Multiple vector writers without locking

Question

I have a few threads writing in a vector. It's possible that different threads try to write the same byte. There is no reads. Can I use only an atomic_fecth_or(), like in the example, so the vector will become thread safe? It compiled with GCC without errors or warnings.

    std::vector<std::atomic<uint8_t>> MapVis(1024*1024); 

    void threador()
    {
    ...
    std::atomic_fetch_or(&MapVis[i], testor1); 
    }

Answer 1

It compiled with GCC without errors or warnings

That doesn't mean anything because compilers don't perform that sort of concurrency analysis. There are dedicated static analysis tools that may do this with varying levels of success.

Can I use only an atomic_fetch_or ...

you certainly can, and it will be safe at the level of each individual std::atomic<uint8_t>.

... the vector will become thread safe?

it's not sufficient that each element is accessed safely. You specifically need to avoid any operation that invalidates iterators (swap, resize, insert, push_back etc.).

I'd hesitate to say vector is thread-safe in this context - but you're limiting yourself to a thread-safe subset of its interface, so it will work correctly.

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Note that as VTT suggests, keeping a separate partial vector per thread is better if possible. Partly because it's easier to prove correct, and partly because it avoids false sharing between cores.