Create va_list dynamically

Question

I have a function

void foo(int cnt, va_list ap);

I need to use it, but requirement is quite strict, number of va_list vary an

Answer 1

What you are trying to use is alloca. A va_list object can not store variables, the function call stores them, and you can only access it via va_list. These variables are only valid during the call, and they get ovverwriten afterwards.

THIS WILL NOT WORK:

va_list func(int dummy, ...)
{
   va_list result;
   va_start(result, dummy);
   return result;
}

To allocate memory on the stack, without having to write a variadic functions use alloca. It works more or less like malloc, but you don't have to call free, it automagically frees itself when you leave the scope.

int * local = (int *) alloca(3 * sizeof(int));
local[0] = 10;
local[1] = 20;
local[2] = 30;

It's fundamentally the same as writing

int local[3];
local[0] = 10;
local[1] = 20;
local[2] = 30;

But with alloca 3 does not need to be a constant. Again you can only use it inside the enclosing scope, so do not return it from the function.

if what you want from a va_list is the multiple types in one list consider writing a union like this:

union variant
{
    int          i;
    unsigned int u;
    float        f;
    double       d;
    const char * s;
    void *       v;
};