如何让 wc 接受管道文件而不是标准输入来获取输入？

Question

这是一道作业题。任务是使用 execlp、fork 和管道复制命令：ls | wc -l 在 C 程序中。

我的方法

我觉得问题可以这样解决：

1. 创建管道文件：pipe.txt
2. 使用 fork() 创建子进程
   • 将子进程的stdout映射到pipe.txt
   • 使用execlp
   执行ls
   • 这会将 ls 的输出放入 pipe.txt
3. 父进程内部
   • 将父进程的stdin映射到pipe.txt
   • 使用 execlp 执行 wc -l 而不提供任何进一步的参数，因此它从标准输入读取
   • 由于这个父进程的stdout还是终端本身，所以应该在终端打印出行数

我的代码

#include <stdio.h>
#include <unistd.h>
#include <stdlib.h>
#include <sys/types.h>
#include <unistd.h> 
#include <fcntl.h> 
#include <errno.h> 
#include <sys/wait.h>

int main() {
        int pipefds[2];
        int returnstatus;
        int pid;

        char argArr[30] = {'\n'};

        returnstatus = pipe(pipefds);
        if (returnstatus == -1) {
                printf("Unable to create pipe\n");
                return 1;
        }

        int file_desc = open("pipe.txt", O_RDWR | O_APPEND | O_CREAT); 

        pid = fork();

        if (pid == 0) {
                int copy_desc = dup2(file_desc, 1);
                execlp("ls", "ls", NULL);
        } else {
                int copy_desc = dup2(file_desc, 0);
                close(copy_desc);
                execlp("wc", "wc", "-l", NULL);
        }
        return 0;
}

实际输出

main.cpp blabla.cpp main pipe.txt
>

问题

这有两点错误：

1. 既然我把child的stdout设置成了pipe.txt文件，为什么还是在终端输出？注意：它确实也将输出放在 pipe.txt 文件中。但是为什么它也显示在终端上呢？

2. 它开始等待用户提供输入？它不应该从管道文件而不是用户获取输入吗？

预期输出

5

*如果当前目录下有5个文件

尝试过的解决方案

1. 仅使用管道：（出现错误的文件描述符错误）

int main() {
        int pipefds[2];
        int returnstatus;
        int pid;

        returnstatus = pipe(pipefds);
        if (returnstatus == -1) {
                printf("Unable to create pipe\n");
                return 1;
        }

        pid = fork();

        if (pid == 0) {
                dup2(pipefds[0], 1);
                close(pipefds[1]);
                execlp("ls", "ls", NULL);
        } else {
                dup2(pipefds[1], 0);
                close(pipefds[0]);
                execlp("wc", "wc", "-l", NULL);
        }
        return 0;
}

Answer 1

感谢您的宝贵意见。

代码中的问题是我根本没有使用管道。我正在使用我创建的文件完成所有工作。这就是基本问题。

这是新代码：

#include <stdio.h>
#include <unistd.h>
#include <stdlib.h>
#include <sys/types.h>
#include <unistd.h> 
#include <fcntl.h> 
#include <errno.h> 
#include <sys/wait.h>

int main() {
    // Step1. Create pipe file descriptors: pipefd[0] for reading from pipe, pipefd[1] for writing to the pipe
    int pipefds[2];

    // Helping variables
    int returnstatus;
    int pid;

    // Step2. Create a pipe with the file descriptors
    returnstatus = pipe(pipefds);

    // Check if pipe was successfully created
    if (returnstatus == -1) {
        printf("Unable to create pipe\n");
        return 1;
    }

    // Step3. Fork to create a child process
    pid = fork();

    if (pid == 0) {
        // Inside the child process

        // Step4. Duplicate the file descriptor of the write end of the pipe and set it equal to the stdout of the process 
        dup2(pipefds[1], 1);

        // Step5. Close both ends of the pipe
        close(pipefds[0]);
        close(pipefds[1]);

        // Step6. Execute the LS command. It ouputs to stdout which we set equal to the pipe in Step4.
        // So essentially, we send all output of ls to our pipe 
        returnstatus = execlp("ls", "ls", NULL);

        // Error checking the execlp command
        if (returnstatus == -1){
            perror("Error executing execlp: ");
        }
    } else {
        // Inside the parent process

        // Step7. Duplicate the file descriptor the READ end of the pipe and set it equal to the stdin of the process
        dup2(pipefds[0], 0);

        // Step8. Close the both ends of the pipe
        close(pipefds[0]);
        close(pipefds[1]);

        // Step9. Execute the WC command. It takes the file as an argument usually but if no file is given, it will take
        // stdin as input. Since the stdin is the pipe, therefore it will read all the data from the pipe.
        // The output of the wc command is stdout which is the terminal for this process so we will get the number of
        // files/directories in the current directory as an output on the terminal
        returnstatus = execlp("wc", "wc", "-l", NULL);

        // Error checking the execlp command
        if (returnstatus == -1){
            perror("Error executing execlp: ");
        }
    }
    return 0;
}