Atomic Heart Rucksack Overloaded

22 Atomic vs. Non-Atomic Operations "An operation acting on shared memory is atomic if it completes in a single step relative to other threads. When an atomic store is performed on a shared memory, no other thread can observe the modification half-complete.

Objects of atomic types are the only C++ objects that are free from data races; that is, if one thread writes to an atomic object while another thread reads from it, the behavior is well-defined. In addition, accesses to atomic objects may establish inter-thread synchronization and order non-atomic memory accesses as specified by std::memory_order.

When can 64-bit writes be guaranteed to be atomic, when programming in C on an Intel x86-based platform (in particular, an Intel-based Mac running MacOSX 10.4 using the Intel compiler)? For exampl...

The definition of atomic is hazy; a value that is atomic in one application could be non-atomic in another. For a general guideline, a value is non-atomic if the application deals with only a part of the value. Eg: The current Wikipedia article on First NF (Normal Form) section Atomicity actually quotes from the introductory parts above.

The AVR-libc user manual in the section backs up my claim that 8-bit types on AVR, when compiled by gcc, already have naturally atomic reads and naturally atomic writes when it implies that 8-bit reads and writes are already atomic by saying (emphasis added):

Which types on a 64-bit computer are naturally atomic in gnu C and gnu ...

Yes an operation can be atomic or not, but there are specific data types that are referred to as being atomic types because they allow atomic operations due to their memory size requirements in regards to the word size on the system.