Path: utzoo!attcan!uunet!decwrl!elroy.jpl.nasa.gov!jpl-devvax!lwall
From: lwall@jpl-devvax.JPL.NASA.GOV (Larry Wall)
Newsgroups: comp.lang.perl
Subject: Re: (f)flush?
Message-ID: <8992@jpl-devvax.JPL.NASA.GOV>
Date: 3 Aug 90 16:42:05 GMT
References: <7193@star.cs.vu.nl> <8936@jpl-devvax.JPL.NASA.GOV> <1990Aug2.225410.2587@NCoast.ORG>
Reply-To: lwall@jpl-devvax.JPL.NASA.GOV (Larry Wall)
Organization: Jet Propulsion Laboratory, Pasadena, CA
Lines: 47

In article <1990Aug2.225410.2587@NCoast.ORG> allbery@ncoast.ORG (Brandon S. Allbery KB8JRR) writes:
: I would like to have a per-filehandle immediate-flush flag to replace the
: current $| variable.  There are times when I want buffered output on one
: filehandle but not the others.

Er, we seem to laboring under a slight misapprehension here.  The $| variable
already IS a per-filehandle immediate-flush flag.  The actual flag is stored
out in the I/O structure--look at IOF_FLUSH in stab.h.  When you refer to
the $| varialbe, it's referring to that flag for the currently selected
output filehandle, and not to the flag for any other filehandle.

: Additionally, I'd like a "setbuf" function, again per-filehandle.
: 
: Reason?  I sat down and wrote a cpio in Perl in response to the tar in Perl
: that was posted a few weeks ago, including a library to do tape buffering...
: and then remembered that Perl uses stdio and its default buffers.  I can turn
: off buffering with $|, but it then goes character at a time instead of print
: statement at a time, no?

Depends on your implementation of STDIO, and what kind of thing you're writing
to.  Turning off $| merely makes it not do flushes.  If it was unbuffered
before, having $| on doesn't help you, and if it was buffered before, turning
$| off won't make stdio unbuffered.  On most systems, output to (raw) tape and
disks is buffered.

: And I don't necessarily want to unbuffer stdout
: while I'm doing this, and setting $| every time I want to do tape I/O is a
: major pain.  Worse is to have a setbuf and have it apply to all filehandles
: via $|; the standard tape block size on our machine is 512K bytes....

$| is not a global, and it has nothing to do with setbuf.

: This buffering problem is the reason I haven't tried to post my cpio library;
: as is, it has a lot of excess baggage via the nonfunctional tape-buffering
: routines, and it won't write tapes with compatible block sizes.  Worse, the
: cpio-reading part will break on machines with stupid tape drivers which deal
: with reading a small chunk of a large block (i.e. stdio-buffered read of a
: 20-blocked cpio tape) by returning the requested amount of data and discarding
: the rest of the block.  And yes, there are machines which do that!
: Mercifully, I've forgotten which one it was, but it screwed me up for a while
: back when I was free-lance consulting.

On machines with syscall(), you can get around dumb stdio limitations using
it.  It might be worthwhile for me to add a sysread() and syswrite()
builtin to bypass stdio.

Larry