Provided by: perl-doc_5.26.1-6_all bug


       IPC::Open2 - open a process for both reading and writing using open2()


           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;


       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.  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.


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


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