Computer Science

Introduction to Deadlock in Operating Systems (OS)

Published: 23rd August, 2023

Vibha Gupta

Technical Content Writer at almaBetter

In this article, we will explain deadlock in operating system, explore causes and characteristics of deadlock, and discuss strategies for handling deadlocks.

What is deadlock in operating system? Deadlock is a critical issue that can occur in operating systems when multiple processes cannot proceed because each process is waiting for a resource that is being held by another process. In this article, we will explain deadlock in operating systems, explore the causes and characteristics of deadlock in OS example, and discuss various strategies for handling deadlocks in OS and preventing deadlocks in operating systems.

Table of Contents

  1. Define Deadlock in OS
  2. Causes of Deadlock
    • Mutual Exclusion
    • Hold and Wait
    • No Preemption
    • Circular Wait
  3. Deadlock Detection
  4. Deadlock Prevention
  5. Deadlock Avoidance
  6. Deadlock Recovery
  7. Difference Between Deadlock and Starvation
  8. Advantages and Disadvantages of Deadlock
  9. Conclusion

1. What is Deadlock in OS?

Deadlock is a scenario in operating systems where two or more processes are unable to proceed because each process is waiting for a resource that is being held by another process. This situation creates a standstill where no progress can be made until the deadlock is resolved. Deadlocks can occur in various systems, including computer networks, distributed systems, and multi-threaded applications.

In a deadlock, each process is stuck in a waiting state, unable to proceed with its execution. This occurs when a process requests a resource held by another process, which is waiting for a resource held by the first process. The result is a deadlock cycle, where the processes are stuck in a circular dependency, waiting indefinitely for the resources they need to continue.

Deadlocks can have severe consequences, leading to system crashes, frozen applications, and unresponsive user interfaces. Therefore, it is crucial to understand the causes of deadlock and implement strategies to prevent and handle them effectively.

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2. Necessary Conditions for Deadlock in OS

Deadlocks occur due to the fulfillment of four necessary conditions for deadlock in operating system known as the Coffman conditions. Let's explore each of these conditions in detail:

Mutual Exclusion

The mutual exclusion condition states that some resources can only be accessed by one process at a time. This means that once a process acquires a resource, other processes are denied access until it is released. For example, in a multi-threaded application, a critical section of code may be protected by a lock that can only be held by one thread at a time.

Hold and Wait

The hold and wait condition arises when a process is holding at least one resource and is waiting to acquire additional resources. In this scenario, a process may acquire a resource but cannot proceed with its execution because it requires other resources that are currently held by other processes. This leads to a situation where processes are stuck, waiting for resources to be released.

No Preemption

The no preemption condition states that resources cannot be forcibly taken away from a process. Once a process acquires a resource, it can only release it voluntarily. This means that a process cannot be interrupted or preempted to free up its resources and allow other processes to use them. Preemption could potentially lead to data corruption and inconsistencies if a process is forcefully interrupted during its execution.

Circular Wait

The circular wait condition occurs when a set of processes is circularly waiting for resources. In other words, each process waits for a resource that another process holds in the set, creating a circular dependency. This circular wait cycle prevents any process from acquiring the necessary resources to continue its execution.

By fulfilling these four conditions simultaneously, a deadlock in OS can occur. It is important to note that all four conditions must be present for a deadlock to happen. If any one of these conditions is not met, a deadlock cannot occur.

3. Deadlock Detection - Types of Deadlock in OS

Detecting a deadlock is essential to handle and resolve the issue effectively. Deadlock detection involves periodically monitoring the system's state to identify if a deadlock has occurred. Several algorithms, such as the resource-allocation graph and the banker's algorithm, can be used to detect deadlocks.

The resource-allocation graph is a graphical representation of the resources and processes in the system. It consists of nodes representing processes and resources and edges representing resource requests and allocations. By analyzing the graph, it is possible to identify cycles that indicate the occurrence of a deadlock in OS.

