


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(\*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');


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
       ">&", then the child will send output directly to that
       file handle.  If $wtrfh is a string that begins with "<&",
       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 "/^open2:/".  However, "exec" failures in the
       child are not detected.  You'll have to trap SIGPIPE your-
       self.

       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.
       Failing to do this can result in an accumulation of
       defunct or "zombie" processes.  See the waitpid entry in



2000-04-06                 perl v5.6.1                          1





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


       the perlfunc manpage 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 pre-
       sumably safe because you "know" that commands like bc will
       read a line at a time and output a line at a time.  Pro-
       grams 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 pro-
       cess, you can't control what it does with pipe buffering.
       Thus you can't just open a pipe to "cat -v" and continu-
       ally 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 the IPC::Open3 manpage for an alternative that handles
       STDERR as well.  This function is really just a wrapper
       around open3().





























2000-04-06                 perl v5.6.1                          2


