Semaphores in OS (Operating System)

To get rid of the problem of wasting the wake-up signals, Dijkstra proposed an approach which involves storing all the wake-up calls. Dijkstra states that, instead of giving the wake-up calls directly to the consumer, producer can store the wake-up call in a variable. Any of the consumers can read it whenever it needs to do so.

Semaphore is the variables which storesthe entire wake up calls that are being transferred from producer to consumer. It is a variable on which read, modify and update happens automatically in kernel mode.

Semaphore cannot be implemented in the user mode because race condition may always arise when two or more processes try to access the variable simultaneously. It always needs support from the operating system to be implemented.

According to the demand of the situation, Semaphore can be divided into two categories.

1. Counting Semaphore
2. Binary Semaphore or Mutex

We will discuss each one in detail.