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   1  package IPC::Open2;
   2  
   3  use strict;
   4  our ($VERSION, @ISA, @EXPORT);
   5  
   6  require 5.000;
   7  require Exporter;
   8  
   9  $VERSION    = 1.02;
  10  @ISA        = qw(Exporter);
  11  @EXPORT        = qw(open2);
  12  
  13  =head1 NAME
  14  
  15  IPC::Open2, open2 - open a process for both reading and writing
  16  
  17  =head1 SYNOPSIS
  18  
  19      use IPC::Open2;
  20  
  21      $pid = open2(\*CHLD_OUT, \*CHLD_IN, 'some cmd and args');
  22        # or without using the shell
  23      $pid = open2(\*CHLD_OUT, \*CHLD_IN, 'some', 'cmd', 'and', 'args');
  24  
  25      # or with handle autovivification
  26      my($chld_out, $chld_in);
  27      $pid = open2($chld_out, $chld_in, 'some cmd and args');
  28        # or without using the shell
  29      $pid = open2($chld_out, $chld_in, 'some', 'cmd', 'and', 'args');
  30  
  31  =head1 DESCRIPTION
  32  
  33  The open2() function runs the given $cmd and connects $chld_out for
  34  reading and $chld_in for writing.  It's what you think should work 
  35  when you try
  36  
  37      $pid = open(HANDLE, "|cmd args|");
  38  
  39  The write filehandle will have autoflush turned on.
  40  
  41  If $chld_out is a string (that is, a bareword filehandle rather than a glob
  42  or a reference) and it begins with C<< >& >>, then the child will send output
  43  directly to that file handle.  If $chld_in is a string that begins with
  44  C<< <& >>, then $chld_in will be closed in the parent, and the child will
  45  read from it directly.  In both cases, there will be a dup(2) instead of a
  46  pipe(2) made.
  47  
  48  If either reader or writer is the null string, this will be replaced
  49  by an autogenerated filehandle.  If so, you must pass a valid lvalue
  50  in the parameter slot so it can be overwritten in the caller, or
  51  an exception will be raised.
  52  
  53  open2() returns the process ID of the child process.  It doesn't return on
  54  failure: it just raises an exception matching C</^open2:/>.  However,
  55  C<exec> failures in the child are not detected.  You'll have to
  56  trap SIGPIPE yourself.
  57  
  58  open2() does not wait for and reap the child process after it exits.
  59  Except for short programs where it's acceptable to let the operating system
  60  take care of this, you need to do this yourself.  This is normally as
  61  simple as calling C<waitpid $pid, 0> when you're done with the process.
  62  Failing to do this can result in an accumulation of defunct or "zombie"
  63  processes.  See L<perlfunc/waitpid> for more information.
  64  
  65  This whole affair is quite dangerous, as you may block forever.  It
  66  assumes it's going to talk to something like B<bc>, both writing
  67  to it and reading from it.  This is presumably safe because you
  68  "know" that commands like B<bc> will read a line at a time and
  69  output a line at a time.  Programs like B<sort> that read their
  70  entire input stream first, however, are quite apt to cause deadlock.
  71  
  72  The big problem with this approach is that if you don't have control 
  73  over source code being run in the child process, you can't control
  74  what it does with pipe buffering.  Thus you can't just open a pipe to
  75  C<cat -v> and continually read and write a line from it.
  76  
  77  The IO::Pty and Expect modules from CPAN can help with this, as they
  78  provide a real tty (well, a pseudo-tty, actually), which gets you
  79  back to line buffering in the invoked command again.
  80  
  81  =head1 WARNING 
  82  
  83  The order of arguments differs from that of open3().
  84  
  85  =head1 SEE ALSO
  86  
  87  See L<IPC::Open3> for an alternative that handles STDERR as well.  This
  88  function is really just a wrapper around open3().
  89  
  90  =cut
  91  
  92  # &open2: tom christiansen, <tchrist@convex.com>
  93  #
  94  # usage: $pid = open2('rdr', 'wtr', 'some cmd and args');
  95  #    or  $pid = open2('rdr', 'wtr', 'some', 'cmd', 'and', 'args');
  96  #
  97  # spawn the given $cmd and connect $rdr for
  98  # reading and $wtr for writing.  return pid
  99  # of child, or 0 on failure.  
 100  # 
 101  # WARNING: this is dangerous, as you may block forever
 102  # unless you are very careful.  
 103  # 
 104  # $wtr is left unbuffered.
 105  # 
 106  # abort program if
 107  #    rdr or wtr are null
 108  #     a system call fails
 109  
 110  require IPC::Open3;
 111  
 112  sub open2 {
 113      local $Carp::CarpLevel = $Carp::CarpLevel + 1;
 114      return IPC::Open3::_open3('open2', scalar caller,
 115                  $_[1], $_[0], '>&STDERR', @_[2 .. $#_]);
 116  }
 117  
 118  1