Special return value for application-supplied callbacks

There is a special-purpose return value which is not returned by any function in the actual Cryptoki API, but which may be returned by an application-supplied callback function. It is:

CKR_CANCEL: When a function executing in serial with an application decides to give the application a chance to do some work, it calls an application-supplied function with a CKN_SURRENDER callback (see Section 11.17). If the callback returns the value CKR_CANCEL, then the function aborts and returns CKR_FUNCTION_CANCELED.

RSA Security Inc. Public-Key Cryptography Standards - PKCS#11 - v210