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

On Thu, Nov 26, 2009 at 7:46 PM, Junio C Hamano [off-list ref] wrote:

Erik Faye-Lund [off-list ref] writes:

quoted

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