdout

        // let's pretend I'm reading commands here in a shell prompt
        READ_COMMAND_FROM_PROMPT();

        for(i=1; i<argc; i++) {
            // do we have a previous command?
            if(i > 1) {
                dup2(aux, 0);
                close(aux);
            }

            // do we have a next command?
            if(i < argc-1) {
                pipe(fd);

                aux = fd[0];
                dup2(fd[1], 1);
                close(fd[1]);
            }

            // last command? restore stdout...
            if(i == argc-1) {
                dup2(std1, 1);
                close(std1);
            }

            if(!fork()) {
                // if not last command, close all pipe ends
                // (the child doesn't use them)
                if(i < argc-1) {
                    close(std0);
                    close(std1);
                    close(fd[0]);
                }

                execlp(argv[i], argv[i], NULL);
                exit(0);
            }
        }

        // restore stdin to be able to keep using the shell
        dup2(std0, 0);
        close(std0);
    }

    return 0;
}

这通过像bash这样的管道模拟一系列命令,例如:cmd1 | cmd2 | ...... | cmd_n.我说"模拟",因为,正如您所看到的,命令实际上是从参数中读取的.只是为业余时间编写一个简单的shell提示符......

当然,有一些问题需要修复,并添加像错误处理,但这不是重点.我觉得我有点得到代码,但它仍然让我很困惑这整个事情的工作原理.

我错过了什么或这真的有效,这是解决问题的一个很好的清洁解决方案吗？如果没有,有人能指出这个代码的关键问题吗？

Answer 1

看起来合理,虽然它确实需要修复泄漏std和aux孩子以及循环后,父母的原始文件stdin永远丢失.

颜色可能会更好......

./a.out foo bar baz <stdin >stdout
std = dup(stdout)     ||     |+==========================std
                      ||     ||                          ||
pipe(fd)              ||     ||    pipe1[0] -- pipe0[1]  ||
                      ||     ||       ||          ||     ||
aux = fd[0]           ||     ||      aux          ||     ||
                      ||     XX       ||          ||     ||
                      ||      /-------++----------+|     ||
dup2(fd[1], 1)        ||     //       ||          ||     ||
                      ||     ||       ||          ||     ||
close(fd[1])          ||     ||       ||          XX     ||
                      ||     ||       ||                 ||
fork+exec(foo)        ||     ||       ||                 ||
                      XX     ||       ||                 ||
                       /-----++-------+|                 ||
dup2(aux, 0)          //     ||       ||                 ||
                      ||     ||       ||                 ||
close(aux)            ||     ||       XX                 ||
                      ||     ||                          ||
pipe(fd)              ||     ||    pipe2[0] -- pipe2[1]  ||
                      ||     ||       ||          ||     ||
aux = fd[0]           ||     ||      aux          ||     ||
                      ||     XX       ||          ||     ||
                      ||      /-------++----------+|     ||
dup2(fd[1], 1)        ||     //       ||          ||     ||
                      ||     ||       ||          ||     ||
close(fd[1])          ||     ||       ||          XX     ||
                      ||     ||       ||                 ||
fork+exec(bar)        ||     ||       ||                 ||
                      XX     ||       ||                 ||
                       /-----++-------+|                 ||
dup2(aux, 0)          //     ||       ||                 ||
                      ||     ||       ||                 ||
close(aux)            ||     ||       XX                 ||
                      ||     ||                          ||
pipe(fd)              ||     ||    pipe3[0] -- pipe3[1]  ||
                      ||     ||       ||          ||     ||
aux = fd[0]           ||     ||      aux          ||     ||
                      ||     XX       ||          ||     ||
                      ||      /-------++----------+|     ||
dup2(fd[1], 1)        ||     //       ||          ||     ||
                      ||     ||       ||          ||     ||
close(fd[1])          ||     ||       ||          XX     ||
                      ||     XX       ||                 ||
                      ||      /-------++-----------------+|
dup2(std, 1)          ||     //       ||                 ||
                      ||     ||       ||                 ||
fork+exec(baz)        ||     ||       ||                 ||

• foo得到stdin=stdin,stdout=pipe1[1]
• bar得到stdin=pipe1[0],stdout=pipe2[1]
• baz得到stdin=pipe2[0],stdout=stdout

我的建议是不同的,因为它避免了修改父级,stdin并且stdout只在子级内操纵它们,并且永远不会泄漏任何FD.但是,图表有点难度.

for cmd in cmds
    if there is a next cmd
        pipe(new_fds)
    fork
    if child
        if there is a previous cmd
            dup2(old_fds[0], 0)
            close(old_fds[0])
            close(old_fds[1])
        if there is a next cmd
            close(new_fds[0])
            dup2(new_fds[1], 1)
            close(new_fds[1])
        exec cmd || die
    else
        if there is a previous cmd
            close(old_fds[0])
            close(old_fds[1])
        if there is a next cmd
            old_fds = new_fds

parent
    cmds = [foo, bar, baz]
    fds = {0: stdin, 1: stdout}

cmd = cmds[0] {
    there is a next cmd {
        pipe(new_fds)
            new_fds = {3, 4}
            fds = {0: stdin, 1: stdout, 3: pipe1[0], 4: pipe1[1]}
    }

