Mutex vs.semaphore: What are the differences?

This article talks about Mutex vs. semaphore. It also includes advantages and disadvantages of mutex and semaphore.

A semaphore is a non negetive variable value used for access control mechanism. It is used as a synchronization tool to share information among several processes. A mutex is a type of semaphore that supports mutual exclusion, preventing two threads from operating simultaneously on a particular resource. In addition, it allows only one thread at a time to access the resource. Both semaphores and mutexes are used in computer programming and systems management. In this article, we are going to study Mutex vs.semaphore.

Table of contents

• Key differences between Mutex and semaphore
• What is Mutex?
• Advantages of Mutex
• Disadvantages of Mutex
• What is a semaphore?
• Advantages of semaphore
• Disadvantage of semaphore
• Conclusion

Mutex vs semaphore

A mutex is a locking and unlocking mechanism, and a semaphore is a signaling mechanism. Both are used for critical section and mutual exclusion problems.

• Mutex uses a locking mechanism. When a process uses a resource, it locks the resource, uses it, then releases it. On the other hand, a semaphore uses a signaling mechanism using the wait() and signal() methods to indicate whether a process is releasing or acquiring a resource. 
• A mutex is an object while a semaphore is an integer variable. Semaphores have wait() and signal() functions. However, Mutex has no such functionality. 
• A Mutex object allows multiple process threads to access a shared resource, but only one at a time. On the other hand, Semaphores allow multiple process threads to access a finite instance of a resource until a finite instance of the resource becomes available. 
• A mutex allows the same process to acquire and release the lock simultaneously. However, the value of a semaphore variable can be changed by any process that needs some resources, but only one process at a time can change the value.

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What is Mutex?

A Mutex or Mutual Exclusion Object is used to allow only one of the processes access to the resource at a time. The Mutex object allows all processes to use the same resource, but the resource is accessed by one process at a time. Mutex uses a lock-based approach to handle critical section issues.

Each time a process requests a resource from the system, the system creates a mutex object with a unique name or ID. So whenever a process wants to use that resource, it acquires a lock on the object. After locking, the process uses the resource and eventually releases the mutex object. Other processes can then create and use mutex objects in the same way.

Advantages of Mutex

• Mutex is to create a barrier that prevents two different threads from accessing a resource simultaneously. This prevents resources from being unavailable when another thread needs them.
• Mutex is that it can help with code reliability. Resources accessed by a thread can become unavailable if the CPU’s memory management fails. By preventing access to a resource at this time, the system can recover from any errors that cause a failure in memory management and still have the resource available, and Mutex helps here.

Disadvantages of Mutex

• It cannot be locked or unlocked by any context other than the context that acquired it.
• Typical implementations can result in busy wait states that waste CPU time.
• If one thread acquires the lock, goes to sleep, or becomes preemptive, the other thread may get stuck further. This can lead to hunger.
• In the critical section, he should only allow one thread at a time.

What is a semaphore?

A semaphore is an integer variable S that is initialized with the number of resources present in the system and used for process synchronization. Change the value of S using two functions: wait() and signal(). These two functions are used to change the value of a semaphore, but they allow only one process to change the value at any given time. Two processes cannot change the value of a semaphore at the same time. Semaphores have two categories: counting semaphores and binary semaphores.

Advantages of semaphore

• Efficient Allocation: Semaphores allow you to allocate system resources more efficiently. This means that memory can be used more effectively.
• Controlling multiple processes: Semaphores allow you to control multiple processes. This means you can allocate memory to specific tasks as needed.
• Improved performance: Semaphore-based memory management improves performance and improves system responsiveness.

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Disadvantage of semaphore

• Semaphores are prone to programming errors.
• Mutual exclusion is not possible due to the complexity of programming semaphores.
• Implementing semaphores can be expensive in terms of memory and CPU usage.
• One of the most critical limitations of semaphores is priority inversion. 
• Low-priority processes enter the critical section, and high-priority processes continue to wait.

Conclusion

Note that the basic difference between a mutex and a semaphore is that a semaphore is a signaling mechanism whereas a mutex is a locking mechanism. A semaphore is a variable used to control access to a shared resource within the operating system, and a mutex is simply a lock acquired before entering a critical section and releasing it. A semaphore is better for multiple instances of a resource, but a mutex is better for a single shared resource.