stdatomic

How do memory_order_seq_cst and memory_order_acq_rel differ?

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http://en.cppreference.com/w/cpp/atomic/memory_order has a good example at the bottom that only works with memory_order_seq_cst. Essentially memory_order_acq_rel provides read and write orderings relative to the atomic variable, while memory_order_seq_cst provides read and write ordering globally. That is, the sequentially consistent operations are visible in the same order across all threads. The example boils down to this: bool … Read more

Does std::atomic work appropriately?

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The standard does not specify a specialization of std::atomic<std::string>, so the generic template <typename T> std::atomic<T> applies. 29.5 [atomics.types.generic] p1 states: There is a generic class template atomic. The type of the template argument T shall be trivially copyable (3.9). There is no statement that the implementation must diagnose violations of this requirement. So either … Read more

Why don’t compilers merge redundant std::atomic writes?

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The C++11 / C++14 standards as written do allow the three stores to be folded/coalesced into one store of the final value. Even in a case like this: y.store(1, order); y.store(2, order); y.store(3, order); // inlining + constant-folding could produce this in real code The standard does not guarantee that an observer spinning on y … Read more

What does the [[carries_dependency]] attribute mean?

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[[carries_dependency]] is used to allow dependencies to be carried across function calls. This potentially allows the compiler to generate better code when used with std::memory_order_consume for transferring values between threads on platforms with weakly-ordered architectures such as IBM’s POWER architecture. In particular, if a value read with memory_order_consume is passed in to a function, then … Read more

Must I call atomic load/store explicitly?

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Are assignment and access operations for non-reference types also atomic? Yes, they are. atomic<T>::operator T and atomic<T>::operator= are equivalent to atomic<T>::load and atomic<T>::store respectively. All the operators are implemented in the atomic class such that they will use atomic operations as you would expect. I’m not sure what you mean about “non-reference” types? Not sure … Read more

c++, std::atomic, what is std::memory_order and how to use them?

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The std::memory_order values allow you to specify fine-grained constraints on the memory ordering provided by your atomic operations. If you are modifying and accessing atomic variables from multiple threads, then passing the std::memory_order values to your operations allow you to relax the constraints on the compiler and processor about the order in which the operations … Read more

When do I really need to use atomic instead of bool? [duplicate]

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No type in C++ is “atomic by nature” unless it is an std::atomic*-something. That’s because the standard says so. In practice, the actual hardware instructions that are emitted to manipulate an std::atomic<bool> may (or may not) be the same as those for an ordinary bool, but being atomic is a larger concept with wider ramifications … Read more

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