Okay, maybe I should rephrase it. It takes more than an hour to carefully think and analyze, and then implement the thing, and then make sure there aren't any bugs. Thinking things through beforehand makes the possibility of errors less likely, but even then you may have made a silly mistake at some point or another. That is why we have things like unit tests and code reviews, because even the best programmers make mistakes. I certainly wouldn't trust C code written by a single developer in one hour to be production-ready.
I can rephrase as well. I'm not going to rush your interview code to production, but the review cycle needs to start with a first version. We'll refine as needed, but this is a simple enough problem to expect well-structured code in one hour.
