Explain deadlock prevention vs deadlock avoidance strategies with examples.

• A. Prevention uses wait-for graphs; avoidance prevents deadlocks by always aborting transactions.
• B. Prevention detects deadlocks after they occur and aborts processes.
• C. Prevention ensures one of the necessary conditions cannot hold; avoidance uses resource allocation strategies like wait-die or detection.
• D. Prevention and avoidance are the same concept.

Answer: (C)

Deadlock prevention and deadlock avoidance tackle the problem in different ways. Prevention works by making sure that at least one of the conditions that allow a deadlock to happen can never hold. Practically, that means changing how resources are requested and held—for example, requiring a process to request all resources it will need at once or requiring preemption so a resource can be taken away if needed. With these guarantees, the system can never reach a state where a circular wait, mutual exclusion, hold-and-wait, and no preemption all occur.

Avoidance, by contrast, makes allocation decisions on the fly to keep the system in a safe state. After each request, the system checks whether granting it could lead to a deadlock and only grants if there is a possible sequence of future events that lets every process finish. Banker's algorithm is the classic example. Policies like wait-die (older requests wait, younger ones abort) are also used to prevent deadlocks, while detection-based approaches let deadlocks occur and then detect and recover from them. The key idea is that avoidance works by staying out of unsafe states through careful allocation, whereas prevention prevents the fundamental conditions for deadlock from ever forming.