Re: Under Cygwin ZSH, "diff -" fails unless forced into text mode

[Peter Stephenson <p.w.stephenson@xxxxxxxxxxxx>]

On Thu, 17 Jul 2008 08:59:29 +0900
henman@xxxxxxxxxxxxxxxx wrote:
> I saw the following question at the cygwin group and figure that it is
> a zsh related question.   
> 
> I tried to see if setopt NO_MULTIOS had any bearing on the problem,
> but it didn't.
> 
> The question follows:
> 
> 
> --------  extract
> From: <Gerald.Williams@xxxxxxxxxxxx>
> To: <cygwin@xxxxxxxxxx>
> Date: Wed, 16 Jul 2008 23:19:02 +0200
> Subject: Under Cygwin ZSH, "diff -" fails unless forced into text mode
> 
> Does anyone know why, under Cygwin ZSH, the following fails?
> 
>  zsh% cat foo | diff - foo
>  diff: -: Illegal seek
>  zsh%

The short, but unsatisfactory, answer is that it's not required to
work.  diff expects to have a seekable file, for which a pipe doesn't
necessarily qualify: it depends partly on the OS and also partly on how
far back you need to seek, so you sometimes get away with it.

This is why zsh has both <(...) and =(...) process subtitution forms:
the second is a bit less efficient because it's guaranteed to expand to
a real file.

> It passes if I force it into text mode:
> 
>  zsh% cat foo | diff -a - foo
>  zsh%

There's some Cygwin-specific code in main.c to force read-only files
into text mode.  This is a fairly murky area and I have no idea how that
affects pipes.

-- 
Peter Stephenson <p.w.stephenson@xxxxxxxxxxxx>
Web page now at http://homepage.ntlworld.com/p.w.stephenson/