Variable number of arguments in C functions

Question

Hello. I know how to achieve creating a function that accepts a variable number of arguments. For example:
#include <stdio.h>
#include <stdarg.h>
int add (int x, ...);
int main (int argc, const char * argv[])
int result = add(3, 5, 3, 7);
printf("%d", result);
return 0;
int add (int x, ...)
va_list argList;
va_start(argList, x);
int sum = 0;
int i;
for (i = 0; i < x; ++i)
sum += va_arg(argList, int);
va_end(argList);
return sum;
The first argument, x, is sent to the function and represents how many additional optional arguments will be sent to the function (3 in the above example). Then, the definition of the add function totals those remaining (3) arguments, returning a value of (in this case) 15, which main then prints to the console. Simple enough, but here's my question:
What if I want to achieve this same optional arguments concept without having to send the function the number of optional arguments it will be accepting. For example, the printf() function takes an optional number of arguments, and nowhere there do you have to specify an extra argument that represents the number of additional optional arguments being passed (unless maybe the number of formatting specifiers in the first argument determines this number). Can this be done? Does anyone have any input here? Thanks in advance.

Answer 1

Hi Tron -
I looked over my first response again, and it needs to be corrected. Fortunately Bob and Hansz straightened everything out nicely, but I still need to fix my post:
RayNewbie wrote:
Yes, the macros are designed to walk a list of args when neither the number of args or their type is known.
The above should have said. "The macros are designed to walk a list of args when neither the number of args or their type is known _at compile time_".
If I may both paraphrase and focus your original question, I think you wanted to know if there was any way the function could run without knowing the number of args to expect. The answer to this question is "No". In fact at runtime, the function must know both the number of args and the type of each.
Tron55555 wrote:
... the printf() function takes an optional number of arguments, and nowhere there do you have to specify an extra argument that represents the number of additional optional arguments being passed (unless maybe the number of formatting specifiers in the first argument determines this number).
As both Bob and Hansz have explained, the underlined statement is correct. Similarly, the example from the manual gives the number and types of the args in the first string arg.
Hansz also included an alternative to an explicit count or format string, which is to terminate the arg list with some pre-specified value (this is sometimes called a "sentinel" value. However when using a sentinel, the called function must know the data types. For example, you could never simply terminate the args with a sentinel and then pass a double followed by an int. The combined length of these args is 8 + 4 => 12 bytes, so unless the function knew which type was first at compile time, it wouldn't be possible to determine whether the list was 4+8 or 8+4 at runtime.
If you're interested in knowing why a variable arg function is limited in this way, or in fact how any C function finds its args, its parameters, and how to return to the calling function, you might want to do some reading about the "stack frame". You can learn the concept without getting into any assembler code. Here's an article that might be good to start with: [http://en.citizendium.org/wiki/Stack_frame].
After you have some familiarity with the stack frame, take a look at the expansions of the stdarg macros and see if you can figure out how they work, especially how va_arg walks down the stack, and what info is required for a successful trip. Actually, I don't think you can find these expansions in the stdarg.h file for Darwin; it looks like the #defines point to built-in implementations, so here are some typical, but simplified defs:
// these macros aren't usable; necessary type casts have been removed for clarity
typedef va_list char*;
#define va_start(ap,arg1) ap = &arg1 + sizeof(arg1)
#define va_arg(ap,type) *(ap += sizeof(type))
#define va_end(ap) 0
Note that I"m trusting you not to start asking questions about the stack or the above macros until you've studied how a stack works and how the C stack frame works in particular.
- Ray