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NAME

       IPC::Open3 - open a process for reading, writing, and error handling using open3()

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 under Windows
       and OS/2, 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 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 capture.

       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().