Calling a function with va_list argument needs va_start() at the beggining?

Question

A function with the following header:

int max(int n, va_list vals)

Called inside the function:

int max_first(int n, ...)

Needs a va_start(vals, n) invocation at the body's beginning? I tried without and it works but I don't understand which is the correct way of doing it.

int max(int n, va_list vals)
{
    va_start(vals, n);
    // etc
}

Answer 1

The question is if

int max(int n, va_list vals)

Called inside the function:

int max_first(int n, ...)
 

Needs a va_start(vals, n) invocation at the body's beginning?

No, it does not and it must not, the correct pattern is as follows:

int max_first(int n, ...) {
    va_list vals;
    va_start(vals, n);
    int rv = max(n, vals);
    va_end(vals);
    return rv;
}

then

int max(int n, va_list vals) {
    for (int i = 0; i < n; i ++) {
        int val = va_arg(vals, int);
        ...
    }

    ...
}

i.e. you can call va_start only in a function that has ... and you need to pass in the argument immediately preceding the ..., and each call to va_start must be always followed by va_end for the same value, and if you pass it to a function then you must call va_end right afterwards without using it in the calling function; if you want to process the arguments again, you must then call va_start again.