lib/perl5/5.6.1/IPC/Open2.pm

   1 package IPC::Open2;
   2
   3 use strict;
   4 our ($VERSION, @ISA, @EXPORT);
   5
   6 require 5.000;
   7 require Exporter;
   8
   9 $VERSION        = 1.01;
  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(\*RDRFH, \*WTRFH, 'some cmd and args');
  22       # or without using the shell
  23     $pid = open2(\*RDRFH, \*WTRFH, 'some', 'cmd', 'and', 'args');
  24
  25     # or with handle autovivification
  26     my($rdrfh, $wtrfh);
  27     $pid = open2($rdrfh, $wtrfh, 'some cmd and args');
  28       # or without using the shell
  29     $pid = open2($rdrfh, $wtrfh, 'some', 'cmd', 'and', 'args');
  30
  31 =head1 DESCRIPTION
  32
  33 The open2() function runs the given $cmd and connects $rdrfh for
  34 reading and $wtrfh 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 $rdrfh 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 $wtrfh is a string that begins with
  44 C<< <& >>, then $wtrfh will be closed in the parent, and the child will read
  45 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