How to get parameters of different type from an ellipsis in C

Question

I have a function which accepts variable number of argumets of different data types using ellipsis. I wanted to know is there any way by which I can get each of them. If not all, atleast separate a particular argument (either first or last parameter) from the rest and va_list from the remaining.
Declaration goes like this :

    int foo (char *a , ...)

Usage :

    result = foo ("I'm a String",2,34.56,"one more String",2323232323);

    result = foo ("I'm another String",3,"again one more String",34.62,111111111);

So, here(usage 1) I want to remove parameter '2' from the va_list got and make another va_list with the rest. Function declaration is flexible, and can be changed accordingly.
Thanks in advance

Answer 1

Note that variable argument functions must have a way to determine the types of the arguments in the ellipsis (va_list). The printf() and scanf() families of functions have the format string to specify the types; the open() system call has an optional third argument but it is always an int; and the execl() function takes a list of arguments all of the same type, char *, terminated by a NULL pointer).

If you have a function:

void foo_tail(va_list args);

You can write your function foo() as:

int foo(char *a, ...)
{
    va_list args;
    va_start(args, a);
    int arg2 = va_arg(args, int);
    foo_tail(args);
    va_end(args);
    return arg2;
}

This achieves what the question asks for:

remove parameter '2' from the va_list got and make another va_list with the rest.

Whether foo_tail() can work out what to do with the va_list it is given is a separate discussion; it is not clear how it would distinguish between the two calls shown in the question, but to handle both, that information would have to be made available somehow. A variable list of mixed types as shown is very problematic.

Answer 2

When you use va_arg the argument you get is actually removed from the va_list. So the rest of the arguments are already in your va_list.

However, you can not, say, get an arbitrary argument from the list. They have to be taken in order. You also must know the actual types of the arguments you're getting.