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Date: Tue, 6 May 2008 01:35:04 -0400
From: David Schultz <das@FreeBSD.ORG>
To: John Baldwin <jhb@FreeBSD.ORG>
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Cc: Peter Jeremy <peterjeremy@optushome.com.au>,
	Alfred Perlstein <alfred@FreeBSD.ORG>,
	src-committers@FreeBSD.ORG, cvs-all@FreeBSD.ORG, cvs-src@FreeBSD.ORG
Subject: Re: cvs commit: src/include stdio.h src/lib/libc/stdio clrerr.c
	feof.c ferror.c fileno.c getc.c getchar.c local.h putc.c
	putchar.c xprintf.c
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On Mon, May 05, 2008, John Baldwin wrote:
> On Monday 05 May 2008 03:24:17 pm Peter Jeremy wrote:
> > On Mon, May 05, 2008 at 02:59:28PM -0400, John Baldwin wrote:
> > >On Monday 05 May 2008 02:40:03 pm Alfred Perlstein wrote:
> > >> I'm _not_ objecting, just interested in why.
> > >> 
> > >> Any references to discussions on this?  Are we now safe for
> > >> future compat or something?
> > >
> > >Having FILE be opaque broke just about every 'configure' script on the 
> > >planet. :(
> > 
> > Either autoconf and friends are _intended_ as impediments to
> > portability or they are completely broken by design.
> 
> It appears that autoconf only believes a type is real if you can typedef it to 
> another type, cast 0 to a valid pointer to the new typedef'd type, and do a 
> sizeof() of the typdef'd type.  The last is where having an opaque type 
> breaks down for scripts that want to make sure FILE is a real type.

I believe FILE is required to be a complete type, but it need not
expose any usable fields to applications. For instance,

       typedef struct { char __pad[__FILE_LENGTH]; } FILE;

would be valid. I don't know whether that breaks lots of apps or
not. Of course that still makes it possible to write bogus apps
that depend on sizeof(FILE).