I have a global std::unordered_map<int, int> m. I also have exactly one thread that is type A and multiple threads are type B running concurrently.

Thread type A: call insert(), erase() to add/remove some elements (guarantee not the elements read/write in the thread type B concurrently) of m

Thread type B: call operator[] of m, do something like m[key] = value. (guarantee thread type B will not modify the same element concurrently, and the key exists in m already)

Is it safe to do these operations concurrently without a lock?

CodePudding user response：

No. insert can rehash the array which reallocates the bucket array. If that happens right before operator[] accesses it, that's a use-after-free.

This probably isn't the only reason it's unsafe, but we only have to find one, to prove that it's unsafe.

CodePudding user response：

No, it is not safe. None of the methods of std::unordered_map, nor any other container in the C library, are thread safe. Therefore if one execution thread is modifying the container, all access to the container, from all execution threads, must be properly synchronized.