Difference between binary semaphore and mutex

Question

Is there any difference between a binary semaphore and mutex or are they essentially the same?

Answer 1

They are NOT the same thing. They are used for different purposes!
While both types of semaphores have a full/empty state and use the same API, their usage is very different.

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

Here is a simple example:

   Thread A                     Thread B    Take Mutex      access data      ...                        Take Mutex  <== Will block      ...    Give Mutex                     access data  <== Unblocks                                   ...                                 Give Mutex 

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.

    Task A                      Task B    ...                         Take BinSemaphore   <== wait for something    Do Something Noteworthy    Give BinSemaphore           do something    <== unblocks 

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.

Answer 2

• A mutex can be released only by the thread that had acquired it.
  
• A binary semaphore can be signaled by any thread (or process).

so semaphores are more suitable for some synchronization problems like producer-consumer.

On Windows, binary semaphores are more like event objects than mutexes.