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.\" ========================================================================
.\"
.IX Title "IPC::Open2 3perl"
.TH IPC::Open2 3perl "2014-12-27" "perl v5.20.2" "Perl Programmers Reference Guide"
.\" For nroff, turn off justification.  Always turn off hyphenation; it makes
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.SH "NAME"
IPC::Open2 \- open a process for both reading and writing using open2()
.SH "SYNOPSIS"
.IX Header "SYNOPSIS"
.Vb 1
\&    use IPC::Open2;
\&
\&    $pid = open2(\e*CHLD_OUT, \e*CHLD_IN, \*(Aqsome cmd and args\*(Aq);
\&      # or without using the shell
\&    $pid = open2(\e*CHLD_OUT, \e*CHLD_IN, \*(Aqsome\*(Aq, \*(Aqcmd\*(Aq, \*(Aqand\*(Aq, \*(Aqargs\*(Aq);
\&
\&    # or with handle autovivification
\&    my($chld_out, $chld_in);
\&    $pid = open2($chld_out, $chld_in, \*(Aqsome cmd and args\*(Aq);
\&      # or without using the shell
\&    $pid = open2($chld_out, $chld_in, \*(Aqsome\*(Aq, \*(Aqcmd\*(Aq, \*(Aqand\*(Aq, \*(Aqargs\*(Aq);
\&
\&    waitpid( $pid, 0 );
\&    my $child_exit_status = $? >> 8;
.Ve
.SH "DESCRIPTION"
.IX Header "DESCRIPTION"
The \fIopen2()\fR function runs the given \f(CW$cmd\fR and connects \f(CW$chld_out\fR for
reading and \f(CW$chld_in\fR for writing.  It's what you think should work 
when you try
.PP
.Vb 1
\&    $pid = open(HANDLE, "|cmd args|");
.Ve
.PP
The write filehandle will have autoflush turned on.
.PP
If \f(CW$chld_out\fR is a string (that is, a bareword filehandle rather than a glob
or a reference) and it begins with \f(CW\*(C`>&\*(C'\fR, then the child will send output
directly to that file handle.  If \f(CW$chld_in\fR is a string that begins with
\&\f(CW\*(C`<&\*(C'\fR, then \f(CW$chld_in\fR will be closed in the parent, and the child will
read from it directly.  In both cases, there will be a \fIdup\fR\|(2) instead of a
\&\fIpipe\fR\|(2) made.
.PP
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.
.PP
\&\fIopen2()\fR returns the process \s-1ID\s0 of the child process.  It doesn't return on
failure: it just raises an exception matching \f(CW\*(C`/^open2:/\*(C'\fR.  However,
\&\f(CW\*(C`exec\*(C'\fR failures in the child are not detected.  You'll have to
trap \s-1SIGPIPE\s0 yourself.
.PP
\&\fIopen2()\fR 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 \f(CW\*(C`waitpid $pid, 0\*(C'\fR when you're done with the process.
Failing to do this can result in an accumulation of defunct or \*(L"zombie\*(R"
processes.  See \*(L"waitpid\*(R" in perlfunc for more information.
.PP
This whole affair is quite dangerous, as you may block forever.  It
assumes it's going to talk to something like \fBbc\fR, both writing
to it and reading from it.  This is presumably safe because you
\&\*(L"know\*(R" that commands like \fBbc\fR will read a line at a time and
output a line at a time.  Programs like \fBsort\fR that read their
entire input stream first, however, are quite apt to cause deadlock.
.PP
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
\&\f(CW\*(C`cat \-v\*(C'\fR and continually read and write a line from it.
.PP
The IO::Pty and Expect modules from \s-1CPAN\s0 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.
.SH "WARNING"
.IX Header "WARNING"
The order of arguments differs from that of \fIopen3()\fR.
.SH "SEE ALSO"
.IX Header "SEE ALSO"
See IPC::Open3 for an alternative that handles \s-1STDERR\s0 as well.  This
function is really just a wrapper around \fIopen3()\fR.