CODEWITHSUNDEEP
c++c++11atomiccompare-and-swap
Seves
Seves

Reputation: 165

How to atomically set a value if a condition holds in C/C++?

I need to atomically assign a = b if condition c holds, and do not assign if the condition does not hold. Is there a way to do this in C/C++?

Clarification: I meant atomically "test and assign", rather than "atomically assign".

Upvotes: 6

Views: 5341

Answers (2)

Yakk - Adam Nevraumont
Yakk - Adam Nevraumont

Reputation: 275585

If c is some property of a itself, then compare-exchange atomic operations can solve your problems. You have indicated in comments that this is not the case.

Failing that, wrap all access of c (anything that reads its state or modifies its state) and a within a locks of a common std::mutex.

Test c and assign to a within a lock of that std::mutex.

Failing that, write a single-threaded program.

Failing that, use OS-level APIs to suspend every thread in your process except yourself, then test c and possibly assign to a. Note that this could see c or a in a state where it is half-written (ie, in an incoherant state), and with the thread suspension plan there really tends not to be a practical way to avoid it, unless both c and a are lock-free atomic themselves.

Note that this plan is extremely dangerous and fragile.

Failing that, describe your problem less abstractly and ask another question. The odds are a less abstract version of your problem will have solutions is reasonable.

Upvotes: 2

Smeeheey
Smeeheey

Reputation: 10326

There isn't a single operation that accomplishes what you require in general. The common case that is covered if is the case where your condition c tests if a is already set to some particular value. In this case, the operation you need is a compare-and-exchange, for example:

std::atomic<int> value;
...
int expected = 9;
bool result = std::atomic_compare_exchange_strong(&value, &expected, 10);

Here value is set to 10 only if the condition value == 9 holds true, the whole operation being atomic.

If your condition doesn't take this form, then you would have to uses mutexes (or some other multiple-operation mechanism) to achieve atomicity.

Upvotes: 16

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