    fork             => child
                        there is a next cmd {
                            close(new_fds[0])
                                fds = {0: stdin, 1: stdout, 4: pipe1[1]}
                            dup2(new_fds[1], 1)
                                fds = {0: stdin, 1: pipe1[1], 4: pipe1[1]}
                            close(new_fds[1])
                                fds = {0: stdin, 1: pipe1[1]}
                        }
                        exec(cmd)

    there is a next cmd {
        old_fds = new_fds
            old_fds = {3, 4}
    }
}

cmd = cmds[1] {
    there is a next cmd {
        pipe(new_fds)
            new_fds = {5, 6}
            fds = {0: stdin, 1: stdout, 3: pipe1[0], 4: pipe1[1],
                                        5: pipe2[0], 6: pipe2[1]}
    }

    fork             => child
                        there is a previous cmd {
                            dup2(old_fds[0], 0)
                                fds = {0: pipe1[0], 1: stdout,
                                       3: pipe1[0], 4: pipe1[1],
                                       5: pipe2[0], 6: pipe2[1]}
                            close(old_fds[0])
                                fds = {0: pipe1[0], 1: stdout,
                                                    4: pipe1[1],
                                       5: pipe2[0]  6: pipe2[1]}
                            close(old_fds[1])
                                fds = {0: pipe1[0], 1: stdout,
                                       5: pipe2[0], 6: pipe2[1]}
                        }
                        there is a next cmd {
                            close(new_fds[0])
                                fds = {0: pipe1[0], 1: stdout, 6: pipe2[1]}
                            dup2(new_fds[1], 1)
                                fds = {0: pipe1[0], 1: pipe2[1], 6: pipe2[1]}
                            close(new_fds[1])
                                fds = {0: pipe1[0], 1: pipe1[1]}
                        }
                        exec(cmd)

    there is a previous cmd {
        close(old_fds[0])
            fds = {0: stdin, 1: stdout,              4: pipe1[1],
                                        5: pipe2[0], 6: pipe2[1]}
        close(old_fds[1])
            fds = {0: stdin, 1: stdout, 5: pipe2[0], 6: pipe2[1]}
    }

    there is a next cmd {
        old_fds = new_fds
            old_fds = {3, 4}
    }
}

cmd = cmds[2] {
    fork             => child
                        there is a previous cmd {
                            dup2(old_fds[0], 0)
                                fds = {0: pipe2[0], 1: stdout,
                                       5: pipe2[0], 6: pipe2[1]}
                            close(old_fds[0])
                                fds = {0: pipe2[0], 1: stdout,
                                                    6: pipe2[1]}
                            close(old_fds[1])
                                fds = {0: pipe2[0], 1: stdout}
                        }
                        exec(cmd)

    there is a previous cmd {
        close(old_fds[0])
            fds = {0: stdin, 1: stdout,              6: pipe2[1]}
        close(old_fds[1])
            fds = {0: stdin, 1: stdout}
    }
}

编辑

您更新的代码确实修复了以前的FD泄漏...但是添加了一个:您现在正在泄漏std0给孩子们.正如乔恩所说,这对大多数程序来说可能并不危险......但是你仍然应该写一个比这更好的shell.

即使它是临时的,我强烈建议不要修改你自己的shell标准输入/输出/错误(0/1/2),只在exec之前的子进程中这样做.为什么？假设您printf在中间添加了一些调试,或者由于错误情况需要挽救.如果你不先清理乱糟糟的标准文件描述符,你就会遇到麻烦.请为了让事情在意外情况下按预期运行,在你需要之前不要捣乱.

编辑

正如我在其他评论中提到的那样,将其拆分为更小的部分会使其更容易理解.这个小助手应该很容易理解并且没有错误:

/* cmd, argv: passed to exec
 * fd_in, fd_out: when not -1, replaces stdin and stdout
 * return: pid of fork+exec child
 */
int fork_and_exec_with_fds(char *cmd, char **argv, int fd_in, int fd_out) {
    pid_t child = fork();
    if (fork)
        return child;

    if (fd_in != -1 && fd_in != 0) {
        dup2(fd_in, 0);
        close(fd_in);
    }

    if (fd_out != -1 && fd_in != 1) {
        dup2(fd_out, 1);
        close(fd_out);
    }

    execvp(cmd, argv);
    exit(-1);
}

应该这样:

void run_pipeline(int num, char *cmds[], char **argvs[], int pids[]) {
    /* initially, don't change stdin */
    int fd_in = -1, fd_out;
    int i;

    for (i = 0; i < num; i++) {
        int fd_pipe[2];

        /* if there is a next command, set up a pipe for stdout */
        if (i + 1 < num) {
            pipe(fd_pipe);
            fd_out = fd_pipe[1];
        }
        /* otherwise, don't change stdout */
        else
            fd_out = -1;

        /* run child with given stdin/stdout */
        pids[i] = fork_and_exec_with_fds(cmds[i], argvs[i], fd_in, fd_out);

        /* nobody else needs to use these fds anymore
         * safe because close(-1) does nothing */
        close(fd_in);
        close(fd_out);

        /* set up stdin for next command */
        fd_in = fd_pipe[0];
    }
}

你可以看到猛砸的execute_cmd.c#execute_disk_command被称为从execute_cmd.c#execute_pipeline,XSH的process.c#process_run被称为从jobs.c#job_run,甚至每一个人的BusyBox的各种 小 和 最小的 贝壳拆分起来.