GNU bug report logs - #17145
head fails with implicit stdin on darwin

Previous Next

Full log

View this message in rfc822 format

From: Paul Eggert <eggert <at> cs.ucla.edu>
To: Pádraig Brady <P <at> draigBrady.com>,  Denis Excoffier <gcc <at> Denis-Excoffier.org>
Cc: 17145 <at> debbugs.gnu.org
Subject: bug#17145: head fails with implicit stdin on darwin
Date: Mon, 31 Mar 2014 12:43:48 -0700

[Message part 1 (text/plain, inline)]

On 03/31/2014 05:32 AM, Pádraig Brady wrote:
> It seems we might be using non portable code here.

Yes.

> In dd we have:

I haven't looked at dd carefully, but I assume it's OK. It's a low-level 
program intended for people who want to exercise system calls pretty 
much directly.  If someone wants to lseek on a device, dd should 
probably do just that.

> is there a way we can robustly determine if we have a seekable device 
> or not?

Not as far as I know.  In fact, I expect much of the coreutils code uses 
the word "seekable" to mean "lseek returns a nonnegative value", which 
isn't the same thing that POSIX means. Quite possibly we should clean up 
coreutils' terminology here.

> head -n -1 /dev/zero I.E. we currently treat such devices as empty, 
> and return immediately with success status, whereas treating as a 
> stream of NULs, would result in memory exhaustion while buffering 
> waiting for a complete line. That is probably the more consistent 
> operation at least.

Yes, that sounds right.

> So the attached uses this more conservative test for the --lines=-N case.

Inoticed other instances of the problem, and in general the whole lseek 
business needs a cleanup; the code's too complicated.  Attached is a 
proposed cleanup+fix patch.

[0001-head-port-to-Darwin-and-use-simpler-seeks.patch (text/x-patch, attachment)]

Previous Next

GNU bug tracking system
Copyright (C) 1999 Darren O. Benham, 1997,2003 nCipher Corporation Ltd, 1994-97 Ian Jackson.