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Deadlock in Operating Systems - Definition, Conditions, Examples & Solutions

Deadlock in Operating Systems

Deadlock is one of the most critical and frequently asked topics in Operating Systems. It is widely covered in computer science exams, interviews, and competitive tests. In this article, you’ll learn deadlock in a simple, practical, and exam-ready way.

What is Deadlock?

A deadlock is a situation where two or more processes are permanently blocked, each waiting for a resource that is currently held by another process.

In simple words: everyone is waiting, but no one can proceed.

Deadlock Definition

Deadlock is a condition in which a set of processes are blocked because each process is holding a resource and waiting for another resource held by another process.

🌍 Real-Life Example of Deadlock

  • Person A has a pen and needs a notebook
  • Person B has a notebook and needs a pen
  • Neither person releases their resource

Both keep waiting forever — this situation is called deadlock.

Deadlock in Operating Systems

In operating systems, deadlock occurs when processes compete for limited resources such as CPU, memory, files, or I/O devices.

Example:

  • Process P1 holds Resource R1 and waits for R2
  • Process P2 holds Resource R2 and waits for R1

This creates a circular dependency, resulting in deadlock.

⚠️ Necessary Conditions for Deadlock

Deadlock occurs only if all four conditions hold simultaneously.

1️⃣ Mutual Exclusion

At least one resource must be non-sharable (only one process can use it at a time).

2️⃣ Hold and Wait

A process is holding one resource while waiting for additional resources.

3️⃣ No Preemption

Resources cannot be forcibly taken from a process; they must be released voluntarily.

4️⃣ Circular Wait

A circular chain of processes exists where each process waits for a resource held by the next.

🔄 Deadlock Example Table

Process Resource Held Resource Requested
P1 R1 R2
P2 R2 R1

Circular wait exists — hence deadlock occurs.

Methods for Handling Deadlock

🔹 1. Deadlock Prevention

Deadlock prevention ensures that at least one of the four necessary conditions never occurs.

  • Eliminate hold and wait
  • Allow resource preemption
  • Apply strict resource ordering

🔹 2. Deadlock Avoidance

The system checks resource allocation requests before granting them.

Banker’s Algorithm

Banker’s Algorithm ensures the system always remains in a safe state.

🔹 3. Deadlock Detection and Recovery

The system allows deadlock to occur, detects it, and recovers by:

  • Terminating processes
  • Preempting resources

🔹 4. Ignoring Deadlock (Ostrich Algorithm)

Some operating systems ignore deadlocks because they occur rarely and handling them is costly.

Deadlock vs Starvation

Feature Deadlock Starvation
Cause Circular resource dependency Unfair scheduling
Resolution System intervention needed Priority adjustment

Frequently Asked Questions (FAQ)

Can deadlock occur with one process?

No. Deadlock requires at least two processes.

Which algorithm avoids deadlock?

Banker’s Algorithm.

Is deadlock same as livelock?

No. In livelock, processes keep changing state but make no progress.

Summary

  • Deadlock is a permanent blocking situation
  • It requires four necessary conditions
  • Handled using prevention, avoidance, detection, or ignoring
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