Difference between Mutex and Semaphore

While developing different operating systems, developers are usually confused about the difference between mutex and semaphore.

If you are one of them then you are at the right place to get to know about their differences. We are here to answer all of your confusion related to this topic.

The main Difference Between Mutex And Semaphore

Mutex and semaphore are basic kernel resources used to have synchronization services during development. But they both differ in their approach and operations.

A mutex has a locking mechanism and processes only a single critical piece of code at one time. The semaphore has a signaling mechanism and processes multiple threads at one time. Mutex applies lock and unlocks operations whereas wait and signal operations are applied by a semaphore. To develop any of these operating systems, we need to understand clearly the difference between them so we can efficiently design and develop them. 

To go through a deep analysis of the difference between a mutex and a semaphore, you have to scroll down to the article.

In this article, we are going to have a detailed explanation of the comparison of their properties, working, and a lot more.

Mutex vs. Semaphore: One-to-one comparison

During the explanation of the difference between mutex and semaphore comes the point where we have to discuss one-to-one differences in their features, properties, operations, threads, and management.

MutexSemaphore
It is an object.It is a non-negative integer variable.
It can include multiple threads but not at the same time.It includes multiple threads and also at the same time.
It is all about locking. The process shall acquire a lock on the mutex object if the resource has to be acquired.It is all about wait and signal. This indicates is a process is acquiring a resource or releasing a resource.
Object lock is released only by the thread filling it.Its value can be modified by any one of the threads having access to it.
It includes lock and unlocks operations.Its value is changed by signal and the wait operation.
It is modified by the thread that controls it.It is modified by signal and the wait operation.

What is Mutex?

The acronym of mutex stands for Mutual Exclusion Object. A mutex is an object that allows multiple threads to access the single resource one at a time one by one.

Mutex includes mutual exclusion, meaning either the producer thread or the consumer thread can access the key i.e. mutex. The buffer can be filled by one at a time, either the producer or the consumer, and prevents others access to it, and so it has to wait. It is mostly used to process the critical sections of a program by locking them as a single thread.

How does mutex work?

A mutex has a lock-based approach. Being a lock mechanism, it allows access to only a single thread to fill the buffer making the other threads wait.

Either consumer or producer, only one can use the resources at the same time. It locks the buffer once the producer or the consumer occupies it and the other has to wait till the one using it is done. For example, for one door there is only one key for one person no matter how many people need the room.

Operations of Mutex

Mutex uses acquire() and release() operations to work. A basic algorithm for a mutex is given below.

The acquire() method is applied first.

acquire()
while !available do
  busy! wait!
end
available=false 

When the process is done the release() method is applied.

release()
available=true

In the above algorithm, the acquire() operation is applied first to lock the thread. It will display the message of ‘busy! Wait!’ until the processor is not empty.

After the task completion, the release() operation will be applied to unlock the processor. It shows that the processor is now available to process the next thread.

What are the Types of Mutex?

It has generally no subtypes rather it is sometimes referred to as a subtype of semaphore i.e. binary semaphore, but that is not entirely true even though their functions are nearly the same.

But in Linux, it is divided into three subtypes i.e. Normal mutex, Recursive mutex, and Errorcheck mutex.

Normal mutex is simply a default mutex while recursive mutex can apply multiple locks to a resource, without creating a deadlock, if it is accessed by another thread continuously.

An Errorcheck mutex checks whether a resource is being locked again without unlocking it first and gives an error

What are mutexes used for?

A mutex can be used to save a shared resource from being accessed by simultaneous threads. Each of the processes or threads has to wait for ownership to accomplish the code for having access to the resource.

Advantages of Mutex

The pros of a mutex are as follows.

  • They are simple locks that are accessed and obtained before entering the critical section.
  • Data remains consistent due to no race issues as mutex allows only one thread at a time.

Disadvantages of Mutex

The cons of a mutex are as follows.

  • Only one thread has access to the buffer causing wastage of time.
  • Only the thread filling the buffer can lock, unlock it.
  • It cannot help other threads move forward if a thread having the lock sleeps or is preempted and leads to starvation.

What is Semaphore

Semaphore is a non-negative integer variable that allows access to multiple threads to the buffer’s identical resources at the same time. For example,  it will convert a 5kb buffer into five buffers of 1kb each and in this way, multiple threads will be able to work on the identical resources.

What is the Working of Semaphore

Semaphore has a signaling approach. Being a signaling mechanism it allows access to multiple threads simultaneously to the buffers.

It allows different consumers and producers to work on identical resources at the same time. For example, for multiple users, it will create multiple keys of one door for each user.

Operations of Semaphore

Semaphore uses wait() and signal() operations to work. A basic algorithm for a semaphore is given below.

The wait() method is applied first.

wait(S)
while S<=0 do
  busy! wait!
End
S-- 

When the process is done the signal() method is applied.

signal(S)
S++

In the above algorithm, the wait() operation is applied first. It will display the message of ‘busy! Wait!’ until the processor is not empty.

After the task completion, the signal() operation will be applied which signals that the next threads get ready to enter the processor. S is the value of the semaphore in this algorithm.

What are the two types of Semaphore?

Its subtypes include binary semaphore and counting semaphore. A binary semaphore can only use the values of zero and one while the counting semaphore can be assigned any non-negative integer value.

Why semaphore is used?

Semaphore is mainly used to control or manage access to a device that is being used mutually by multiple tasks or systems. For example, a computer lab has only one printer connected with many computers.

A semaphore controls the access of every computer to the printer and manages it.

Advantages of Semaphore

The merits of a semaphore are stated below.

  • It allows access to more than one thread.
  • They take care not to enter multiple processes into the critical section.
  • They provide efficient management of resources.
  • There is little wastage of time as less of it is wasted in waiting such as that in mutex.
  • They can be run and implemented in the machine-independent code of the microkernel as it is machine-independent.

Disadvantages of Semaphore

The demerits of a semaphore are stated below.

  • The operating system, to wait and signal semaphore, keeps a record of all the calls.
  • It faces priority inversion as well as loss of modularity.
  • It is at more risk of error of the programmer that causes deadlock.

Which is better, mutex or semaphore?

Mutex and semaphore, both are highly useful and efficient in their works and able to fulfill the requirements of their users.

However, if you want to know which is the better one, you need to carefully analyze your needs and requirements according to the given situation and deduce which one of the two would suit you better. 

Concluding thoughts

So after the detailed analysis of differences between mutex and semaphore, we can easily judge by their facts and figures that each of these is helpful according to your suitable requirements.

I hope that this article may help you figure out which one of them would be a better option for your project according to your needs.

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