IPC::Open2

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



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 read-
       ing 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 excep-
       tion 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 nor-
       mally 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 "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 com-
       mands 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 v5.8.6                       2001-09-21                     IPC::Open2(3)