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Is there any difference between a binary semaphore and mutex or are they essentially the same?
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Using both the semaphore, a process is able to enter the critical section, so a progress is made. A Binary Semaphore is a semaphore whose integer value range over 0 and 1. A counting semaphore is a semaphore that has multiple values of the counter. The value can range over an unrestricted domain.
Multiple number of threads can acquire binary semaphore at a time concurrently. Binary semaphore have no ownership. There is ownership associated with mutex because only owner can release the lock.
A mutex is used when only one thread or process is allowed to access a resource and a semaphore is used when only a certain set limit of threads or processes can access the shared resource. Essentially a mutex is a semaphore where the limit is set to 1.
A binary semaphore is restricted to values of zero or one, while a counting semaphore can assume any nonnegative integer value. A binary semaphore can be used to control access to a single resource. In particular, it can be used to enforce mutual exclusion for a critical section in user code.
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 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.
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so semaphores are more suitable for some synchronization problems like producer-consumer.
On Windows, binary semaphores are more like event objects than mutexes.
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