[voidUpdate]

I don't use C enough to know what the convention is for throwing an error when the function can return a number anyway. You'd have to ask someone else

[zbentley]

In C, errors are usually indicated by a negative return value constant, crashing the program with abort, or setting the errno global (thread-local, but whatever) and expecting callers to check it. Sometimes multiple of those.

[QuercusMax]

One reasonably common pattern is to have the return value indicate success / error, and you pass in a pointer to the value which will be mutated if successful.

[jamesfinlayson]

Yep, lots of the Windows API does it this way.