In deadlock theory, what are the necessary conditions for a deadlock to occur among competing processes in a system?

Introduction / Context:
Deadlocks are situations where a set of processes are blocked forever because each is waiting for resources held by others. Classical deadlock theory identifies four necessary conditions that must all be present for a deadlock to occur. Understanding these conditions is fundamental to designing systems that can prevent, avoid or recover from deadlocks. This question asks you to recall those necessary conditions.

Given Data / Assumptions:

• Processes may request and hold multiple resources.
• Resources may be held in non sharable mode.
• Processes may be unable to proceed while waiting for more resources.
• The system does not forcibly preempt resources in all cases.

Concept / Approach:
The four necessary conditions for deadlock are mutual exclusion, hold and wait, no preemption and circular wait. Mutual exclusion means that at least one resource is non sharable; only one process can use it at a time. Hold and wait means processes already holding some resources can request additional ones and block while holding the originals. No preemption means resources cannot always be taken away; they are released only voluntarily. Circular wait means there is a cycle in the resource allocation graph where each process waits for a resource held by the next process in the cycle. All four must be present for deadlock to be possible.

Step-by-Step Solution:
Step 1: List the first condition, mutual exclusion, stating that some resources cannot be shared safely. Step 2: Add the second condition, hold and wait, where processes hold resources while waiting for new ones. Step 3: Add the third condition, no preemption, indicating that the system does not always forcibly take resources back. Step 4: Add the fourth condition, circular wait, where a cycle of processes exists, each waiting for a resource held by another in the cycle. Step 5: Choose the option that correctly names all four of these conditions as necessary for deadlock.

Verification / Alternative check:
Most operating system textbooks and exam syllabi present the same four Coffman conditions and emphasise that they are necessary but not individually sufficient. This standard result appears in many diagrams and examples, such as resource allocation graphs with cycles. The correct option reproduces this list accurately, while the other options mention unrelated states or trivial configurations.

Why Other Options Are Wrong:
Option B talks about a ready process and an idle printer, which has nothing to do with the formal conditions for deadlock. Option C describes a situation where processes never request resources, in which case deadlock due to resource contention cannot occur. Option D restricts execution to a single process that never requests resources; again, this eliminates resource contention and therefore cannot describe the necessary conditions for deadlock.

Common Pitfalls:
A frequent mistake is to forget one of the four conditions or to misname them, for example by omitting circular wait. Another pitfall is to think that breaking any one of the conditions always eliminates all performance issues; it only guarantees that deadlock, in the strict theoretical sense, cannot occur. Resource contention and starvation can still be problems even when deadlocks are impossible.

Final Answer:
The necessary conditions for deadlock are mutual exclusion, hold and wait, no preemption and circular wait all holding at the same time.