1. Binary Semaphores:
* Concept: A binary semaphore can only be in one of two states: "locked" or "unlocked". It acts like a single-use lock, ensuring only one thread can access a resource at a time.
* Mechanism: It is initialized with a value of 1 (unlocked). When a thread wants to access the resource, it checks the semaphore. If the semaphore is unlocked (value 1), the thread acquires the semaphore (sets it to 0 - locked) and gains access to the resource. Other threads wanting to access the resource will find the semaphore locked and have to wait until the first thread releases the semaphore.
* Use Cases:
* Mutual Exclusion: Ensuring that only one thread can access a critical section of code at a time.
* Protecting shared resources: Preventing multiple threads from accessing a shared resource simultaneously.
* Controlling access to a single resource: A simple way to manage access to a single resource, like a printer or a shared file.
2. Counting Semaphores:
* Concept: A counting semaphore can have an integer value representing the number of units of a resource that are available. It allows a limited number of threads to access a resource concurrently.
* Mechanism: It is initialized with a positive integer value representing the initial number of available resources. Threads can acquire a semaphore (decrementing its value) to access a resource, and release the semaphore (incrementing its value) when they are finished.
* Use Cases:
* Resource Pooling: Managing a pool of limited resources (like database connections, threads, etc.) and ensuring that only a specific number of threads can access them at any given time.
* Concurrency Control: Controlling the maximum number of concurrent tasks that can be executed in a system, preventing resource exhaustion.
* Producer-Consumer Problem: Coordinating between threads producing data and threads consuming data by ensuring that the consumer never tries to access data that is not yet available.
Important Note:
While these are the main types, there are other variations like "strong" and "weak" semaphores based on how they handle the wait queue. However, the key distinction lies in the way binary and counting semaphores manage resource access.