Mutex vs Semaphore (2024)

Mutex and Semaphore both provide synchronization services but they are not the same. Details about both Mutex and Semaphore are given below −

Mutex is a mutual exclusion object that synchronizes access to a resource. It is created with a unique name at the start of a program. The Mutex is a locking mechanism that makes sure only one thread can acquire the Mutex at a time and enter the critical section. This thread only releases the Mutex when it exits the critical section.

A Mutex is different than a semaphore as it is a locking mechanism while a semaphore is a signalling mechanism. A binary semaphore can be used as a Mutex but a Mutex can never be used as a semaphore.

A semaphore is a signalling mechanism and a thread that is waiting on a semaphore can be signaled by another thread. This is different than a mutex as the mutex can be signaled only by the thread that called the wait function.

A semaphore uses two atomic operations, wait and signal for process synchronization.

The wait operation decrements the value of its argument S, if it is positive. If S is negative or zero, then no operation is performed.

The signal operation increments the value of its argument S.

There are mainly two types of semaphores i.e. counting semaphores and binary semaphores.

Counting Semaphores are integer value semaphores and have an unrestricted value domain. These semaphores are used to coordinate the resource access, where the semaphore count is the number of available resources.

The binary semaphores are like counting semaphores but their value is restricted to 0 and 1. The wait operation only works when the semaphore is 1 and the signal operation succeeds when semaphore is 0.

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Now, delving into the content of the article, the key concepts revolve around Mutex and Semaphore, two fundamental synchronization mechanisms used in concurrent programming. Let's break down the information provided:

1. Mutex (Mutual Exclusion):

   • Definition: A Mutex is a mutual exclusion object designed to synchronize access to a shared resource.
   • Purpose: It ensures that only one thread can acquire the Mutex at any given time, preventing concurrent access to the critical section.
   • Characteristics: Created with a unique name at the start of a program, it is a locking mechanism that is released only when the thread exits the critical section.
   • Differentiation from Semaphore: Unlike a semaphore, a Mutex is solely a locking mechanism, and it cannot be used as a signaling mechanism.

2. Semaphore:

   • Definition: A Semaphore is a signaling mechanism used for process synchronization.
   • Signaling Mechanism: Unlike a Mutex, a semaphore can signal a waiting thread, and any thread can perform this signaling action.
   • Operations: Semaphore employs two atomic operations - wait and signal.
   • Wait Operation: Decreases the value of the semaphore by 1 if it is positive; no operation is performed if the value is zero or negative.
   • Signal Operation: Increments the value of the semaphore.

3. Types of Semaphores:

   • Counting Semaphores:

     • Definition: Integer value semaphores with an unrestricted value domain.
     • Usage: Coordinating resource access, where the semaphore count represents the number of available resources.

   • Binary Semaphores:

     • Definition: Similar to counting semaphores but with a restricted value domain (0 and 1).
     • Wait Operation: Only works when the semaphore is 1.
     • Signal Operation: Succeeds when the semaphore is 0.

In summary, Mutex and Semaphore serve distinct purposes in synchronization. A Mutex focuses on exclusive access to a resource through locking, while a Semaphore serves as a signaling mechanism for process synchronization, with variations such as counting semaphores and binary semaphores, each suited for specific synchronization scenarios.