使用 Fork 和 Dup 的 Unix 管道

Question

假设在一个程序中我想执行两个进程，一个执行ls -al 命令，然后将结果传送到wc 命令，并在终端上显示输出。如何使用管道文件描述符来做到这一点？到目前为止我写的代码：

int main(int argc, char* argv[]) {
    int pipefd[2];
    int pipefd2[2];

    pipe(pipefd2);
    if ((fork()) == 0) {
       dup2(pipefd2[1], STDOUT_FILENO);
       close(pipefd2[0]);
       close(pipefd2[1]);
       execl("ls", "ls", "-al", NULL);
       exit(EXIT_FAILURE);
    } 

    if ((fork()) == 0){
        dup2(pipefd2[0], STDIN_FILENO);
        close(pipefd2[0]);
        close(pipefd2[1]);
        execl("/usr/bin/wc", "wc", NULL);
        exit(EXIT_FAILURE);
    }
    close(pipefd[0]);
    close(pipefd[1]);
    close(pipefd2[0]);
    close(pipefd2[1]);
}

举个例子会很有帮助。

Answer 1

您的示例代码在语法和语义上被破坏了（例如 pipefd2 没有被 decared，pipefd 和 pipefd2 之间的混淆等）因为这闻起来像家庭作业，请确保您理解我下面的注释，并在需要时询问更多信息。我省略了对 pipe、fork 和 dup 的错误检查，但理想情况下它们应该存在。

int main(int argc, char *argv[]) {
    int pipefd[2];
    pid_t ls_pid, wc_pid;

    pipe(pipefd);

    // this child is generating output to the pipe
    //
    if ((ls_pid = fork()) == 0) {
        // attach stdout to the left side of pipe
        // and inherit stdin and stdout from parent
        dup2(pipefd[1],STDOUT_FILENO);
        close(pipefd[0]);              // not using the right side

        execl("/bin/ls", "ls","-al", NULL);
        perror("exec ls failed");
        exit(EXIT_FAILURE);
    } 

    // this child is consuming input from the pipe
    //
    if ((wc_pid = fork()) == 0) {
        // attach stdin to the right side of pipe
        // and inherit stdout and stderr from parent
        dup2(pipefd[0], STDIN_FILENO);

        close(pipefd[1]);              // not using the left side
        execl("/usr/bin/wc", "wc", NULL);
        perror("exec wc failed");
        exit(EXIT_FAILURE);
    }

    // explicitly not waiting for ls_pid here
    // wc_pid isn't even my child, it belongs to ls_pid

    return EXIT_SUCCESS;
}