Function rkrdb_select_basic 

#[unsafe(no_mangle)]
pub unsafe extern "C" fn rkrdb_select_basic(
    id: usize,
    traj_id: i64,
    symbol: *const c_char,
    natoms_min: u32,
    natoms_max: u32,
    limit: u32,
) -> c_int

Select by required symbol (optional) and natoms range (use 0, UINT32_MAX for any). Results stored internally; use rkrdb_result_count / rkrdb_result_key.