使用 2 个管道执行 cat|grep|grep

Question

标题不言自明，这是我当前的代码：

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

int main() {

  pid_t pid ;
  int fd[2], fd2[2];
  pipe(fd);
  pipe(fd2);
  pid = fork(); 

  if(pid == 0) {
    close(1);
    dup(fd[1]);
    close(fd[0]);
    close(fd[1]);

    char *exp[] = {"cat", "filename.txt", NULL};
    execvp("cat", exp);
    exit(EXIT_FAILURE);
  } 
  else if(pid > 0) {
    close(1);
    dup(fd2[1]);
    close(fd2[0]);
    close(fd2[1]);

    char *exp[] = {"grep", "-w", "stringname", NULL};
    execvp(exp[0], exp);

    pid_t pid2=fork();

    close(0);
    dup(fd[0]);
    close (fd[1]);
    close(fd[0]);

  char *exp2[] = {"grep", "-c", "stringname", NULL};
  execvp(exp2[0], exp2);

  exit(EXIT_FAILURE);
  }

  else {
    printf("Error in forking");
    exit(EXIT_FAILURE);
  }
  close(fd[0]);
  close(fd[1]);
  close(fd2[0]);
  close(fd2[1]);
  return 0;
}

目前程序正在编译但未执行（它在执行时卡在某个地方，我没有得到任何输出），任何关于我做错了什么的帮助以及如何解决它？

Answer 1

你的代码有一些问题：

1. 第一个 grep 之后的 execvp(exp[0], exp); 将在第二个 fork() 之前执行。这是一个逻辑错误。
2. 我不太明白您如何使用管道的文件描述符。您应该将标准输入和标准输出替换为适当的管道末端并关闭所有其他末端。

我用这些更改重写了您的代码，使用dup2 使其更简洁：

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

int main()
{
    int pipes[4];

    // Create two pipes
    pipe(pipes);
    pipe(pipes + 2);

    // Execute cat
    if (fork() == 0) {
        // Replace output with write end of first pipe
        dup2(pipes[1], 1);

        // Close all ends of pipes (we already copied it with `dup2`)
        close(pipes[0]);
        close(pipes[1]);
        close(pipes[2]);
        close(pipes[3]);

        char *exp[] = {"cat", "filename.txt", NULL};
        execvp(exp[0], exp);
        perror("cat failed");
        exit(EXIT_FAILURE);
    } else {
        // Execute first grep
        if (fork() == 0) {
            // Replace input with read end of 1st pipe
            dup2(pipes[0], 0);

            // Replace output with write end of 2nd pipe
            dup2(pipes[3], 1);

            // Close all ends of pipes
            close(pipes[0]);
            close(pipes[1]);
            close(pipes[2]);
            close(pipes[3]);

            char *exp[] = {"grep", "-w", "stringname", NULL};
            execvp(exp[0], exp);
            perror("first grep failed");
            exit(EXIT_FAILURE);
        } else {
            // Execute second grep
            if (fork() == 0) {
                // Replace input with read end of 2nd pipe
                dup2(pipes[2], 0);

                // Close all ends of pipes
                close(pipes[0]);
                close(pipes[1]);
                close(pipes[2]);
                close(pipes[3]);

                char *exp[] = {"grep", "-c", "stringname", NULL};
                execvp(exp[0], exp);
                perror("second grep failed");
                exit(EXIT_FAILURE);
            }
        }
    }

    // Close all ends of pipes
    close(pipes[0]);
    close(pipes[1]);
    close(pipes[2]);
    close(pipes[3]);

    for (int i = 0; i < 3; i++) {
        wait(NULL);
    }

    return 0;
}