Quiz Entry - updated: 2026.09.25
What is the difference between a semaphore, a mutex and a queue in an RTOS?
A semaphore signals that something happened or counts available resources; a mutex gives one task exclusive ownership of a shared resource; a queue passes data between tasks.
All three let tasks coordinate, and all three can make a task block until the thing it waits for is available:
| Mechanism | Purpose | Typical use |
|---|---|---|
| Binary semaphore | Signalling: "an event happened" | An ISR gives it when data arrives; a task waiting on it wakes up |
| Counting semaphore | Counts available units or pending events | Three free buffers; five unprocessed button presses |
| Mutex | Mutual exclusion with an owner: only the task that took it may give it back | Protecting an I2C bus or a shared struct used by two tasks |
| Queue | Copies data from one task (or ISR) to another in FIFO order | A sensor task sends readings to a Wi-Fi upload task |
The mutex/semaphore distinction matters. A mutex has an owner, which lets the RTOS apply priority inheritance (see priority inversion) and prevents another task from releasing a lock it doesn't hold. A binary semaphore has no owner, which is exactly what makes it suitable for an ISR to signal a task.
Tip: semaphore = signal, mutex = lock, queue = mailbox.
Go deeper:
FreeRTOS — Mutexes — how FreeRTOS mutexes differ from binary semaphores, including priority inheritance.- Wikipedia — Semaphore (programming) — Dijkstra's original concept, counting vs binary.