Uploaded November 2025 | Updated September 2026, 3 weeks ago

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A semaphore is a synchronization primitive similar to a mutex, but it can count beyond just 0 and 1 (what mutexes are limited to). While a mutex acts like a single key to a restroom, a semaphore is like having multiple keys that can unlock multiple bathrooms in a shopping complex. Semaphores are used to control access to critical resources or code sections by maintaining a counter that gets decremented when a task acquires it and incremented when a task releases it. When the counter reaches 0, any additional tasks trying to acquire the semaphore must wait until another task releases it.
In practice, semaphores are commonly used to synchronize threads in producer/consumer patterns. Producer threads increment the semaphore when they add items to a shared buffer, signaling to consumer threads that data is ready to be read. Consumer threads decrement the semaphore when they remove items. This pattern often uses multiple semaphores together: a mutex to protect buffer access, an "empty" semaphore to track available slots, and a "full" semaphore to track items ready to be consumed.
To learn more about semaphores, check out this video in my Introduction to Real-Time Operating Systems series here:
youtube.com/watch?v=5JcMtbA9QEE&list=PLEBQazB0HUyQ4hAPU1cJED6t3DU0h34bz
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