Skip to main content

Semaphore vs Mutex

Mutex​

A synchronization primitive that can also be used for interprocess synchronization.

Mutual Exclusion Semaphores​

Mutual Exclusion semaphores are used to protect shared resources (data structure, file, etc..).

A Mutex semaphore is "owned" by the task that takes it. If Task B attempts to semGive a mutex currently held by Task A, Task B's call will return an error and fail.

Mutexes always use the following sequence:

  • SemTake
  • Critical Section
  • SemGive

Semaphore​

Limits the number of threads that can access a resource or pool of resources concurrently.

Binary Semaphore​

Binary Semaphore address a totally different question:

  • Task B is pended waiting for something to happen (a sensor being tripped for example).
  • Sensor Trips and an Interrupt Service Routine runs. It needs to notify a task of the trip.
  • Task B should run and take appropriate actions for the sensor trip. Then go back to waiting.

Note that with a binary semaphore, it is OK for B to take the semaphore and A to give it. Again, a binary semaphore is NOT protecting a resource from access. The act of Giving and Taking a semaphore are fundamentally decoupled. It typically makes little sense for the same task to so a give and a take on the same binary semaphore.

Difference​

Mutex is for exclusive access to a resource. A Binary semaphore should be used for Synchronization (i.e. "Hey Someone! This occurred!").