


IPC::Open2(3p)   Perl Programmers Reference Guide  IPC::Open2(3p)


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

SYNOPSIS
           use IPC::Open2;

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

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

           waitpid( $pid, 0 );
           my $child_exit_status = $? >> 8;

DESCRIPTION
       The open2() function runs the given $cmd and connects
       $chld_out for reading and $chld_in 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 $chld_out is a string (that is, a bareword filehandle
       rather than a glob or a reference) and it begins with
       ">&", then the child will send output directly to that
       file handle.  If $chld_in is a string that begins with
       "<&", then $chld_in 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 "/^open2:/".  However, "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
       "waitpid $pid, 0" when you're done with the process.



perl v5.10.1                2009-10-12                          1





IPC::Open2(3p)   Perl Programmers Reference Guide  IPC::Open2(3p)


       Failing to do this can result in an accumulation of
       defunct or "zombie" processes.  See "waitpid" in perlfunc
       for more information.

       This whole affair is quite dangerous, as you may block
       forever.  It assumes it's going to talk to something like
       bc, both writing to it and reading from it.  This is
       presumably safe because you "know" that commands like bc
       will read a line at a time and output a line at a time.
       Programs like 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 "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.

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

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



























perl v5.10.1                2009-10-12                          2


