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Explain the difference between a mutex and a semaphore in two short paragraphs, and say when to use each.

A mutex is a locking mechanism designed for mutual exclusion: only the thread that locked it may unlock it, and it has a notion of ownership. It's binary (locked/unlocked) and is used to protect a shared resource so that only one thread accesses it at a time. A semaphore is a signaling mechanism built around a counter: it tracks how many permits are available, any thread can wait (decrement) or post (increment) it, and it doesn't care which thread does what. A binary semaphore looks similar to a mutex, but lacks ownership, so it can be released by a different thread—useful for signaling between threads rather than guarding a critical section. Use a mutex when you need to protect a shared resource (a data structure, file, or device) from concurrent access, which is the common case for locking critical sections. Use a semaphore when you need to control access to a pool of N identical resources (e.g., limiting concurrent database connections) or to signal between threads, such as a producer notifying a consumer that data is ready or a task-completion event. As a rule of thumb: mutex for mutual exclusion, semaphore for counting or signaling.

2026-09-28 に生成。回答は実行ごとに異なります。