What is Semaphore? Counting, Binary Types with Example

What is Semaphore?

Semaphore is simply a variable that is non-negative and shared between threads. A semaphore is a signaling mechanism, and a thread that is waiting on a semaphore can be signaled by another thread. It uses two atomic operations, 1) Wait, and 2) Signal for the process synchronization.

A semaphore either allows or disallows access to the resource, which depends on how it is set up.

Characteristics of Semaphore

Here, are the characteristics of a semaphore:

• It is a mechanism that can be used to provide synchronization of tasks.
• It is a low-level synchronization mechanism.
• Semaphore will always hold a non-negative integer value.
• Semaphore can be implemented using test operations and interrupts, which should be executed using file descriptors.

Types of Semaphores

The two common kinds of semaphores are:

• Counting semaphores
• Binary semaphores

Counting Semaphores

This type of Semaphore uses a count that helps a task to be acquired or released numerous times. If the initial count = 0, the counting semaphore should be created in the unavailable state.

However, If the count is > 0, the semaphore is created in the available state, and the number of tokens it has equals to its count.

Binary Semaphores

The binary semaphores are quite similar to counting semaphores, but their value is restricted to 0 and 1. In this type of semaphore, the wait operation works only if semaphore = 1, and the signal operation succeeds when semaphore = 0. It is easier to implement than counting semaphores.

Example of Semaphore

The below-given program is a step-by-step implementation, which involves the usage and declaration of a semaphore.

Shared var mutex: semaphore = 1;
Process i
    begin
    .
    .
    P(mutex);
    execute CS;
    V(mutex);
    .
    .
    End;

Wait and Signal Operations in Semaphores

Both of these operations are used to implement process synchronization. The goal of this semaphore operation is to get mutual exclusion.

Wait for Operation

This type of semaphore operation helps you to control the entry of a task into the critical section. However, If the value of wait is positive, then the value of the wait argument X is decremented. In the case of negative or zero value, no operation is executed. It is also called P(S) operation.

After the semaphore value is decreased, which becomes negative, the command is held up until the required conditions are satisfied.

P(S)
{
    while (S<=0);
    S--;
}

Signal operation

This type of Semaphore operation is used to control the exit of a task from a critical section. It helps to increase the value of the argument by 1, which is denoted as V(S).

P(S)
{
    while (S>=0);
    S++;
}

Counting Semaphore vs. Binary Semaphore

Here, are some major differences between counting and binary semaphore:

Difference between Semaphore vs. Mutex

The table below compares a semaphore with a mutex across several parameters:

Advantages of Semaphores

Here, are pros/benefits of using Semaphore:

• It allows more than one thread to access the critical section.
• Semaphores are machine-independent.
• Semaphores are implemented in the machine-independent code of the microkernel.
• They do not allow multiple processes to enter the critical section.
• As there is busy waiting in semaphore, there is never a wastage of process time and resources.
• They are machine-independent, which should be run in the machine-independent code of the microkernel.
• They allow flexible management of resources.

Disadvantages of Semaphores

Here, are cons/drawbacks of semaphore:

• One of the biggest limitations of a semaphore is priority inversion.
• The operating system has to keep track of all calls to wait and signal semaphore.
• Their use is never enforced, but it is by convention only.
• In order to avoid deadlocks in semaphore, the Wait and Signal operations require to be executed in the correct order.
• Semaphore programming is complicated, so there are chances of not achieving mutual exclusion.
• It is also not a practical method for large scale use as their use leads to loss of modularity.
• Semaphore is more prone to programmer error.
• It may cause deadlock or violation of mutual exclusion due to programmer error.

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