What are the advantages and disadvantages of using semaphores in resource access control?

Semaphores are synchronization primitives that allow limiting the number of threads simultaneously accessing a resource.

Advantages:

• Enable control over access to limited resources (e.g., connection pool).
• Can be used to implement queues and manage execution order.
• Flexibility: you can set the number of permissions (count), which allows managing the level of parallelism.

Disadvantages:

• Possibility of deadlocks if semaphores are used improperly.
• Complexity in debugging and understanding, especially with a large number of semaphores.
• Risk of errors when incorrectly increasing/decreasing the counter, which can lead to infinite waiting or race conditions.

In iOS, GCD (DispatchSemaphore) is often used for access control, which is convenient but requires caution to avoid main thread deadlocks.