Semaphores in Operating System: Types & Advantages

Semaphore is essentially a non-negative integer that is used as a signal to solve the critical section problem. It is used to solve critical section problems by synchronizing processes utilizing two atomic operations, wait and signal.

Key Terms: Semaphores, Wait, Signal, Operating System, Deadlocks, Binary system, Counting 

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What are Semaphores in Operating Systems?

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Semaphores are integer variables that are primarily used to solve the critical section problem by combining two atomic processes for process synchronization, wait and signal. The definition of wait and signal are given below. 

Wait

If the value of its A argument is positive, it decrements it. If S is zero or negative, no operation will be done.

wait(A)

{

while (A<=0);

A–;

}

Signal

This operation raises the value of its argument A.

signal(A)

{

A++;

}

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History of Semaphores in Operating Systems

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The semaphore concept was developed by a Dutch computer scientist named Edsger Dijkstra and his team while working on an operating system for the Electrologica X8. Over time, this technique became known as THE multiprogramming system. 

• Dijkstra proposed a solution to the problem of wasting wake-up signals, which includes storing all wake-up calls. 
• Instead of immediately providing the wake-up calls to the client, the producer, according to Dijkstra, can keep them in a variable.
• Any of the consumers can read it whenever they want.
• The variable semaphore stores all of the wake-up calls sent from the producer to the consumer. 
• It is a variable in kernel mode that is automatically modified, read, and updated.
• A semaphore cannot be implemented in user mode because race situations are always possible when multiple processes try to access the variable at the same time. 
• For implementation, it is always dependent on the operating system.

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Types of Semaphores

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Semaphores are classified into two types: counting semaphores and binary semaphores. The following details are provided:

Counting Semaphores

These are integer value semaphores with an unrestricted value domain. 

• These semaphores are used to coordinate resource access, with the number of semaphores representing the number of accessible resources. 
• The semaphore count is automatically incremented when resources are added and decremented when resources are removed.

Binary Semaphores

Binary semaphores are similar to counting semaphores, except their values are limited to 0 and 1.

• Waiting only works when the semaphore is 1, and signaling only works when the semaphore is 0. 
• Binary semaphores are sometimes easier to implement than counting semaphores.

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Advantages of Semaphores

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The advantages are mentioned below:

• Because of semaphores, only one process is permitted to enter the vital part.
• They strictly adhere to the idea of mutual exclusion. 
• They are also significantly more efficient than other synchronization methods.
• Because no processor time is lost in determining if a condition is met for a process to access its crucial area, there is no resource waste due to busy waiting in the semaphores.
• Semaphores are implemented in the machine-independent code of the microkernel. 
• As a result, they are machine-independent.

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Disadvantages of Semaphores

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The disadvantages are mentioned below:

• Because semaphores are so complex, the wait and signal actions must be implemented in the correct order to avoid deadlocks.
• Semaphores are inefficient at the final scale because they reduce modularity. This is mostly due to the wait, as well as the signal protocols, preventing the system from creating a structured layout.
• Semaphores can induce a priority inversion, with low-priority processes accessing the critical piece first and high-priority processes accessing it later.

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Things To Remember

• Semaphores are synchronization primitives used in concurrent programming to control access to shared resources and avoid race conditions.
• A semaphore is essentially a non-negative integer variable.
• Two fundamental operations on semaphores: Wait (P) and Signal (V).
• The signal (V) operation increments the semaphore value. If there are blocked processes/threads waiting on the semaphore, one of them is unblocked.
• Semaphores can be implemented as binary (0 or 1) or counting (integer values greater than or equal to 0).
• Binary semaphores can be used for mutual exclusion (locking) and signaling between two processes/threads.
• Counting semaphores can represent the availability of a certain resource with a fixed capacity.
• Semaphore misuse can lead to deadlocks or race conditions, so careful management is crucial.

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