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Difference Between Semaphore and Mutex
In operating systems, semaphore and mutex are two kernel resources that are used to provide synchronization services. A semaphore is an integer variable, while a mutex is an object. Read this article to learn more about semaphore and mutex and how they are different from each other.
What is Semaphore?
Semaphore is a signalling mechanism. It is basically an integer variable. A semaphore uses two operations, namely wait and signal for process synchronization. Therefore, the wait and signal operations can modify a semaphore. There are two types of semaphores namely, Counting Semaphore and Binary Semaphore.
Counting Semaphore is the type of semaphore that has an integer value and an unrestricted value domain. A counting semaphore is mainly used in the coordination of the resource accesses, where the count of the semaphore is the number of available resources.
On the other hand, a binary semaphore is similar to the counting semaphore, but its value is restricted to 0 and 1. Hence, the wait operation of binary semaphore only works when the semaphore is 1 and the signal operation works when semaphore value is 0.
What is Mutex?
Mutex is a locking mechanism. It is basically a mutual exclusion object that can synchronize resource access. Mutex is defined with a unique name at the beginning of a program.
A Mutex makes sure multiple program threads cannot acquire the mutex simultaneously. One program thread releases the mutex only when it exits the critical section. Therefore, the process is kept in a queue when mutex is locked and can be accessed only after the mutex has been unlocked.
Now, let us discuss the differences between semaphore and mutex in detail.
Difference between Semaphore and Mutex
The following table highlights all the important differences between semaphore and mutex −
S.No. |
Semaphore |
Mutex |
|---|---|---|
1. |
It is a signalling mechanism. |
It is a locking mechanism. |
2. |
It is an integer variable. |
It is an object. |
3. |
The 'wait' and 'signal' operations can be used to modify a semaphore. |
It is modified by that process (only) that requests or releases a resource. |
4. |
The process will have to 'wait' until the count of the semaphore is higher than 0. |
If the mutex is locked, then the process will have to wait. |
5. |
If no resources are available, the process requires that a resource that needs to be executed has to 'wait' |
The process is kept in a queue when mutex is locked. This can be accessed only after the mutex has been unlocked. |
6. |
Multiple program threads can be used with a semaphore. |
Multiple program threads can be used in mutex (but not simultaneously). |
7. |
The value of a semaphore can be changed by any process that releases or obtains the resource. |
The lock on the object can be released by the process only, which would have obtained the lock on it. |
8. |
There are different kinds of semaphores: Counting semaphore and Binary semaphore. |
It doesn't have any subtypes. |
9. |
The value of a semaphore can be changed using 'wait' operation and 'signal' operation. |
The object is either locked or unlocked. |
10. |
The process is occupied if all resources are used. |
If the object has already been locked, the process that requests the resources will wait. |
11. |
The process requesting the resource uses the 'wait' operation and blocks itself until the count of the semaphore becomes greater than 1. |
It will be queued by the system before the lock has been released. |
Conclusion
The most significant difference that you should note here is that a semaphore is a signaling mechanism, whereas a mutex is a locking mechanism.
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