The banker's algorithm is a resource allocation and deadlock avoidance algorithm. It ensures that a state is safe by simulating the execution of processes and checking if they can complete their execution without entering a deadlock state. If a safe state cannot be reached, a deadlock is detected.

Once a deadlock is detected, appropriate measures can be taken to resolve it and allow the processes to proceed with their execution. Deadlock detection is an important aspect of deadlock management, as it provides insights into the system's current state and helps make informed decisions for deadlock resolution.

4. Deadlock Prevention

Preventing deadlocks is a proactive approach that focuses on eliminating one or more of the Coffman conditions to ensure that deadlocks cannot occur. Let's explore some strategies for preventing characteristics of deadlock in operating system:

Mutual Exclusion Elimination

Mutual exclusion is a necessary condition for deadlock, as it restricts resources to be accessed only one process at a time. The possibility of deadlocks can be reduced by eliminating mutual exclusion, such as allowing multiple processes to access a resource simultaneously. However, this strategy may not be feasible in all scenarios, especially when resources are not shareable.

Hold and Wait Avoidance

The hold and wait condition can be avoided by adopting a strategy where a process requests all the required resources before starting its execution. This ensures that a process does not hold any resources while waiting for additional resources, reducing the chances of a deadlock. However, this approach may result in resource underutilization and decreased system efficiency.

No Preemption

To prevent deadlocks, resources can be preempted or forcefully taken away from a process when required by another process. This approach allows the system to reallocate resources dynamically and prioritize processes based on their needs. However, preemption can introduce complexity and potential data integrity issues, making it challenging to implement in certain contexts.

Circular Wait Resolution

By enforcing a total ordering of resource requests, circular waits can be eliminated. This means that processes must request resources in a predetermined order, preventing the formation of circular dependency chains. However, this approach requires careful resource management and coordination to ensure the correct order of resource requests.

Preventing deadlocks can be challenging, as it involves carefully analyzing the system's resource allocation policies and making changes to the design and implementation of the system. Prevention strategies should be evaluated based on their feasibility, impact on system performance, and ability to eliminate the conditions necessary for deadlock.

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5. Deadlock Avoidance: Prevention of Deadlock in OS

Deadlock avoidance is a more flexible approach to dealing with deadlocks. It focuses on dynamically granting resource requests based on the predicted behavior of processes. By analyzing the resource needs of processes and predicting future resource requests, the system can avoid granting resources that may lead to a deadlock. You will learn more about methods of handling deadlock in os further.

One commonly used algorithm for deadlock avoidance is the banker's algorithm. This algorithm uses a mathematical model to determine if a resource allocation will result in a deadlock in OS. It simulates the execution of processes and checks if they can complete their execution without entering a deadlock state. If a safe state can be reached, the resource allocation is considered safe and can be granted.

The banker's algorithm requires information about the maximum resource requirements of each process, the currently allocated resources, and the available resources in the system. Based on this information, the algorithm analyzes whether a deadlock is possible. If a deadlock is detected, the system can choose to delay granting resources until a safe state can be achieved.

Deadlock avoidance algorithms can be complex and require substantial computational resources. They need to balance the need for resource allocation with the prevention of deadlocks. While avoidance strategies can be effective in certain scenarios, they may not be suitable for all systems due to the overhead of predicting resource needs and ensuring a safe state.

6. Deadlock Recovery - Methods For Handling Deadlock in OS

Deadlock recovery becomes necessary when deadlock prevention or avoidance strategies are not applicable or unsuccessful. Deadlock recovery involves taking corrective actions to resolve the deadlock and allow the processes to continue their execution. There are two primary approaches for deadlock recovery:

1. Process Termination

In this approach, the operating system terminates one or more processes involved in the deadlock. The resources held by those processes are released by terminating the processes, allowing other processes to proceed. However, process termination can result in data loss or inconsistency, as the terminated processes may have made partial progress before the deadlock occurred.

When selecting processes for termination, several factors can be considered, such as the process's priority, the progress made by the process, and the resources consumed by the process. By carefully selecting the processes to terminate, the impact on the system can be minimized.

