What is the difference between Mutex and Semaphore in Operating System

Mutex is essentially a locking and releasing mechanism and however, Semaphore is a signalling mechanism. Both are used for Critical section and mutual exclusion problems.

Most people think that Binary Semaphore and Mutex are essentially the same but they are not.

We first recommend reading the post about Semaphores and mutex here before proceeding further.

Differences

Mutex

Example – Imagine mutex as a key to a single toilet. Only one person can be inside the toilet at a time and he would want to lock the toilet and others waiting for toilet access must wait for person already inside to release the toilet. When finished the person gives the key to next person in the queue.

• Mutex is for thread.
• Mutex is essentially atomic and singular in nature.
• Mutex is binary in nature.
• Operations like Lock and Release are possible
• Mutex works in userspace
• Only one thread can acquire a mutex at a time
• Mutex is an object

Semaphore

Example – Imagine a bathroom with 4 identical toilets here, identical keys can open the bathrooms. Here as many as 4 people can use the bathroom simultaneously and they wait and signal one another about the occupancy of the bathrooms

• Semaphore is for processes
• Semaphores are also atomic but not singular in nature
• Semaphores are of two types that are binary and counting
• Semaphores work in kernel space
• Only one process can acquire binary semaphore at a time but multiple processes can simultaneously acquire semaphore in case of counting semaphore
• Semaphore is an integer variable