Function semq_diff_id 

pub unsafe extern "C" fn semq_diff_id(
    diff: *const semq_diff_t,
    list: u32,
    i: u64,
    out_u64: *mut u64,
    bytes: *mut *const u8,
    len: *mut u64,
    err: *mut semq_error_t,
) -> semq_status_t

Id i of a list, in canonical order. For u64 kinds *out_u64 is set; for utf8 kinds *bytes/*len are set.