Path: utzoo!utgpu!jarvis.csri.toronto.edu!clyde.concordia.ca!rutgers!tut.cis.ohio-state.edu!zaphod.mps.ohio-state.edu!uakari.primate.wisc.edu!ames!elroy.jpl.nasa.gov!jpl-devvax!lwall
From: lwall@jpl-devvax.JPL.NASA.GOV (Larry Wall)
Newsgroups: comp.lang.perl
Subject: Re: Problems using ptys in perl.
Message-ID: <6989@jpl-devvax.JPL.NASA.GOV>
Date: 5 Feb 90 23:04:40 GMT
References: <2276@uvaarpa.virginia.edu> <7115@ogicse.ogc.edu>
Reply-To: lwall@jpl-devvax.JPL.NASA.GOV (Larry Wall)
Organization: Jet Propulsion Laboratory, Pasadena, CA
Lines: 28

In article <7115@ogicse.ogc.edu> hakanson@ogicse.UUCP (Marion Hakanson) writes:
: I haven't worked out the nitty-gritty details, but the behavior I have
: seen can be described as the parent writing to a forked filehandle,
: after which the parent sees both what it wrote to that filehandle as
: well as what the child wrote to it.  In other words, the data gets
: duplicated, instead of being consumed by (just) the child.  I'm not
: sure that you are seeing the same thing, but it looks similar.
: 
: My intuition tells me that the implementation of the stdio subsystem
: is at fault here, but I have not examined any source code to confirm
: that hypothesis.  Larry, I must say that this is one issue that makes
: me wish Perl used Unix file descriptors instead of C file pointers,
: but I certainly don't blame you for wanting to avoid implementing
: your own buffered I/O system.  But it certainly would make forking
: a little less painful.

Note the disclaimer in the manual entry on fork:

     fork    Does a fork() call.  Returns the child pid to the
             parent process and 0 to the child process.  Note:
             unflushed buffers remain unflushed in both
             processes, which means you may need to set $| to
             avoid duplicate output.

The disclaimer should probably be propagated to other constructs that
do fork as well, such as open().

Larry