What conditions are necessary for deadlock, and how can software avoid it?

The classic necessary conditions are mutual exclusion, hold-and-wait, no preemption, and circular wait. A program can reduce deadlock risk by breaking a condition, for example acquiring multiple locks in a consistent global order. Keep critical sections small, avoid waiting for exte al work while holding locks, and use suitable higher-level concurrency primitives. Timeouts can help detect a problem but do not automatically restore a valid state. Distinguish deadlock from starvation or livelock, where progress fails for different reasons and requires a different remedy.