NSLock

Implements a mutex lock—a lock that enforces mutually exclusive access to a shared resource. Only one thread can acquire the lock and use the resource, thereby blocking other threads until it releases the lock.

NSRecursiveLock

Implements a recursive lock—a mutex lock that can be acquired multiple times by the thread that currently owns it. The lock remains in place until each recursive acquisition of the lock has been balanced with a call that releases the lock.

NSConditionLock

NSCondition

Both of these classes implement condition locks. This type of lock combines a semaphore with a mutex lock to implement locking behavior that is effective based on a program-defined condition. A thread blocks and waits for another thread to signal the condition, at which point it (or another waiting thread) is unblocked and continues execution. You can signal multiple threads simultaneously, causing them to unblock and continue execution.