Cardiff will allow long-serving Kevin McNaughton to leave this summer along with Danny Gabbidon and Nicky Maynard.
Highly-rated Atletico Madrid midfielder Saul Niguez has signed a new five-year contract.
Bolton striker Michael O'Halloran has joined Tranmere on loan until January.
The Scotland Under-21 international has made two first-team appearances for Bolton, whilst also enjoying loan spells at Sheffield United and Carlisle.
O'Halloran's only league start came for the Blades in a 2-2 draw against Stevenage in April.