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(libc.info)Old Varargs

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Old-Style Variadic Functions
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   Before ISO C, programmers used a slightly different facility for
writing variadic functions.  The GNU C compiler still supports it;
currently, it is more portable than the ISO C facility, since support
for ISO C is still not universal.  The header file which defines the
old-fashioned variadic facility is called `varargs.h'.

   Using `varargs.h' is almost the same as using `stdarg.h'.  There is
no difference in how you call a variadic function; see Note: Calling
Variadics.  The only difference is in how you define them.  First of
all, you must use old-style non-prototype syntax, like this:

     tree
     build (va_alist)
          va_dcl
     {

   Secondly, you must give `va_start' only one argument, like this:

       va_list p;
       va_start (p);

   These are the special macros used for defining old-style variadic
functions:

 - Macro: va_alist
     This macro stands for the argument name list required in a variadic
     function.

 - Macro: va_dcl
     This macro declares the implicit argument or arguments for a
     variadic function.

 - Macro: void va_start (va_list AP)
     This macro, as defined in `varargs.h', initializes the argument
     pointer variable AP to point to the first argument of the current
     function.

   The other argument macros, `va_arg' and `va_end', are the same in
`varargs.h' as in `stdarg.h'; see Note: Argument Macros, for details.

   It does not work to include both `varargs.h' and `stdarg.h' in the
same compilation; they define `va_start' in conflicting ways.