What are the main mechanisms for creating low-level system locks?

At a low level, lock mechanisms in iOS systems are implemented using several core mechanisms:

• Mutexes — ensure mutual exclusion, allowing only one thread to own a resource at a time.
• Semaphores — limit the number of threads that can access a resource simultaneously.
• Spinlocks — locks that actively wait for the resource to be released without sleeping.
• Atomic operations — operations that are performed atomically without interruption, often used to implement lightweight locks.

In iOS, these mechanisms are implemented at the POSIX level (e.g., pthread_mutex_t) and through low-level APIs, as well as via high-level constructs like GCD (Grand Central Dispatch) and NSLock, which utilize these mechanisms under the hood.