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

[Alejandro Colomar]

Hi Joseph,

On Mon, Nov 10, 2025 at 06:32:38PM +0000, Joseph Myers wrote:
> On Mon, 10 Nov 2025, Alejandro Colomar wrote:
> 
> > GCC supports __extension__ since at least GCC 2.8.
> 
> This is a file shared with gnulib, but __extension__ is generally only 
> relevant with -pedantic; there's no problem with supporting its absence 
> with non-GCC compilers, so no real advantage in removing this code.
> 
> In fact, GCC 2.7 (the oldest version supporting ELF for any target 
> supported now by glibc) *does* support __extension__, so the more 
> interesting question about such a condition is whether there are specific 
> sub-cases of __extension__ that are used in installed headers and are the 
> reason for the 2.8 version here.

git-blame(1) in gnulib points to this:

	commit 6421fd46004655f39afca254ad02b72ab97f5ecb
	Author: Paul Eggert <eggert@cs.ucla.edu>
	Date:   Thu Mar 27 13:38:41 2014 -0700

	    obstack: Remove ancient NeXTSTEP gcc support conditional
	    
	    This change will ease merging with glibc.  The "#if ... __NEXT__"
	    causes a warning with -Wundef which glibc now enables by default.
	    Problem reported by Will Newton in
	    <http://lists.gnu.org/archive/html/bug-gnulib/2014-03/msg00032.html>.
	    glibc <sys/cdefs.h> now uses __extension__ for GCC 2.8 or later,
	    so go with that.
	    * lib/obstack.h (__extension__):

	[...]

	diff --git a/lib/obstack.h b/lib/obstack.h
	index f847a53706..f92492fc68 100644
	--- a/lib/obstack.h
	+++ b/lib/obstack.h
	@@ -258,10 +258,7 @@ extern int obstack_exit_failure;
	 #define obstack_memory_used(h) _obstack_memory_used (h)
	 
	 #if defined __GNUC__
	-/* NextStep 2.0 cc is really gcc 1.93 but it defines __GNUC__ = 2 and
	-   does not implement __extension__.  But that compiler doesn't define
	-   __GNUC_MINOR__.  */
	-# if __GNUC__ < 2 || (__NeXT__ && !__GNUC_MINOR__)
	+# if ! (2 < __GNUC__ + (8 <= __GNUC_MINOR__))
	 #  define __extension__
	 # endif

That commit message probably refers to this in glibc:

	$ cat ./misc/sys/cdefs.h | grep -C2 __extension__
	/* It is possible to compile containing GCC extensions even if GCC is
	   run in pedantic mode if the uses are carefully marked using the
	   `__extension__' keyword.  But this is not generally available before
	   version 2.8.  */
	#if !(__GNUC_PREREQ (2,8) || defined __clang__)
	# define __extension__		/* Ignore */
	#endif

However, it seems to ignore this:

	commit bc244b778ac3e17089454050ac86662ad51f7fd0
	Author: Jim Meyering <jim@meyering.net>
	Date:   Fri Nov 4 16:50:15 1994 +0000

	    merge with 3.9u1

	[...]
	diff --git a/lib/obstack.h b/lib/obstack.h
	index 01767195da..840a63baaa 100644
	--- a/lib/obstack.h
	+++ b/lib/obstack.h
	[...]
	@@ -269,7 +269,10 @@ int obstack_chunk_size (struct obstack *obstack);
	 #define obstack_blank_fast(h,n) ((h)->next_free += (n))
	 ^L
	 #if defined (__GNUC__) && defined (__STDC__)
	-#if __GNUC__ < 2
	+/* NextStep 2.0 cc is really gcc 1.93 but it defines __GNUC__ = 2 and
	+   does not implement __extension__.  But that compiler doesn't define
	+   __GNUC_MINOR__.  */
	+#if __GNUC__ < 2 || !__GNUC_MINOR__
	 #define __extension__
	 #endif
	 
Which claims that GCC 2 is fine with __extension__.

The commit that added the 2.8 check in <sys/cdefs.h> is
8325d82cf391 (1998-10-05; "Update."), whose ChangeLog only says:

    1998-10-05  Ulrich Drepper  <drepper@cygnus.com>
    
            * misc/sys/cdefs.h (__extension__): Define as empty if no gcc or
            gcc before 2.8 is used.

so, I don't really know why Ulrich did that.  There's some possibility
that he was wrong.  Or maybe it was really not widely available on some
GCC forks that claimed different __GNUC__ and __GNUC_MINOR__ values.
Who knows.


Have a lovely night!
Alex

-- 
<https://www.alejandro-colomar.es>
Use port 80 (that is, <...:80/>).
-------------- next part --------------
A non-text attachment was scrubbed...
Name: signature.asc
Type: application/pgp-signature
Size: 833 bytes
Desc: not available
URL: <https://sourceware.org/pipermail/libc-alpha/attachments/20251111/9bf5065f/attachment.sig>