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Date: Fri, 16 Apr 2021 20:00:13 -0400
From: Rich Felker <dalias@...c.org>
To: James Y Knight <jyknight@...gle.com>
Cc: musl@...ts.openwall.com,
Érico Nogueira <ericonr@...root.org>
Subject: Re: [PATCH 2/2] include <stdc-predef.h> in <features.h>
On Fri, Apr 16, 2021 at 06:12:52PM -0400, James Y Knight wrote:
> Ugh, I thought Clang had added support for this years ago. But it looks
> like the change (https://reviews.llvm.org/D34158) never actually made it
> in; it ran into some test failures after being committed and was
> reverted, and then never reapplied. :(
The story of LLVM.. Random junk getting committed and kept, actually
important and correct changes getting reverted because of bogus tests.
Rich
> On Fri, Apr 16, 2021 at 2:51 PM Rich Felker <dalias@...c.org> wrote:
>
> > On Fri, Apr 16, 2021 at 02:57:21PM -0300, Érico Nogueira wrote:
> > > Em 16/04/2021 11:26, Rich Felker escreveu:
> > > >On Thu, Apr 15, 2021 at 09:35:21PM -0300, Érico Nogueira wrote:
> > > >>GCC source code does contain a function to pre-include the
> > > >><stdc-predef.h> header for glibc targets, but even so glibc still
> > >
> > > I seem to have been mistaken about the feature being glibc specific;
> > using
> > >
> > > echo "" | cc -xc - -E
> > >
> > > it seems the file does end up being included automatically.
> > >
> > > However, when using clang instead of gcc, it isn't included
> > > automatically. I don't know if this is something that clang ought to
> > > fix, is there some sort of standard about <stdc-predef.h>? Michael
> > > Forney's cproc compiler doesn't seem to touch it either.
> >
> > It's not a standard, but given that it's established I don't see any
> > reasonable argument for other compilers not to just do the same. You
> > can always fix them manually with CC="clang -include stdc-predef.h" or
> > similar though.
> >
> > > >>includes it in their own <features.h> header. furthermore, even if GCC
> > > >>implemented this for musl targets, it is still necessary for other
> > > >>compilers or previous versions of GCC.
> > > >>---
> > > >> include/features.h | 2 ++
> > > >> 1 file changed, 2 insertions(+)
> > > >>
> > > >>diff --git a/include/features.h b/include/features.h
> > > >>index 85cfb72a..f3d53cbe 100644
> > > >>--- a/include/features.h
> > > >>+++ b/include/features.h
> > > >>@@ -1,6 +1,8 @@
> > > >> #ifndef _FEATURES_H
> > > >> #define _FEATURES_H
> > > >>+#include <stdc-predef.h>
> > > >>+
> > > >> #if defined(_ALL_SOURCE) && !defined(_GNU_SOURCE)
> > > >> #define _GNU_SOURCE 1
> > > >> #endif
> > > >>--
> > > >>2.31.1
> > > >
> > > >I've hesitated to do this because features.h is not consistently
> > > >included from all standard headers (only if it's needed), and the
> > > >result would be inconsistent exposure of these macros. (Also
> > > >inconsistent if they're checked before any standard headers are
> > > >included, which is unfixable.) I think it makes more sense to just add
> > > >"-include stdc-predef.h" to the compiler specfile or equivalent if it
> > > >doesn't auto-include it, so that you get behavior that actually
> > > >matches the spec.
> > >
> > > Do you know if clang can use the specfile? That would make it worth
> > > it adding the entry, since GCC has the expected behavior already.
> >
> > No; specfiles are highly tied to GCC's compiler driver architecture.
> > clang might have some other equivalent mechanism though.
> >
> > Rich
> >
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