in reply to
XS/Inline::C and ellipsis syntax
I'll agree that it introduces too many problems and should be avoided, especially if the code needs to be portable.
However, if absolutely necessary, there's the option to create a stack, then pass the pointer off to a va_arg() call. The stack can be made by testing SvNOK, SvPOK, or SvIOK and incrementing the pointer accordingly. Some compilers want a pointer that increments, others want pointers that de-increment. On retrieval, the format string indicates how the va_arg should be taken off the stack (int, char, double).
This example is the clearest thing I've found on forged va_lists. It also explains how the alignment can cause problems: