open() and pipes

David Cantrell david at
Tue Aug 1 14:12:32 BST 2006

On Tue, Aug 01, 2006 at 12:49:40PM +0100, Dominic Mitchell wrote:

> It's just far to easy to get into a situation where both ends of the
> pipe ending up calling read() at the same time.  At which point, you
> have two deadlocked processes.

Sure.  You have that with IPC::Open2 as well.  It's mentioned
prominently in the docs.  Provided that it was as well-documented in
perlfunc I don't see what the problem would be.

In my particular application this is not an issue as the external
program *can not under any circumstances other than a fatal error* send
anything until I've finished writing to it.

> In a past life, I used the expect extension for TCL a lot to deal with
> this sort of thing. is probably a good thing to look at in
> the Perl world.

I'm not convinced that Expect is what I want.  I want to do something
exactly the same as how procmail handles filters.  Stuff goes in, gets
munged, and comes back out.  I want all of what comes out, and I don't
need to do any pattern matching.  Indeed, Expect doesn't even appear to
*have* a method for "gimme all the stuff what comes out".

>                  It looks complicated.  That's because it's doing
> complicated stuff.  :-)

Defnitly not what I need then.  All I want to do is replace this ugly
hack ...

  open(FOO), '|myprogram outputfile=foo.tmp');
  print FOO 'munge this';
  # at this point some naughty chap replaces foo.tmp just to piss me off
  open(FOORESULTS, 'foo.tmp');
  my $munged = <FOORESULTS>;
  unlink 'foo.tmp';

with this ...

  open(FOO, '|myprogram|');
  print FOO, 'munge this';
  my $munged = <FOO>;

David Cantrell | Nth greatest programmer in the world

Feature: an incorrectly implemented bug

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