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Semaphore

A semaphore is a non-negative integer variable that is shared between various threads. Semaphore works upon signaling mechanism, in this a thread can be signaled by another thread. Semaphore uses two atomic operations for process synchronisation: 

  • Wait (P)
  • Signal (V)

Advantages of Semaphore

  • Multiple threads can access the critical section at the same time.
  • Semaphores are machine-independent.
  • Only one process will access the critical section at a time, however, multiple threads are allowed.
  • Semaphores are machine-independent, so they should be run over microkernel.
  • Flexible resource management.

Disadvantages of Semaphore

  • It has priority inversion.
  • Semaphore’s operation (Wait, Signal) must be implemented in the correct manner to avoid deadlock.
  • It leads to a loss of modularity, so semaphores can’t be used for large-scale systems.
  • Semaphore is prone to programming error and this can lead to deadlock or violation of mutual exclusion property.
  • Operating System has to track all the calls to wait and signal operations.

Using Semaphore

The producer-consumer problem: Consider the standard producer-consumer problem. Assume, we have a buffer of 4096-byte length. A producer thread collects the data and writes it to the buffer. A consumer thread processes the collected data from the buffer. The objective is, both the threads should not run at the same time. 

Solution- A semaphore is a generalized mutex. In lieu of a single buffer, we can split the 4 KB buffer into four 1 KB buffers (identical resources). A semaphore can be associated with these four buffers. The consumer and producer can work on different buffers at the same time. 

Note: The content is a generalized explanation. Practical details vary with implementation. 

Mutex vs Semaphore

In the Operating System, Mutexes and Semaphores are kernel resources that provide synchronization services (also known as synchronization primitives). Synchronization is required when multiple processes are executing concurrently, to avoid conflicts between processes using shared resources.

Similar Reads

Mutex

Mutex is a specific kind of binary semaphore that is used to provide a locking mechanism. It stands for Mutual Exclusion Object. Mutex is mainly used to provide mutual exclusion to a specific portion of the code so that the process can execute and work with a particular section of the code at a particular time....

Semaphore

A semaphore is a non-negative integer variable that is shared between various threads. Semaphore works upon signaling mechanism, in this a thread can be signaled by another thread. Semaphore uses two atomic operations for process synchronisation:...

Difference Between Mutex and Semaphore

Mutex Semaphore A mutex is an object. A semaphore is an integer. Mutex works upon the locking mechanism. Semaphore uses signaling mechanism Operations on mutex: Lock Unlock Operation on semaphore: Wait Signal Mutex doesn’t have any subtypes. Semaphore is of two types: Counting Semaphore Binary Semaphore A mutex can only be modified by the process that is requesting or releasing a resource. Semaphore work with two atomic operations (Wait, signal) which can modify it. If the mutex is locked then the process needs to wait in the process queue, and mutex can only be accessed once the lock is released. If the process needs a resource, and no resource is free. So, the process needs to perform a wait operation until the semaphore value is greater than zero....

Misconception

There is an ambiguity between binary semaphore and mutex. We might have come across that a mutex is a binary semaphore. But it is not! The purposes of mutex and semaphore are different. Maybe, due to similarity in their implementation a mutex would be referred to as a binary semaphore....

Mutex vs Semaphore – FAQ’s

Q.1: Can a thread acquire more than one lock (Mutex)?...

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