Function memcpy 

#[unsafe(no_mangle)]
pub unsafe extern "C" fn memcpy(
    s1: *mut c_void,
    s2: *const c_void,
    n: size_t,
) -> *mut c_void

See https://pubs.opengroup.org/onlinepubs/9799919799/functions/memcpy.html.

Safety

The caller must ensure that either:

• n is 0, or
  • s1 is convertible to a &mut [MaybeUninit<u8>] with length n, and
  • s2 is convertible to a &[MaybeUninit<u8>] with length n.