经典 C. 在 execvp 函数中使用管道，stdin 和 stdout 重定向 [英] Classic C. Using pipes in execvp function, stdin and stdout redirection

问题描述

我想在我的 Linux C 程序中使用管道和 execvp 函数模拟 bash.例如

I want to simulate bash in my Linux C program using pipes and execvp function. e.g

ls -l | wc -l  

有我的程序:

if(pipe(des_p) == -1) {perror("Failed to create pipe");}

if(fork() == 0) {    //first fork
  close(1);          //closing stdout
  dup(des_p[1]);     //replacing stdout with pipe write 
  close(des_p[0]);   //closing pipe read
  close(des_p[1]);   //closing pipe write

  if(execvp(bash_args[0], bash_args)) // contains ls -l
    /* error checking */
}
else {
  if(fork() == 0) {  //creating 2nd child
    close(0);        //closing stdin
    dup(des_p[0]);   //replacing stdin with pipe read
    close(des_p[1]); //closing pipe write
    close(des_p[0]); //closing pipe read

    if(execvp(bash_args[another_place], bash_args)) //contains wc -l
      /* error checking */
  }

  close(des_p[0]);
  close(des_p[1]);
  wait(0);
  wait(0);
}

这段代码实际上可以运行，但没有做正确的事情.这段代码有什么问题?这不起作用，我不知道为什么.

This code actually runs, but doesn't do the right thing. What's wrong with this code? That's not working and I don't have a clue why.

推荐答案

您需要关闭父级中的管道 fds，否则子级将不会收到 EOF，因为管道仍处于打开状态以供在父级中写入.这将导致第二个 wait() 挂起.对我有用:

You need to close the pipe fds in the parent, or the child won't receive EOF, because the pipe's still open for writing in the parent. This would cause the second wait() to hang. Works for me:

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


int main(int argc, char** argv)
{
        int des_p[2];
        if(pipe(des_p) == -1) {
          perror("Pipe failed");
          exit(1);
        }

        if(fork() == 0)            //first fork
        {
            close(STDOUT_FILENO);  //closing stdout
            dup(des_p[1]);         //replacing stdout with pipe write 
            close(des_p[0]);       //closing pipe read
            close(des_p[1]);

            const char* prog1[] = { "ls", "-l", 0};
            execvp(prog1[0], prog1);
            perror("execvp of ls failed");
            exit(1);
        }

        if(fork() == 0)            //creating 2nd child
        {
            close(STDIN_FILENO);   //closing stdin
            dup(des_p[0]);         //replacing stdin with pipe read
            close(des_p[1]);       //closing pipe write
            close(des_p[0]);

            const char* prog2[] = { "wc", "-l", 0};
            execvp(prog2[0], prog2);
            perror("execvp of wc failed");
            exit(1);
        }

        close(des_p[0]);
        close(des_p[1]);
        wait(0);
        wait(0);
        return 0;
}

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