Re: [PATCH] Fix "identifier redeclared" compilation error with SUN cc.

[Junio C Hamano <gitster@pobox.com>]

Björn Steinbrink <B.Steinbrink@gmx.de> writes:

> Guido, could you please test this patch?
>
> I have no clue which versions of SUN's cc are affected, so I simply enabled
> the workaround for all versions. Someone with more knowledge about that
> should probably limit the check to only do that for the broken versions.
>
>  git-compat-util.h |    2 ++
>  1 files changed, 2 insertions(+), 0 deletions(-)

> diff --git a/git-compat-util.h b/git-compat-util.h
> index ede9408..c3ff4b4 100644
> --- a/git-compat-util.h
> +++ b/git-compat-util.h
> @@ -6,6 +6,8 @@
>  #ifndef FLEX_ARRAY
>  #if defined(__GNUC__) && (__GNUC__ < 3)
>  #define FLEX_ARRAY 0
> +#elif defined(sun) || defined(__SUN__)
> +#define FLEX_ARRAY 1
>  #else
>  #define FLEX_ARRAY /* empty */
>  #endif

This feels a bit too narrow and too broad at the same time,
doesn't it?

As I suspect there are other compilers that do not implement
flexible array members (so you cannot use "member[]") nor older
gcc extension of zero sized member (so you cannot use
"member[0]" either), this checking specifically for Sun is too
narrow.

On the other hand, as you said, this is too broad, because not
everybody may be using the SUN compiler on Sun, nor the version
that does not understand flexible array members.

But being broad should always be safer, albeit a bit wasteful.

How about doing it this way?

  # ifndef FLEX_ARRAY
  #   if defined(__GNUC__)
  #     if (__GNUC__ < 3)
  #       define FLEX_ARRAY 0
  #     else
  #       define FLEX_ARRAY /* empty */
  #     endif
  #   else
        /* more cases we know we can use 0 or empty can come here */
  #   endif
  # endif

  /* if still undefined, default to the safe, old fashioned way */
  # ifndef FLEX_ARRAY
  #   define FLEX_ARRAY 1
  # endif

The basic idea is:

 * The user (from Makefile command line, config.mak, or you
   could add autoconf test) can pass -DFLEX_ARRAY=... to specify
   exactly what should happen;

 * Otherwise, if we happen to know for sure that we can use "0"
   or "/* empty */" with the compiler, we define FLEX_ARRAY;
   currently we know such things for gcc.

 * For everybody else, we use safer default of "1".  IOW, if you
   know your compiler does not grok "/* empty */" nor "0", you
   do not have to do anything special but use the default case
   as everybody else.