How do I fill a va_list

Question

If I have a va_list I know how to extract all its elements:

void printInts(int n,...)
{
    va_list va;
    va_start(va, n);
    for(unsigned int i=0; i<n; i++)
    {
        int arg=va_arg(va, int);
        printf("%d",arg);
    }
    va_end(va);
} 

So when I call printInts(3,1,2,3) the va_list get filled of all the parameters.
But how do I manually fill a va_list without using va_start? I mean that I want something like:

va_list va;
push_arg(va, int, 5); // And so on until I fill all parameters
...

I need this because there is a function that accept a va_list as argument, and I don't know how to fill that va_list of all its parameters.

Answer 1

There's no ability to fill a va_list explicitly.

You should write a wrapper function. Say you need to call your function foo, instead of manually filling in a va_list, you define a new function like so:

void call_foo(int arg1, ...)
{
   va_list ap;
   va_start(ap, arg1);
   foo(arg1, ap);
   va_end(ap);
}

Now you can call foo, which takes a va_list, however you like, by doing e.g. call_foo(1,2,3,4);, call_foo(1, 1, "Hello"); etc.

This will only allow you to specify the arguments at compile time, you can't build the arguments at runtime.

Answer 2

Normally, these functions come in pairs. If you have a "va-accepting" function, it is easy to create another one:

void printInts_v(int n, va_list ap)
{
    unsigned int i=0;
    for(i=0; i<n; i++)
    {
        int arg=va_arg(ap, int);
        printf("%d", arg);
    }
}

This function can be called this way:

void printInts(int n,...)
{
    va_list ap;
    va_start(ap, n);
    printInts_v(n, ap);
    va_end(va);
}

But I don't think there is a way to portably pre-fill a va_list for later use.

If you work on one architecture and portability is not an issue, you could craft something on your own, however. How exactly to do that is platform-specific.