CODEWITHSUNDEEP
c++multithreading
khuttun
khuttun

Reputation: 733

Is it a data race to access shared data after joining a thread?

Does the following code result in a data race:

int f()
{
    int i = 0;
    std::thread t{[&]{ i = 1; }};
    t.join();
    return i;
}

No mutex or atomics are used for i access.

Documentation of std::thread::join talks about "completion of the thread synchronizing with the corresponding successful return from join()", but I'm not sure would it be relevant in this context.

Upvotes: 2

Views: 260

Answers (2)

Brian Bi
Brian Bi

Reputation: 119069

As the linked page says,

The completion of the thread identified by *this synchronizes with the corresponding successful return from join().

This implies that anything that happens in t before t exits (that is, anything that happens in t) happens before anything that happens in the main thread after t.join() returns. Therefore, the write to i within thread t happens before i is read by the return statement. Therefore, the two accesses to i are not potentially concurrent. Therefore, there is no data race.

Upvotes: 6

NathanOliver
NathanOliver

Reputation: 180415

Does the following code result in a data race

No. The rule is if you have more than one thread, and at least one of them is a writer, you need synchronization when accessing shared data. The key point here is more than one thread. After you call join, you no longer have more than one thread so main's access of i is safe.

Upvotes: 5

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