Pass va_list or pointer to va_list?

Question

Suppose I have a function which takes variadic arguments (...) or a va_list passed from another such function. The main logic is in this function itself (let's call it f1), but I want to have it pass the va_list to another function (let's call it f2) which will determine the next argument type, obtain it using va_arg, and properly convert and store it for the caller to use.

Is it sufficient to pass a va_list to f2, or is it necessary to pass a pointer to va_list. Unless va_list is required to be an array type or else store its position data at the location the va_list object points to (rather than in the object itself), I can't see how passing it by value could allow the calling function (f1) to 'see' the changes the called function made by va_arg.

Can anyone shed light on this? I'm interested in what the standard requires, not what some particular implementation allows.

Answer 1

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. "

Answer 2

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.

Answer 3

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.

Answer 4

Functions in standard C library pass va_list element itself (man 3 vprintf):

   #include <stdarg.h>

   int vprintf(const char *format, va_list ap);
   int vfprintf(FILE *stream, const char *format, va_list ap);
   int vsprintf(char *str, const char *format, va_list ap);
   int vsnprintf(char *str, size_t size, const char *format, va_list ap);