Well, that depends. Some things may have useless return calls, such as those that try and push out a command.

take, say.. something that initiates garbage collection, or flushes buffers. You can't really say it succeeds or fails other than the fact that you've told it to run or not.

I readily admit, there could be a return value if the request for the subsystem to run is fine, but sometimes, you just don't know. Look at sun's sync function for instance :)