MirOS Manual: IPC::Open2(3p)


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');

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 han-
     dle.  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 "wait-
     pid $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.

perl v5.8.8                2005-02-05                           1

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

     This whole affair is quite dangerous, as you may block for-
     ever.  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 v5.8.8                2005-02-05                           2

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