Linux的3.0：与管道标准输入/输出执行子进程 [英] Linux 3.0: Executing child process with piped stdin/stdout

问题描述

在Linux 3.0 / C ++：

Under Linux 3.0 / C++:

我想，做如下的功能：

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

显然，上述方法不工作，但你的想法。我有一个，我想字符串s传递的应用foo的一个子进程执行的标准输入，然后我想它的标准输出记录到串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.

我应该用什么样的组合的Linux系统调用或POSIX功能？

What combination of linux syscalls or posix functions should I use?

推荐答案

由eerpini提供的code没有书面工作。注意，例如，被在父封闭管端部以后使用。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]); 

和该封闭描述后续的写入。这仅仅是换位，但它表明这个code从未被使用。下面是我已经测试版本。不幸的是，我改变了code风格，所以这是不接受eerpini的code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.

唯一的结构性变化是，我只在孩子重定向I / O（请注意dup2调用仅在子路径）。这是非常重要的，否则父母的I / O被搞砸。由于eerpini初始答案，这是我在开发这个应用。

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.

#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) {
      perror("redirecting stdin");
      return -1;
    }

    // redirect stdout
    if (dup2(aStdoutPipe[PIPE_WRITE], STDOUT_FILENO) == -1) {
      perror("redirecting stdout");
      return -1;
    }

    // redirect stderr
    if (dup2(aStdoutPipe[PIPE_WRITE], STDERR_FILENO) == -1) {
      perror("redirecting stderr");
      return -1;
    }

    // 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
    perror("exec of the child process");
    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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