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Reader Writer Problem in OS

Last Updated on December 11, 2023 by Ankit Kochar

The Reader-Writer Problem is a classic synchronization issue in operating systems (OS) and concurrent programming. It revolves around the challenge of managing shared resources, specifically a data structure or a section of code, that is accessed by multiple threads. The problem arises when balancing the need for simultaneous access by multiple readers against exclusive access for a single writer to ensure data consistency and integrity. Various solutions such as locks, semaphores, and other synchronization mechanisms have been proposed to tackle this issue efficiently.

How the Reader-Writer Problem is OS Handled?

In order to handle the problem, it must be ensured that no concurrent processes cause any form of data inconsistency in the operating system. The reader-writer problem in os can be assumed as follows:-

There are multiple processes that can be either readers and writers with a shared resource between them, let us suppose it as a file or a database. In case there are two processes with both trying to access the resource simultaneously at the same instance. Although it does not matter how many readers can access it simultaneously, it must be kept in mind that only one writer can write it at a time.

There are several algorithms that are designed to curb this problem and among the many algorithms available, we are going to use one to solve the reader-writer problem in os.

Various Cases of Reader-Writer Problem

There are certain cases that I must look forward to understanding the reader-writer problem in os and how it impacts the data inconsistency problem that must be avoided.

Case One
Two processes cannot be allowed to write into shared data parallelly thus they must wait to get access to write into it.

Case Two
Even if one process is writing on data and the other is reading then also they cannot be allowed to have the access to shared resources for the reason that a reader will be reading an incomplete value in this case.

Case Three
The other similar scenario where one process is reading from the data and another writing on it, on the shared resource, it cannot be allowed. Because the writer updates some data that is not available to the reader. The solution being that the writer completes successfully and gives access.

Case Four
In the case that both processes are reading the data then sharing of resources among both the processes will be allowed as it is a case free from any such anomaly because reading does not modify the pre-existing data.

The Solution to the Problem

In order to solve the problem, we maintain three variables, namely, mutex, semaphore, and readCount.

Mutex makes the process to release and acquire a lock when the readCount is being updated. The lock is acquired when the readCount is updated and decremented back after it has done performing the operation. The writer waits for the semaphore until it is its turn to write and increments for other processes to write.

The idea remains to use semaphores when a reader enters the critical section up until it exits the critical section such that no writer can enter in between as it can cause data inconsistency. A semaphore is used to prevent writers from accessing the resource while there are one or more readers accessing it.


The given code below is the code for the writer’s side.

while (TRUE) {
    // Wait on the "w" semaphore to acquire access to the resource

    // Perform the necessary write operation(s) on the resource
    // ...

    // Signal the "w" semaphore to allow other writer processes to access the resource

The above code implements a simple solution for the writer process where the writer waits for the "w" semaphore to become available and then performs the write operation on the resource. The writer then signals the "w" semaphore to allow other writer processes to access the resource. Note that the code is in an infinite loop, so the writer process will continuously wait for the "w" semaphore to become available and then perform the write operation.

Given below is the code for readers side:-

while (TRUE) { // Loop indefinitely
    // Acquire lock
    wait(m); // Wait for the mutex semaphore to be available

    readCount++; // Increment the number of processes doing the read operation
    if (readCount == 1) {
        wait(w); // If this is the first reader, wait for writer semaphore to be available

    // Release lock
    signal(m); // Signal mutex semaphore to allow other processes to access the critical section

    /* Perform the reading operation */
    // Here we assume that the necessary code to read from the resource has been added

    // Acquire lock
    wait(m); // Wait for the mutex semaphore to be available

    readCount--; // Decrement the number of processes doing the read operation
    if (readCount == 0) {
        signal(w); // If this is the last reader, signal writer semaphore to allow writer processes to access the critical section

    // Release lock
    signal(m); // Signal mutex semaphore to allow other processes to access the critical section

The code uses three variables: "mutex" to ensure mutual exclusion while updating the "readCount" variable, "w" semaphore to ensure that no writer can access the critical section when a reader is accessing it, and "readCount" to keep track of the number of processes performing the read operation. In the code, each process first acquires the "mutex" lock before updating the "readCount" variable. If this is the first reader process to access the resource, it waits for the "w" semaphore to become available, which means that no writer process is currently accessing the resource.

After performing the read operation, the process again acquires the "mutex" lock and decrements the "readCount" variable. If this is the last reader process accessing the resource, it signals the "w" semaphore to allow writer processes to access the critical section.

Addressing the Reader-Writer Problem is crucial in ensuring efficient and safe concurrent access to shared resources in operating systems. Understanding the nuances and challenges posed by multiple readers and writers accessing the same data concurrently is essential for designing robust and scalable systems. Employing synchronization techniques and algorithms tailored to this problem can significantly enhance system performance while preventing data corruption and inconsistencies.

Frequently Asked Questions on reader Writer Problem in OS

Here are some FAQs related to reader writer problem in OS

Q1: What is the essence of the Reader-Writer Problem?
The Reader-Writer Problem deals with the challenge of managing concurrent access to shared resources by multiple readers and a single writer. It emphasizes the need to allow multiple readers simultaneous access for reading while ensuring exclusive access for writing to maintain data integrity.

Q2: What are the potential issues if the Reader-Writer Problem is not handled properly?
Failure to manage the Reader-Writer Problem adequately can lead to data corruption, inconsistency, and race conditions. If not synchronized properly, simultaneous read and write operations can result in incorrect data and compromise system reliability.

Q3: What are some common synchronization mechanisms used to solve the Reader-Writer Problem?
Several synchronization techniques are employed to address the Reader-Writer Problem, such as locks, semaphores, monitors, and read-write locks. These mechanisms help regulate access to shared resources, allowing concurrent reading and exclusive writing when necessary.

Q4: How do modern operating systems handle the Reader-Writer Problem?
Modern operating systems often provide built-in synchronization primitives and APIs to facilitate efficient management of shared resources. They offer various tools and techniques that programmers can utilize to implement effective solutions for the Reader-Writer Problem.

Q5: What are the key considerations when implementing a solution for the Reader-Writer Problem?
When implementing a solution, it’s essential to consider factors such as fairness, efficiency, and avoiding potential issues like starvation and deadlock. Choosing the appropriate synchronization mechanism and optimizing for performance are crucial aspects of solving the Reader-Writer Problem effectively.

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