Which are the four necessary deadlock conditions, and what is a common prevention strategy?

• A. Mutual exclusion, Hold-and-Wait, No Preemption, Circular Wait; Prevention: enforce a strict resource ordering
• B. Mutual exclusion, Preemption, Random access, Circular wait; Prevention: use larger memory
• C. Deadlock is impossible; Prevention: none
• D. Symmetry, Waiting, Blocking, Preemption; Prevention: disable interrupts

Answer: (A)

Deadlock happens only when four conditions are all present at once, so recognizing these conditions helps you see how a deadlock forms and how to prevent it. The first condition is mutual exclusion: at least one resource must be held in a non-shareable way, meaning only one process can use it at a time. The second is hold-and-wait: a process holds onto some resources while waiting to acquire additional ones. The third is no preemption: resources cannot be forcibly taken away from a process; they must be released voluntarily. The fourth is circular wait: there is a cycle of processes where each holds a resource the next process in the cycle is waiting for. If any one of these is missing, a deadlock cannot occur.

A common prevention strategy is to enforce a strict global order on resource requests. By assigning a universal ordering to all resources and requiring processes to request resources in that order, you prevent the circular wait condition from ever forming. Once the cycle can't form, multiple processes can’t be stuck waiting for each other indefinitely.

The other options mix in terms that aren’t standard deadlock conditions (like random access) or suggest ineffective or inappropriate remedies (such as using larger memory or disabling interrupts).