c++ - Why does atomic_flag.clear() have a sub-optimal default memory_order argument? -

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std::atomic_flag has 2 functions these default std::memory_orders:

void clear(std::memory_order order = std::memory_order_seq_cst); bool test_and_set(std::memory_order order = std::memory_order_seq_cst);

perhaps i'm wrong, shouldn't clear always memory_order_release , test_and_set always memory_order_acquire? maybe misunderstand these do.

the default memory order sequential consistency (std::memory_order_seq_cst) atomic operations in c++11. can specify more relaxed memory order if want, although more complicated reason about.

the std::atomic_flag garanteeded lock-free , can used build other synchronization methods. whereas std::atomic<t> types can implemented locks if compiler/library writer chooses , still conform specification. why std::atomic_flag exists these defaults. depending on trying might make sense specify different memory order defaults set conservatively correct sequential consistency.


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