IPC::Open3(3)

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

SYNOPSIS

    $pid = open3(\*WTRFH, \*RDRFH, \*ERRFH,
                    'some cmd and args', 'optarg', ...);

DESCRIPTION
Extremely similar to open2(), open3() spawns the given $cmd and connects RDRFH for reading, WTRFH for writing, and ERRFH for errors. If ERRFH is '', or the same as RDRFH, then STDOUT and STDERR of the child are on the same file handle. The WTRFH will have autoflush turned on.

If WTRFH begins with "<&", then WTRFH will be closed in the parent, and the child will read from it directly. If RDRFH or ERRFH begins with ">&", then the child will send output directly to that file handle. In both cases, there will be a dup(2) instead of a pipe(2) made.

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(), which means you'll have to use sysread() instead of normal stuff.

open3() returns the process ID of the child process. It doesn't return on failure: it just raises an exception matching /^open3:/.

WARNING
It will not create these file handles for you. You have to do this yourself. So don't pass it empty variables expecting them to get filled in for you.

Additionally, 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.