Re: Calling va_start multiple times vs __va_copy

[Carlo Wood <carlo at alinoe dot com>]

On Wed, Aug 08, 2001 at 01:38:05PM +0200, Sebastian Ude wrote:
> 
> Is it legitime to parse the optional arguments in a variadic function
> multiple times _without_ copying the va_list through (__)va_copy ?
> 
> I mean a situation like this one:
> 
> void foo( int argno, ... )
>   {
>      va_list ap;
> 
>      va_start(ap, argno);
>      /* parse arguments through va_arg */
>      va_end(ap);
> 
>      va_start(ap, argno);
>      /* parse arguments again */
>      va_end(ap);
>   }
> 
> With GCC, that works - but is that against the standards ?

It will work with almost all OS, but there is at least one
that I know of where va_list is not a struct but a pointer to
a struct.  In the latter case it doesn't work.

You could test for the need to use va_copy by testing if
it is a struct.  I wrote the following test for 'ircu'
(http://ircu.sourceforge.net/):

dnl
dnl Macro: ircu_VA_LIST
dnl
dnl Check if va_list is a struct or an array/pointer.
dnl
AC_DEFUN(ircu_VA_LIST,
[dnl Check if va_list is a struct or an array/pointer.
AC_CACHE_CHECK([if va_list is a struct], ircu_cv_sys_va_list_is_struct,
[AC_TRY_RUN([#include <stdarg.h>
void vtest(va_list vl)
{
  va_arg(vl, int);
}
int test(int ret, ...)
{
  va_list vl;
  va_start(vl, ret);
  vtest(vl);
  ret = va_arg(vl, int);
  va_end(vl);
  return ret;
}
int main(void)
{
  exit(test(0, 0, 1));
}], ircu_cv_sys_va_list_is_struct=yes, ircu_cv_sys_va_list_is_struct=no)])
if test $ircu_cv_sys_va_list_is_struct = yes; then
  AC_DEFINE(VA_LIST_IS_STRUCT)
fi])

This test assumes the current language is C.

-- 
Carlo Wood <carlo@alinoe.com>