Re: [PATCH/RFC 02/11] strbuf: add non-variadic function strbuf_vaddf()

[Erik Faye-Lund <kusmabite@googlemail.com>]

On Thu, Nov 26, 2009 at 7:46 PM, Junio C Hamano <gitster@pobox.com> wrote:
> Erik Faye-Lund <kusmabite@googlemail.com> writes:
>> In practice it seems that something like the following works
>> portably-enough for many applications, dunno if it's something we'll
>> be happy with:
>> #ifndef va_copy
>> #define va_copy(a,b) ((a) = (b))
>> #endif
>
> Since an obvious implementation of va_list would be to make it a pointer
> into the stack frame, doing the above would work on many systems.  On
> esoteric systems that needs something different (e.g. where va_list is
> implemented as a size-1 array of pointers, va_copy(a,b) needs to be an
> assignment (*(a) = *(b))), people can add compatibility macro later.
>
> Historically some systems that do have a suitable implementation had it
> under the name __va_copy() instead, so it would have been better to define
> it as something like:
>
>    #ifndef va_copy
>    # ifdef __va_copy
>    # define va_copy(a,b) __va_copy(a,b)
>    # else
>    # /* fallback for the most obvious implementation of va_list */
>    # define va_copy(a,b) ((a) = (b))
>    # endif
>    #endif
>
> But I do not know it still matters in practice anymore.

Perhaps I can do one better: use memcpy instead of standard
assignment. The Autoconf manual[1] suggests that it's more portable.
Something like this:

#ifndef va_copy
# ifdef __va_copy
#  define va_copy(a,b) __va_copy(a,b)
# else
#  define va_copy(a,b) memcpy(&a, &b, sizeof (va_list))
# endif
#endif

I'll add this to git-compat-util.h this for the next round unless
someone yells really loud at me.

*[1] http://www.gnu.org/software/hello/manual/autoconf/Function-Portability.html#index-g_t_0040code_007bva_005fcopy_007d-357
-- 
Erik "kusma" Faye-Lund