Re: [musl] [PATCH 2/2] include <stdc-predef.h> in <features.h>

[Rich Felker <dalias@libc.org>]

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