Function vsnprintf 

#[unsafe(no_mangle)]
pub unsafe extern "C" fn vsnprintf(
    s: *mut c_char,
    n: size_t,
    format: *const c_char,
    ap: VaList<'_>,
) -> c_int

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