lib/IPC/Open2.pm

package IPC::Open2;

use strict;
our ($VERSION, @ISA, @EXPORT);

require 5.000;
require Exporter;

$VERSION        = 1.01;
@ISA            = qw(Exporter);
@EXPORT         = qw(open2);

=head1 NAME

IPC::Open2, open2 - open a process for both reading and writing

=head1 SYNOPSIS

    use IPC::Open2;

    $pid = open2(\*RDRFH, \*WTRFH, 'some cmd and args');
      # or without using the shell
    $pid = open2(\*RDRFH, \*WTRFH, 'some', 'cmd', 'and', 'args');

    # or with handle autovivification
    my($rdrfh, $wtrfh);
    $pid = open2($rdrfh, $wtrfh, 'some cmd and args');
      # or without using the shell
    $pid = open2($rdrfh, $wtrfh, 'some', 'cmd', 'and', 'args');

=head1 DESCRIPTION

The open2() function runs the given $cmd and connects $rdrfh for
reading and $wtrfh for writing.  It's what you think should work 
when you try

    $pid = open(HANDLE, "|cmd args|");

The write filehandle will have autoflush turned on.

If $rdrfh is a string (that is, a bareword filehandle rather than a glob
or a reference) and it begins with C<< >& >>, then the child will send output
directly to that file handle.  If $wtrfh is a string that begins with
C<< <& >>, then $wtrfh will be closed in the parent, and the child will read
from it directly.  In both cases, there will be a dup(2) instead of a
pipe(2) made.

If either reader or writer is the null string, this will be replaced
by an autogenerated filehandle.  If so, you must pass a valid lvalue
in the parameter slot so it can be overwritten in the caller, or
an exception will be raised.

open2() returns the process ID of the child process.  It doesn't return on
failure: it just raises an exception matching C</^open2:/>.  However,
C<exec> failures in the child are not detected.  You'll have to
trap SIGPIPE yourself.

open2() does not wait for and reap the child process after it exits.
Except for short programs where it's acceptable to let the operating system
take care of this, you need to do this yourself.  This is normally as
simple as calling C<waitpid $pid, 0> when you're done with the process.
Failing to do this can result in an accumulation of defunct or "zombie"
processes.  See L<perlfunc/waitpid> for more information.

This whole affair is quite dangerous, as you may block forever.  It
assumes it's going to talk to something like B<bc>, both writing
to it and reading from it.  This is presumably safe because you
"know" that commands like B<bc> will read a line at a time and
output a line at a time.  Programs like B<sort> that read their
entire input stream first, however, are quite apt to cause deadlock.

The big problem with this approach is that if you don't have control 
over source code being run in the child process, you can't control
what it does with pipe buffering.  Thus you can't just open a pipe to
C<cat -v> and continually read and write a line from it.

The IO::Pty and Expect modules from CPAN can help with this, as they
provide a real tty (well, a pseudo-tty, actually), which gets you
back to line buffering in the invoked command again.

=head1 WARNING 

The order of arguments differs from that of open3().

=head1 SEE ALSO

See L<IPC::Open3> for an alternative that handles STDERR as well.  This
function is really just a wrapper around open3().

=cut

# &open2: tom christiansen, <tchrist@convex.com>
#
# usage: $pid = open2('rdr', 'wtr', 'some cmd and args');
#    or  $pid = open2('rdr', 'wtr', 'some', 'cmd', 'and', 'args');
#
# spawn the given $cmd and connect $rdr for
# reading and $wtr for writing.  return pid
# of child, or 0 on failure.  
# 
# WARNING: this is dangerous, as you may block forever
# unless you are very careful.  
# 
# $wtr is left unbuffered.
# 
# abort program if
#       rdr or wtr are null
#       a system call fails

require IPC::Open3;

sub open2 {
    local $Carp::CarpLevel = $Carp::CarpLevel + 1;
    return IPC::Open3::_open3('open2', scalar caller,
                                $_[1], $_[0], '>&STDERR', @_[2 .. $#_]);
}

1
Revision:	1.1
Committed:	Thu Jun 30 14:27:12 2005 UTC (21 years, 3 months ago) by root
Branch:	MAIN
CVS Tags:	PERL-5-8-7, HEAD
Branch point for:	PERL
Log Message:	* empty log message *
#	User	Rev	Content
1	root	1.1	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