C++ std::atomic and shared memory? [duplicate]

Question

I have 4096 bytes of shared memory allocated. How to treat it as an array of std::atomic<uint64_t> objects?

The final goal is to place array of 64-bit variables to shared memory and perform __sync_fetch_and_add (GCC built-in) on these variables. But I would prefer using native C++11 code instead of using GCC built-ins. So how to use the allocated memory as an std::atomic objects? Should I invoke placement new on 512 counters? What If std::atomic's constructor require additional memory allocations in some imeplementations? Should I consider the aligning of std::atomic object in shared memory?

Answer 1

With C++20, you can use std::atomic_ref if you can make sure that your objects are suitably aligned. For C++17 and anything older, boost::atomic_ref might work as well, though I haven't tested it.

If you don't want to use Boost, then compiler builtins are the only solution left. In that case, you should prefer the __atomic builtins over the old __sync functions, as stated on the GCC documentation page for atomics.