如何在子进程中正确使用管道？

Question

//I want to execute pwd|sort > file.txt

char commands[2][30] = {"pwd", "sort"};
char directory [2][30] = {"/usr/bin/pwd", "/usr/bin/sort"}
char outputFile[30] = "file.txt"
int totalCommands = 2;
bool isOutputFilePresent = true;

//creating two pipes
int fd1[2];
int fd2[2];
pipe(fd1);
pipe(fd2);
//Closing the writing ends but keeping the reading ends open as closing them will destroy both the pipes
close (fd1[1]);
close (fd2[1]);

//loop to execute each command by creating a child process in each iteration
for (int i = 0; i < totalCommands; ++i)
{
    pid_t pid = fork();
    if (pid == 0)
    {
        if (i == totalCommands - 1)
        {
            if (i%2 == 0)
            {
                close(fd1[1]);
                close(fd2[0]);
                dup2(fd1[0], 0);
                dup2(fd2[1], 1);
            }
            else
            {
                close(fd2[1]);
                close(fd1[0]);
                dup2(fd2[0], 0);
                dup2(fd1[1], 1);
            }
        }
        else
        {
            if (i%2 == 0)
            { 
                close(fd2[0]);
                close(fd2[1]);
                close(fd1[1]);
                dup2(fd1[0], 0);
            }
            else
            {
                close(fd1[0]);
                close(fd1[1]);
                close(fd2[1]);
                dup2(fd2[0], 0);
            }
            if(isOutputFilePresent)
            {
                int outputFD = open (outputFile, O_WRONLY | O_CREAT);
                dup2(outputFD, 1);
            }

        }
        execv(directory[i], commands[i]) //ignore the fact that i am passing command name instead of argument vector
    }
    wait(NULL);
}

由于子进程将继承它自己的管道 FD，我关闭了每个进程中未使用的管道，并使用 dup2 为其分配了 stdout 和 stdin FD。但是这段代码有一个逻辑错误，我猜是因为没有正确使用管道。那么我在管道的概念上哪里弄错了，这个问题的解决方案是什么。谢谢！

Answer 1

在打开管道后立即关闭管道的写入端。因此，没有任何东西可以写入这些管道：

pipe(fd1);
pipe(fd2);
//Closing the writing ends but keeping the reading ends open as closing them will destroy both the pipes
close (fd1[1]);
close (fd2[1]);

这样做：

1. 创建两个管道：A 和 B
2. 派生/派生新的子进程
3. Parent 关闭 A 的写端和 B 的读端
4. 子级关闭 A 的读取端和 B 的写入端

因此，孩子向 A 写入，而父母从 A 读取。 父母向 B 写入，孩子从 B 读取。

用于 pipe(2) 的 man page 为您提供示例代码，向您展示如何执行此操作：

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

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

   if (argc != 2) {
       fprintf(stderr, "Usage: %s <string>\n", argv[0]);
       exit(EXIT_FAILURE);
   }

   if (pipe(pipefd) == -1) {
       perror("pipe");
       exit(EXIT_FAILURE);
   }

   cpid = fork();
   if (cpid == -1) {
       perror("fork");
       exit(EXIT_FAILURE);
   }

   if (cpid == 0) {    /* Child reads from pipe */
       close(pipefd[1]);          /* Close unused write end */

       while (read(pipefd[0], &buf, 1) > 0)
           write(STDOUT_FILENO, &buf, 1);

       write(STDOUT_FILENO, "\n", 1);
       close(pipefd[0]);
       _exit(EXIT_SUCCESS);

   } else {            /* Parent writes argv[1] to pipe */
       close(pipefd[0]);          /* Close unused read end */
       write(pipefd[1], argv[1], strlen(argv[1]));
       close(pipefd[1]);          /* Reader will see EOF */
       wait(NULL);                /* Wait for child */
       exit(EXIT_SUCCESS);
   }
}