ptrdiff_t c_hash_map_for_each(const c_hash_map *const _hash_map,
                              void (*const _action_key)(const void *const _key),
                              void (*const _action_data)(void *const _data));

ptrdiff_t c_hash_map_for_each(c_hash_map *const _hash_map,
                              void (*const _action_key)(const void *const _key),
                              void (*const _action_data)(void *const _data));

//...
mutable int value;
//...