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IPC::Open2, open2 - open a process for both reading and writing


use IPC::Open2;
$pid = open2(\*RDR, \*WTR, 'some cmd and args');
  # or
$pid = open2(\*RDR, \*WTR, 'some', 'cmd', 'and', 'args');


The open2() function spawns the given $cmd and connects $rdr for reading and $wtr for writing. It's what you think should work when you try

open(HANDLE, "|cmd args|");

The write filehandle will have autoflush turned on.

If $rdr 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 $wtr is a string that begins with "<&", then WTR 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.

open2() returns the process ID of the child process. It doesn't return on failure: it just raises an exception matching /^open2:/.


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.


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