Pass va_list or pointer to va_list?

I find the texts quite ambiguous on this question. The simplest is perhaps to look in the standard how predefined functions with va_list are supposed to receive it, e.g vsnprintf. And this is clearly by value and not by reference.

It looks like you'll need to pass a pointer to the va_list. For more info, see the C99 standard document section 7.15.In particular, bullet point 3 states:

The object ap may be passed as an argument to another function; if that function invokes the va_arg macro with parameter ap, the value of ap in the calling function is indeterminate and shall be passed to the va_end macro prior to any further reference to ap

[my italics]

Edit: Just noticed a footnote in the standard:

215) It is permitted to create a pointer to a va_list and pass that pointer to another function, in which case the original function may make further use of the original list after the other function returns

So you can pass a pointer to the va_list and do va_arg(*va_list_pointer) in the called function.

In my understanding, you're supposed to pass the va_list directly (not a pointer to it). This seems to be supported by comp.lang.c:

"A va_list is not itself a variable argument list; it's really sort of a pointer to one. That is, a function which accepts a va_list is not itself varargs, nor vice versa. "