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Mutex Vs Semaphore in OS

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.



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


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
Mutex Vs Semaphore (1)

What is SpinLock ?

While a process is in critical section, any other process which waits to enter critical section must loop continuously in call to acquire. This is a spinlock as the process “Spins” while waiting for the lock to be available.

Advantages –

  • No Context switch is required for a process waiting on a lock.
  • Good for systems where it’s known that locks are of short duration.
  • Employed in Multiprocess system,  where one thread can spin on 1 processor while another thread processes its critical section in another processor.

Semaphores –

  • It’s  a Generalized Mutex.
  • Semaphores are integer variable , which is accessed through 2 atomic operations, wait() and signal ().
  • It’s a signalling Mechanism.
Operating System Process Synchornization

      Read More

    1. Process Synchronization
    2. Critical Section
    3. Inter-Process Communication
    4. UEFI(Unified Extensible Firmware Interface) and how is it different from BIOS
    5. Mutex
    6. Semaphore
    7. Mutex vs. Semaphore
    8. Atomic Operations in OS
    9. Peterson’s Algorithm for Mutual Exclusion (Only important for Cisco and Arista Networs)
      1. Java
      2. C
      3. Python
    10. Peterson’s Algorithm for Critical Section Problem (Only important for Cisco and Arista Networs)
    11. Readers-Writers Problem