CODEWITHSUNDEEP
c++c++11atomic
Alex
Alex

Reputation: 13116

Is there any sense to make std::atomic<> objects with the qualifier - volatile?

Is there any sense to make atomic objects with the qualifier - volatile?

Use that:

volatile std::atomic<int> i(1);

instead of:

std::atomic<int> i(1);

Upvotes: 8

Views: 450

Answers (2)

Mats Petersson
Mats Petersson

Reputation: 129374

No, there is absolutely no sense in making std::atomic also volatile, as inside the std::atomic, the code will deal with the possibility that the variable may change at any time, and that other processors may need to be "told" that it has changed (the "telling" other processors is not covered by volatile).

The only time you really need volatile is if you have a pointer to piece of hardware that your code is controlling - for example reading a counter in a timer, or a which frame buffer is active right now, or telling a network card where to read the data for the next packet to send. Those sort of things are volatile, because the compiler can't know that the value of those things can change at any time.

Upvotes: 11

Mike Seymour
Mike Seymour

Reputation: 254461

Usually, it doesn't make sense.

Use atomic to allow the variable to be modified on one thread while other threads may be accessing it without explicit synchronisation.

Use volatile to control access to unusual memory locations (such as hardware registers), where every read and write must happen in the order specified by the program. A normal variable, atomic or otherwise, doesn't usually require such control.

The two concepts are unrelated to each other. In particular, do not confuse volatile with a keyword used in other languages to make variables atomic. In C++, volatile has nothing to do with thread interactions whatsoever.

Upvotes: 4

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