c_variadic - The Rust Unstable Book

# `c_variadic`

The tracking issue for this feature is: #44930

The `c_variadic` language feature enables C-variadic functions to be defined in Rust. The may be called both from within Rust and via FFI.

## Examples

    #![allow(unused)]
    #![feature(c_variadic)]

    fn main() {
    pub unsafe extern "C" fn add(n: usize, mut args: ...) -> usize {
        let mut sum = 0;
        for _ in 0..n {
            sum += args.arg::<usize>();
        }
        sum
    }
    }