Are C++ Reads and Writes of an int Atomic?

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Boy, what a question. The answer to which is:

Yes, no, hmmm, well, it depends

It all comes down to the architecture of the system. On an IA32 a correctly aligned address will be an atomic operation. Unaligned writes might be atomic, it depends on the caching system in use. If the memory lies within a single L1 cache line then it is atomic, otherwise it’s not. The width of the bus between the CPU and RAM can affect the atomic nature: a correctly aligned 16bit write on an 8086 was atomic whereas the same write on an 8088 wasn’t because the 8088 only had an 8 bit bus whereas the 8086 had a 16 bit bus.

Also, if you’re using C/C++ don’t forget to mark the shared value as volatile, otherwise the optimiser will think the variable is never updated in one of your threads.

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