A semaphore does not protect anything. A semaphore only affects ObtainSemaphore and ReleaseSemaphore. Once you've called ObtainSemaphore, all other calls to ObtainSemaphore are blocked until you call ReleaseSemaphore. That's all it does, nothing else. Protection only arises implicitly if you combine the semaphore with a piece of data and everybody who wants to access the data has the discipline to call ObtainSemaphore before and ReleaseSemaphore after the access. Really everybody. If only one program accesses the data without obtaining the semaphore, protection is lost. The best way to protect your data is to keep the structure private and provide some read_data() and write_data() functions which handle the semaphore internally. Yes, only if a semaphore already exists for the data, it provides some protection. You cannot make your own semaphore to protect foreign data. Only if all programs know about the semphore and use it, it provides protection.