


IPC::Open3(3p)   Perl Programmers Reference Guide  IPC::Open3(3p)


NAME
       IPC::Open3, open3 - open a process for reading, writing,
       and error handling

SYNOPSIS
           $pid = open3(\*CHLD_IN, \*CHLD_OUT, \*CHLD_ERR,
                           'some cmd and args', 'optarg', ...);

           my($wtr, $rdr, $err);
           use Symbol 'gensym'; $err = gensym;
           $pid = open3($wtr, $rdr, $err,
                           'some cmd and args', 'optarg', ...);

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

DESCRIPTION
       Extremely similar to open2(), open3() spawns the given
       $cmd and connects CHLD_OUT for reading from the child,
       CHLD_IN for writing to the child, and CHLD_ERR for errors.
       If CHLD_ERR is false, or the same file descriptor as
       CHLD_OUT, then STDOUT and STDERR of the child are on the
       same filehandle (this means that an autovivified lexical
       cannot be used for the STDERR filehandle, see SYNOPSIS).
       The CHLD_IN will have autoflush turned on.

       If CHLD_IN begins with "<&", then CHLD_IN will be closed
       in the parent, and the child will read from it directly.
       If CHLD_OUT or CHLD_ERR begins with ">&", then the child
       will send output directly to that filehandle.  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.

       The filehandles may also be integers, in which case they
       are understood as file descriptors.

       open3() returns the process ID of the child process.  It
       doesn't return on failure: it just raises an exception
       matching "/^open3:/".  However, "exec" failures in the
       child (such as no such file or permission denied), are
       just reported to CHLD_ERR, as it is not possible to trap
       them.

       If the child process dies for any reason, the next write
       to CHLD_IN is likely to generate a SIGPIPE in the parent,
       which is fatal by default.  So you may wish to handle this
       signal.

       Note if you specify "-" as the command, in an analogous



perl v5.10.1                2009-10-12                          1





IPC::Open3(3p)   Perl Programmers Reference Guide  IPC::Open3(3p)


       fashion to "open(FOO, "-|")" the child process will just
       be the forked Perl process rather than an external
       command.  This feature isn't yet supported on Win32
       platforms.

       open3() 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.

       If you try to read from the child's stdout writer and
       their stderr writer, you'll have problems with blocking,
       which means you'll want to use select() or the IO::Select,
       which means you'd best use sysread() instead of readline()
       for normal stuff.

       This is very 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.

See Also
       IPC::Open2
           Like Open3 but without STDERR catpure.

       IPC::Run
           This is a CPAN module that has better error handling
           and more facilities than Open3.

WARNING
       The order of arguments differs from that of open2().













perl v5.10.1                2009-10-12                          2


