Semaphores
Introduction
Semaphores are a synchronization primitive that can be used to protect shared resources. They are used to control access to shared resources by multiple threads or processes.
Application
A resource shared across threads or across processes, e.g. a shared memory space, a file (descriptor), or a network socket, and you need to ensure that no more
than 1-n threads (or processes) should access at a time. This is where semaphores come in.
Mutex or Semaphore?
A mutex on the other hand is there to ensure that no more than one thread can access (to read or write) a shared resource at a time. It is akin to a binary semaphore.
Here’s a comparison between the two as provided by ChatGPT:
| Feature | Semaphore | Mutex |
|---|---|---|
| Purpose | Controls access to resources, allowing multiple threads to access (counting semaphore) or one (binary semaphore). | Ensures mutual exclusion, allowing only one thread to access a critical section. |
| Counter | Maintains a count to track available resources. | Binary state: locked or unlocked (no counter). |
| Ownership | No ownership; any thread can signal (release) a semaphore. | Ownership is enforced; only the thread that locks it can unlock it. |
| Concurrency | Allows multiple threads to proceed if the counter is greater than 1 (in a counting semaphore). | Only one thread can proceed at a time. |
| Blocking Behavior | Threads block if the counter is zero (no available resources). | Threads block if the mutex is locked. |
| Use Cases | - Managing a pool of resources (e.g., thread pools, connection pools). - Synchronizing producer-consumer workflows. |
- Protecting critical sections. - Ensuring exclusive access to shared data. |
| Types | - Binary semaphore (similar to a mutex). - Counting semaphore (allows multiple threads). |
Only one type (binary lock). |
| Risk of Deadlock | Higher risk if not used carefully (e.g., incorrect signaling order). | Lower risk due to strict ownership and locking rules. |
| Performance | Slightly slower due to additional counter operations and flexibility. | Slightly faster because it enforces strict mutual exclusion. |
| Platform Support | Available in most operating systems and threading libraries. | Available in most operating systems and threading libraries. |
For a very nice introduction to semaphores and their usage, this post by Vikram Shukla is a must-read.
POSIX Semaphores
Semaphore routines
An overview of POSIX semaphore routines is available via linux man pages.
Linux vs MacOS
Interestingly, macOS does not support unnamed semaphores, quora.
So, if you are writing code that wants to comply with POSIX portability, you will need to use named semaphores.