What is a Semaphore?

Definition
A semaphore is like an integer with three special properties: you can initialize it to any value, you can only increment or decrement it (not read it directly), and if a decrement would make it negative, the thread blocks until another thread increments it.
The Two Operations
Semaphores were invented by Edsger Dijkstra. The original operations were named P (from Dutch "proberen" meaning test) and V ("verhogen" meaning increment). Modern names are wait() and signal(), or acquire() and release().
Semaphore Operations
wait() / PDecrement the counterIf counter > 0: decrement, continueIf counter = 0: BLOCK until > 0signal() / VIncrement the counterAlways succeeds immediatelyWakes one blocked thread
Think Of It Like A Parking Lot
A parking lot has N spaces. When a car enters (wait), it takes a space. If full, cars line up at the entrance. When a car leaves (signal), it frees a space and the next waiting car can enter. The semaphore count is the number of free spaces.
Why Not Just Use An Integer?
The magic is in the blocking. A regular integer does not block threads. With a semaphore, wait() automatically puts your thread to sleep if the count is zero, no busy waiting needed. The operating system wakes you up when another thread signals.
Key Point: Semaphores are the building block for many patterns: signaling, mutual exclusion, resource counting, and more.

💡 Key Takeaways

✓A semaphore is an integer you can only increment (signal) or decrement (wait). You cannot read its value directly.

✓wait() decrements. If result is negative, the calling thread blocks until another thread calls signal().

✓signal() increments. If any threads are blocked, one is woken up to continue execution.

✓Initial value determines behavior. Initialized to 1 acts like a mutex. Initialized to N allows N concurrent accesses.

✓Semaphores are the foundation of most synchronization patterns. Many complex solutions are built from this simple primitive.

📌 Examples

1Binary semaphore (initial value 1): sem.wait() enters critical section, sem.signal() exits. Only one thread at a time.

2Counting semaphore (initial value N): allows up to N threads to proceed, useful for resource pools.

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