# Concurrency

Docs: cppreference "Concurrency support library", `std::memory_order`,
`std::atomic`.

- Shared mutable data is protected by a mutex or is `std::atomic`; otherwise
  it's a data race (UB).
- Use `std::scoped_lock` / `std::lock_guard` / `std::unique_lock`, never manual
  `lock()`/`unlock()`.
- Multiple mutexes locked together with `std::scoped_lock(a, b)` or in a
  consistent order.
- No callbacks, I/O, or user code invoked while holding a lock.
- Condition variables wait with a predicate (spurious wakeups).
- `std::thread` is joined or detached before destruction; prefer `std::jthread`
  (C++20).
- `std::async` futures: the returned future's destructor blocks; don't discard
  it by accident.
- Atomics: `memory_order_relaxed` only when no other memory depends on the
  value.
- Lambdas passed to threads don't capture locals by reference that may die
  first.
- Static local init is thread-safe; other lazy init (double-checked locking)
  needs atomics or `std::call_once`.
- Coroutines: references/pointer parameters may dangle after the first
  suspension.