CODEWITHSUNDEEP
c++
ehds
ehds

Reputation: 785

How to guarantee data dependence with atomic?

How to guarantee data dependence with c++ atomic?

I want to add something into map in muti-thread, I use atomic_uint64_t for indicator. In every add step,the indicator++.

atomic_uint64_t id;
id.fetch_add(1,memory_order_relaxed);
map[id] = "something";

How can I guarantee the id in map[id]="something" is the same as the result of id.fetch_add.

Upvotes: 2

Views: 85

Answers (1)

Some programmer dude
Some programmer dude

Reputation: 409176

You can't.

The atomicity of id doesn't prevent some other thread from increasing the value between your fetch_add call and map[id] assignment/insertion.

Use a mutex to protect the whole section instead.

By using a mutex, you also prevent data-races for the map access as well.

Upvotes: 5

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