Re: [PATCH] 3.0: require C99 flexible-array member syntax

From: Junio C Hamano <gitster@pobox.com>
Date: Dec 12, 2025, 12:54 UTC
Message-ID: <xmqqqzszc13d.fsf@gitster.g>
In-Reply-To: <aTvT8A31tQFvCvtL@pks.im>

Patrick Steinhardt <ps@pks.im> writes:

Show 6 quoted lines

>> As we are pushing more and more C99 language features, let's declare
>> the historical forms of flexible array member support obsolete and
>> require C99 syntax from all compilers that want to compile Git.
>
> Is there any specific reason why this is tied to the 3.0 breaking
> changes document?

Convenience, mostly.

It is not like we weren't allowing use of an advanced syntax and then trying to see if everybody can use it, which was perfect match for our past "weather balloon" approach. Rather, we know that many platforms did not know how to grok fam[] with an empty brackets, and have workarounds fam[0] and fam[1]. This change proposes to break the platforms that has to rely on the latter.

> So I wonder whether we should instead convert one of the sites that
> currently uses FLEX_ARRAY to use C99 flexible arrays unconditionally.

OK, that is easy enough. Or just remove it altogether and see who screams, like the attached.

----- >8 -----
Subject: [PATCH] FLEX_ARRAY: require platforms to support the C99 syntax

Before C99 syntax to express that the final member in a struct is an array of unknown number of elements, i.e.,

	struct {
		...
		T flexible_array[];
	};

came along, GNU introduced their own extension to declare such a member with 0 size, i.e.,

		T flexible_array[0];

and the compilers that did not understand even that were given a way to emulate it by wasting one element, i.e.,

		T flexible_array[1];

As we are using more and more C99 language features, let's see if the platforms that still need to resort to the historical forms of flexible array member support are still there, by forcing all the flex array definitions to use the C99 syntax and see if anybody screams (in which case reverting the changes is rather easy).

Signed-off-by: Junio C Hamano <gitster@pobox.com>
---

 * In the longer term, I'd love to import the fortified memcpy()
   invented for the Linux kernel project that takes advantage of
   advanced compiler features like __builtin_object_dynamic_size()
   and friends, and use of C99 flexible array syntax is a good first
   step to make it happen.

 git-compat-util.h | 33 ++-------------------------------
 1 file changed, 2 insertions(+), 31 deletions(-)

diff --git c/git-compat-util.h w/git-compat-util.h
index 398e0fac4f..8e3f3a58a3 100644
--- c/git-compat-util.h
+++ w/git-compat-util.h
@@ -38,37 +38,8 @@ struct strbuf;
 DISABLE_WARNING(-Wsign-compare)
 #endif
 
-#ifndef FLEX_ARRAY
-/*
- * See if our compiler is known to support flexible array members.
- */
-
-/*
- * Check vendor specific quirks first, before checking the
- * __STDC_VERSION__, as vendor compilers can lie and we need to be
- * able to work them around.  Note that by not defining FLEX_ARRAY
- * here, we can fall back to use the "safer but a bit wasteful" one
- * later.
- */
-#if defined(__SUNPRO_C) && (__SUNPRO_C <= 0x580)
-#elif defined(__GNUC__)
-# if (__GNUC__ >= 3)
-#  define FLEX_ARRAY /* empty */
-# else
-#  define FLEX_ARRAY 0 /* older GNU extension */
-# endif
-#elif defined(__STDC_VERSION__) && (__STDC_VERSION__ >= 199901L)
-# define FLEX_ARRAY /* empty */
-#endif
-
-/*
- * Otherwise, default to safer but a bit wasteful traditional style
- */
-#ifndef FLEX_ARRAY
-# define FLEX_ARRAY 1
-#endif
-#endif
-
+#undef FLEX_ARRAY
+#define FLEX_ARRAY /* empty - weather balloon to require C99 FAM */
 
 /*
  * BUILD_ASSERT_OR_ZERO - assert a build-time dependency, as an expression.