Linux:使用管道标准输入/标准输出执行子进程 [英] Linux: Executing child process with piped stdin/stdout

问题描述

使用 Linux 和 C++，我想要一个执行以下操作的函数:

Using Linux and C++, I would like a function that does the following:

string f(string s)
{
    string r = system("foo < s");
    return r;
}

显然上述方法行不通，但您明白了.我有一个字符串 s，我想将它作为应用程序foo"的子进程执行的标准输入传递，然后我想将其标准输出记录到字符串 r 然后返回.

Obviously the above doesn't work, but you get the idea. I have a string s that I would like to pass as the standard input of a child process execution of application "foo", and then I would like to record its standard output to string r and then return it.

我应该使用哪种 Linux 系统调用或 POSIX 函数组合?我使用的是 Linux 3.0，不需要该解决方案来处理旧系统.

What combination of Linux syscalls or POSIX functions should I use? I'm using Linux 3.0 and do not need the solution to work with older systems.

推荐答案

eerpini 提供的代码无法正常工作.请注意，例如，在父级中闭合的管端将在之后使用.看看

The code provided by eerpini does not work as written. Note, for example, that the pipe ends that are closed in the parent are used afterwards. Look at

close(wpipefd[1]); 

以及对该关闭描述符的后续写入.这只是转置，但它表明此代码从未被使用过.下面是我测试过的一个版本.不幸的是，我改变了代码风格，所以这不被接受为 eerpini 代码的编辑.

and the subsequent write to that closed descriptor. This is just transposition, but it shows this code has never been used. Below is a version that I have tested. Unfortunately, I changed the code style, so this was not accepted as an edit of eerpini's code.

唯一的结构变化是我只重定向了子进程中的 I/O(注意 dup2 调用只在子进程中.)这很重要，否则父进程的 I/O 会被搞砸.感谢 eerpini 的初步答案，我在开发这个答案时使用了它.

The only structural change is that I only redirect the I/O in the child (note the dup2 calls are only in the child path.) This is very important, because otherwise the parent's I/O gets messed up. Thanks to eerpini for the initial answer, which I used in developing this one.

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

#define PIPE_READ 0
#define PIPE_WRITE 1

int createChild(const char* szCommand, char* const aArguments[], char* const aEnvironment[], const char* szMessage) {
  int aStdinPipe[2];
  int aStdoutPipe[2];
  int nChild;
  char nChar;
  int nResult;

  if (pipe(aStdinPipe) < 0) {
    perror("allocating pipe for child input redirect");
    return -1;
  }
  if (pipe(aStdoutPipe) < 0) {
    close(aStdinPipe[PIPE_READ]);
    close(aStdinPipe[PIPE_WRITE]);
    perror("allocating pipe for child output redirect");
    return -1;
  }

  nChild = fork();
  if (0 == nChild) {
    // child continues here

    // redirect stdin
    if (dup2(aStdinPipe[PIPE_READ], STDIN_FILENO) == -1) {
      exit(errno);
    }

    // redirect stdout
    if (dup2(aStdoutPipe[PIPE_WRITE], STDOUT_FILENO) == -1) {
      exit(errno);
    }

    // redirect stderr
    if (dup2(aStdoutPipe[PIPE_WRITE], STDERR_FILENO) == -1) {
      exit(errno);
    }

    // all these are for use by parent only
    close(aStdinPipe[PIPE_READ]);
    close(aStdinPipe[PIPE_WRITE]);
    close(aStdoutPipe[PIPE_READ]);
    close(aStdoutPipe[PIPE_WRITE]); 

    // run child process image
    // replace this with any exec* function find easier to use ("man exec")
    nResult = execve(szCommand, aArguments, aEnvironment);

    // if we get here at all, an error occurred, but we are in the child
    // process, so just exit
    exit(nResult);
  } else if (nChild > 0) {
    // parent continues here

    // close unused file descriptors, these are for child only
    close(aStdinPipe[PIPE_READ]);
    close(aStdoutPipe[PIPE_WRITE]); 

    // Include error check here
    if (NULL != szMessage) {
      write(aStdinPipe[PIPE_WRITE], szMessage, strlen(szMessage));
    }

    // Just a char by char read here, you can change it accordingly
    while (read(aStdoutPipe[PIPE_READ], &nChar, 1) == 1) {
      write(STDOUT_FILENO, &nChar, 1);
    }

    // done with these in this example program, you would normally keep these
    // open of course as long as you want to talk to the child
    close(aStdinPipe[PIPE_WRITE]);
    close(aStdoutPipe[PIPE_READ]);
  } else {
    // failed to create child
    close(aStdinPipe[PIPE_READ]);
    close(aStdinPipe[PIPE_WRITE]);
    close(aStdoutPipe[PIPE_READ]);
    close(aStdoutPipe[PIPE_WRITE]);
  }
  return nChild;
}

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