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List: linux-xfs
Subject: Re: [PATCH 4/8] xfstests: Move fallocate include into global.h
From: Dave Chinner <david () fromorbit ! com>
Date: 2014-02-28 22:31:05
Message-ID: 20140228223105.GE13647 () dastard
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On Fri, Feb 28, 2014 at 11:17:41AM -0600, Eric Sandeen wrote:
> On 2/28/14, 10:11 AM, Lukas Czerner wrote:
> > Move the inclusion of falloc.h with all it's possible defines for the
> > fallocate mode into global.h header file so we do not have to include
> > and define it manually in every tool using fallocate.
> >
> > Signed-off-by: Lukas Czerner <lczerner@redhat.com>
>
> I like the direction, but I think this changes behavior a little bit.
>
> #ifdef FALLOCATE came from an autoconf macro:
>
> AC_DEFUN([AC_PACKAGE_WANT_FALLOCATE],
> [ AC_MSG_CHECKING([for fallocate])
> AC_TRY_LINK([
> #define _GNU_SOURCE
> #define _FILE_OFFSET_BITS 64
> #include <fcntl.h>
> #include <linux/falloc.h> ],
> [ fallocate(0, 0, 0, 0); ],
> [ have_fallocate=true; AC_MSG_RESULT(yes) ],
> [ have_fallocate=false; AC_MSG_RESULT(no) ])
> AC_SUBST(have_fallocate)
> ])
>
> (at least I think so?)
Not quite. autoconf defines "have_fallocate" to match the variable
name in the AC_SUBST() macro above. The makefiles do this:
include/builddefs.in:HAVE_FALLOCATE = @have_fallocate@
include/builddefs.in:HAVE_FALLOCATE = @have_fallocate@
to define HAVE_FALLOCATE at the makefile level, and then they do
this to pass it into the C source:
ltp/Makefile:ifeq ($(HAVE_FALLOCATE), true)
ltp/Makefile:LCFLAGS += -DFALLOCATE
src/Makefile:ifeq ($(HAVE_FALLOCATE), true)
src/Makefile:LCFLAGS += -DHAVE_FALLOCATE
> and so #ifdef FALLOCATE meant that
> an fallocate syscall actually exists. With your changes,
> the test is now whether the fallocate *header* exists.
It actually tests both, because if header doesn't exist, the compile
of the test stub will fail in the macro will fail. So, no change
there, really.
> falloc.h is part of kernel-headers, not glibc. So it's
> possible that there's a divergence between the two.
Right, which is why we need to test both ;)
> I think it's probably ok. Build-time checks should
> determine whether we are able to _build_ and yours do that.
> Each caller of fallocate (or each test using it) then probably
> needs to ensure that the functionality it wants is actually
> available at runtime and handle it if not.
>
> So I'll give this a
>
> Reviewed-by: Eric Sandeen <sandeen@redhat.com>
>
> but maybe the above rambling will ring alarm bells for
> someone else... ;)
I need to look at it all in more detail. I thought I'd just explain
exactly what was happening with autoconf here...
Cheers,
Dave.
--
Dave Chinner
david@fromorbit.com
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