appending last argument to (wrap(ed)) variadic function call

[Alexandre François Garreau]

I want to wrap a call to a variadic function (which have both a really 
variadic version, and one taking a va_list as argument), and let its last 
argument be fixed by this wrapper (accordingly modifying the fixed argument 
that specify about the variable arguments and their types)… how can I do 
that?

That last argument happens to need to be declared (in a local scope, in my 
case), so I can do it with a macro, unless I resort to GNU compound 
statements (“({char * trailing[2]; ...; ret_value;})”).

I want to do something like:
#define escanf(argc, format, ...) ({ char trailing[2], *format = 
astrcat(format, "%1s"); int ret = escanf(format, __VA_ARGS__, &trailing); 
free(format); ret != argc; })

But in a function… possibly itself variadic, so working with a va_list.