Message-ID: <4E906DD1.50107@cornell.edu>
Date: Sat, 08 Oct 2011 11:35:45 -0400
From: Ken Brown <kbrown AT cornell DOT edu>
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To: cygwin <cygwin AT cygwin DOT com>
Subject: Problem redirecting stderr to pipe in subprocess
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The attached STC arose from my attempt to understand the problem 
discussed starting in

   http://cygwin.com/ml/cygwin/2011-09/msg00405.html .

I'm starting a new thread because there appears to be a Cygwin problem 
having nothing to do with emacs.

The STC creates a pipe and then runs `bash -ic ls' in a subprocess, with 
both stdout and stderr sent to the pipe.  The parent process reads from 
the pipe and echoes it to the terminal.

On Linux, I first get the error messages (presumably expected)

bash: cannot set terminal process group (-1): Invalid argument
bash: no job control in this shell

followed by the directory listing.  On Cygwin, the bash process produces 
no output but is simply stopped, and I have to kill it from another 
terminal before the main program will exit.  This happens both with 
cygwin-1.7.9 and the latest snapshot.  Here are a few comments:

1. The problem disappears if I don't send bash's stderr to the pipe.

2. The problem also disappears if I replace -ic by -c in the call to 
bash, presumably because there's nothing sent to stderr in that case.

3. The problem disappears if I don't use a pipe but just have the bash 
subprocess write to the terminal, even if I redirect bash's stderr to 
stdout.

Ken

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#include <sys/types.h>
#include <unistd.h>

int
main()
{
  char  *argv[4];
  char buf;
  int fd[2];
  pid_t  pid;

  pipe (fd);

  argv[0] = "bash";
  argv[1] = "-ic";
  argv[2] = "ls";
  argv[3] = '\0';
  pid = fork();
  if (pid == 0)
    {
      setsid ();
      close (1);
      close (2);
      dup2 (fd[1], 1);
      dup2 (fd[1], 2);
      close (fd[1]);
      execvp (argv[0], argv);
    }
  else
    {
      close (fd[1]);
      while (read (fd[0], &buf, 1) > 0)
      	write (1, &buf, 1);
      close (fd[0]);
    }
}


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