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Date: Fri, 16 Apr 2021 14:57:21 -0300
From: √Črico Nogueira <>
To: Rich Felker <>
Subject: Re: [PATCH 2/2] include <stdc-predef.h> in <features.h>

Em 16/04/2021 11:26, Rich Felker escreveu:
> On Thu, Apr 15, 2021 at 09:35:21PM -0300, √Črico 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.

>> 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.

> Rich

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