2. Resource Preemption

Resource preemption involves forcibly taking resources from one or more processes involved in the deadlock. The operating system can break the circular dependency by preempting resources and allowing the remaining processes to proceed. However, resource preemption can be complex and may require the system to roll back the affected processes to a safe state before resuming their execution.

The selection of processes and resources for preemption requires careful consideration to minimize the impact on system performance and ensure fair resource allocation. Factors such as the number of resources held by a process and the amount of time the process has consumed can be considered when deciding which processes and resources to preempt.

Deadlock recovery strategies aim to resolve deadlocks and restore the system to a consistent state. However, both process termination and resource preemption strategies have their limitations and may result in performance degradation or data loss. Therefore, careful planning and consideration are necessary when implementing deadlock recovery mechanisms.

7. Difference Between Deadlock and Starvation

While deadlock and starvation are both issues that can occur in operating systems, they have distinct characteristics and implications. Here are the key differences between deadlock and starvation:


  • Deadlock occurs when two or more processes are unable to proceed because each process is waiting for a resource that is being held by another process.
  • Deadlock is an infinite waiting situation where processes are stuck in a circular dependency and cannot progress.
  • All the necessary conditions for deadlock, including mutual exclusion, hold and wait, no preemption, and circular wait, must be fulfilled for a deadlock to occur.
  • Deadlock can result in system crashes, frozen applications, and unresponsive user interfaces.


  • Starvation occurs when a low-priority process is continuously denied access to a resource while high-priority processes are granted access.
  • Starvation is a long waiting situation but is not infinite like a deadlock in OS.
  • Starvation can occur due to uncontrolled priority and resource management, where certain processes are consistently prioritized over others.
  • Starvation does not necessarily lead to a system crash or frozen applications, but it can result in decreased system performance and unfair resource allocation.

It is important to distinguish between deadlock and starvation as they require different approaches for resolution. Deadlock in operating system must be detected and resolved to allow processes to proceed, while starvation may require resource allocation policies and priority management adjustments.

8. Advantages and Disadvantages of Deadlock in OS

Like any concept or strategy, deadlocks have both advantages and disadvantages. Let's explore the pros and cons of deadlocks:

Advantages of Deadlock in OS

  • Deadlocks can be useful in certain scenarios, such as processes that perform a single burst of activity and do not require resource sharing.
  • Deadlocks can provide simplicity and efficiency in systems where the correctness of data is more important than overall system performance.
  • Deadlocks can be enforced via compile-time checks, eliminating the need for runtime computation and reducing the chances of unexpected system behavior.

Disadvantages of Deadlock in OS

  • Deadlocks can cause system crashes, frozen applications, and unresponsive user interfaces, leading to a poor user experience.
  • Deadlocks may result in delays in process initiation and overall system performance degradation.
  • Deadlocks can preclude incremental resource requests and disallow processes from making progress.
  • Deadlocks may require inherent preemption, resulting in losses and potential data integrity issues.

It is important to consider the advantages and disadvantages of deadlocks when designing and implementing operating systems. While principles of deadlock in operating system can offer simplicity and correctness in certain scenarios, they require careful resource management and significantly negatively impact system performance and user experience.

9. Conclusion

Deadlock is a critical issue that can occur in operating systems when processes are unable to proceed due to circular dependencies in resource allocation. By understanding the causes of deadlocks and implementing strategies for detection, prevention, avoidance, and recovery from deadlock in OS, operating systems can effectively handle and mitigate the impact of deadlocks.

Mutual exclusion, hold and wait, no preemption, and circular wait are the necessary conditions for deadlocks, and by addressing these conditions, deadlocks can be prevented or avoided. Deadlock detection algorithms and recovery strategies play a crucial role in identifying and resolving deadlocks when prevention and avoidance are not feasible.

By distinguishing deadlock in OS from starvation and considering the advantages and disadvantages of deadlocks, operating system designers can make informed decisions to balance system performance, resource utilization, and user experience.

In conclusion, deadlocks are complex issues that require careful consideration and proactive measures to ensure the stability and efficiency of operating systems.

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