This function cancels an executing blocking call in the current thread. A blocking call is any synchronous function that takes a long time to execute and does not return until complete. There are five blocking calls in this implementation of WinHLLAPI. The blocking calls are Get Key (51), Wait (4), Pause (18), Send File (90), and Receive File (91). If one of these is blocking calls are cancelled, the cancelled function will return WHLLCANCEL.