RE: capturing output from running process crashes gtk2

["Roderich Schupp (ext)" <Roderich Schupp ext BenQ com>]

> Right. Almost there. The following script works, but gives me 
> the warning "Name "main::WRITE" used only once: possible typo 
> at ./test_dialog3 line 23." If I replace \*WRITE with 0, then 
> I get the error message "open3: close(0) failed: Bad file 
> descriptor at ./test_dialog3 line 23".
>
> How do I write it so as to avoid the warning?

Use lexical filehandles (instead of oldstyle global ones), like so:
  
  my ($write, $read, $error);
  my $pid = open3($write, $read , $error, "./test_scan" );
  ...
  Glib::IO->add_watch(fileno($error), 'in', sub {
  ...
                              sysread $error, $line, 1024;

As for the problem with the real scanimage program mentioned by zentara:
- write some character to $write to make the scan continue
  NOTE: make the $write handle autoflush, otherwise 
      print $write "X"
  might not actually write someting into the pipe (that was created by
open3)
  yet, but only when the internal buffer has filled up (you might get
lucky
  with the character "\n" because the filehandle might be line-buffered
  by default, but better to make sure)
- if you want to stop the scan, simply
      close $write
- you should have add_watch for $error also look for "err" conditions
  (esp. eof on $error when the scanimage process exits) and handle
  them in the callback - see the IPC::Open3 man page how to reap 
  the sub process to avoid a zombie

Cheers, Roderich