Function secp256k1::ffi::secp256k1_ecdh

pub unsafe fn secp256k1_ecdh(
    cx: *const Context, 
    out: *mut SharedSecret, 
    point: *const PublicKey, 
    scalar: *const u8, 
    _hashfp: EcdhHashFn, 
    _data: *mut c_void
) -> i32

Sets out to point[0..16]||scalar[0..16]