Thanks. Looks like both CriticalSections and mutexes both do exactly what I want them to. I'll probably go with CriticalSections as I'm only using one process and since this is part of an API, efficiency is always a consideration as it is unclear how often the user may use any functions.

Looked up spinlocks. They could possibly work but the idea of using a busy/waiting loop just spins me the wrong way. And monitors seem a bit too bulky, but thanks for the ideas.