[PATCH v3 2/7] malloc/obstack.h: Assume GCC supports __extension__

[Joseph Myers]

On Tue, 11 Nov 2025, Alejandro Colomar wrote:

> > * a stdarg.h that defines va_list
> 
> The relevant change is:
> 
> 	 #if defined __USE_XOPEN2K || defined __USE_XOPEN2K8
> 	-# ifdef __GNUC__
> 	-#  ifndef _VA_LIST_DEFINED
> 	-typedef __gnuc_va_list va_list;
> 	-#   define _VA_LIST_DEFINED
> 	-#  endif
> 	-# else
> 	+# ifndef __GNUC__
> 	 #  include <stdarg.h>
> 	 # endif
> 	 #endif
> 
> I'm only changing the assumption if __GNUC__ is defined.  This change
> assumes that GCC provides va_list in its own header.  However, I'm not
> changing any assumptions for non-__GNUC__ compilers.

I think this change (to <stdio.h>) is actually incorrect for modern GCC 
(indeed, I don't see how the patched glibc would have passed the conform/ 
tests).

Early on, the header does:

#define __need___va_list
#include <stdarg.h>

The point of that is, if the __need_* protocol is working correctly, to 
define __gnuc_va_list but *not* va_list (because ISO C, for example, does 
not allow <stdio.h> to define va_list, despite having interfaces such as 
vprintf that involve va_list in their types).  So with GCC, or compatible 
compilers, __gnuc_va_list will be defined, but not va_list - unless the 
user had previously included <stdarg.h> themself without __need___va_list 
defined.

But then there is the POSIX requirement (CX-tagged in the 2008 and 2024 
editions, XSI-tagged in the 2004 edition) to define va_list in <stdio.h>.  
That's what the code you're changing implements.  And your change would 
cause <stdio.h> to fail to implement that requirement with current GCC.

-- 
Joseph S. Myers
josmyers@redhat.com