+ :param ptr: pointer to an initialized `memcached_st` struct
+ :param group_key: key namespace
+ :param group_key_length: length of the key namespace without any terminating zero
+ :param key: the key
+ :param key_length: length of the key without any terminating zero
+ :param offset: offset to increment/decrement
+ :param initial: initial value if `key` does not exist and `expiration` is not `MEMCACHED_EXPIRATION_NOT_ADD`
+ :param expiration: expiration as a unix timestamp or as relative expiration time in seconds
+ :param value: the resulting value after initialization/increment/decrement
+ :returns: `memcached_return_t` indicating success