How to capture stdout from a spawned child process

A recent post to ace-users reminded me how hard it was for me to figure out how to capture stdout from a spawned child process in C++. After much research and trial and error, I came up with the following which uses ACE.

Update 7/12/2005: The below code does not work on Windows because Windows does not allow redirection of stdout/stderr to sockets which is what ACE_Pipe uses. See my next post for code that does work on Windows.

  std::string command = ...;

  ACE_Process_Options opt;

  // The child process to spawn.
  opt.command_line(command.c_str());

  // Define the two handles for the pipe,
  // fd[0] will be used for stdin
  // fd[1] will be used for stdout.
  ACE_HANDLE fd[2];

  // Create a pipe from the two file handles.
  // Pipe between stdin and stdout.
  // fd[0] <-> fd[1]
  ACE_Pipe pipe(fd);

  // Set the stdin and stdout of the process to our file handles.
  // fd[0] is used for stdin, fd[1] is used for stdout.
  opt.set_handles(fd[0] /*stdin*/, fd[1] /*stdout*/);

  // Spawn the child process.
  ACE_Process proc;
  if (proc.spawn(opt) == -1) ACE_DEBUG ((LM_ERROR, "%p\n", "spawn failed"));

  // After fork:
  //   fd[0]  <-> fd[1]        child
  //   fd'[0] <-> fd'[1]       parent

  // Need to close parent stdout or parent will be trying to read from it forever.
  // With stdout closed parent will get EOF from the child so that the parent 
  // will know the child is done.
  if (ACE_OS::close(fd[1]) == -1) ACE_DEBUG ((LM_ERROR, "%p\n", "close stdout"));

  // After close:
  //   fd[0]  <-> fd[1]        child
  //   fd'[0] <->              parent
  // which leaves our (parent) stdin piped to child's stdout
  
  // set_handles() dups the handles, so make sure they are released.
  opt.release_handles();

  // Read from parent's stdin, which is connected
  // to the child's stdout.
  std::string child_stdout;
  const size_t BUFFSIZE = 1024;
  char buf[BUFFSIZE + 1];
  ssize_t n = 0;
  while ((n = ACE_OS::read(fd[0], buf, BUFFSIZE)) > 0) {
    buf[n] = 0;
    child_stdout.append(buf, n);
  } 

  ACE_exitcode exitcode = 0;
  proc.wait(&exitcode);
  
  // Close stdin.
  if (ACE_OS::close(fd[0]) == -1) ACE_DEBUG ((LM_ERROR, "%p\n", "close stdin"));

  // Close the pipe, pipe destructor does not close the pipe.
  pipe.close();