In C/C++, are volatile variables guaranteed to have eventually consistent semantics betwen threads?

Question

Is there any guarantee by any commonly followed standard (ISO C or C++, or any of the POSIX/SUS specifications) that a variable (perhaps marked volatile), not guarded by a m

Answer 1

It's going to depend on your architecture. While it is unusual to require an explicit cache flush or memory sync to ensure memory writes are visible to other threads, nothing precludes it, and I've certainly encountered platforms (including the PowerPC-based device I am currently developing for) where explicit instructions have to be executed to ensure state is flushed.

Note that thread synchronisation primitives like mutexes will perform the necessary work as required, but you don't typically actually need a thread synchronisation primitive if all you want is to ensure the state is visible without caring about consistency - just the sync / flush instruction will suffice.

EDIT: To anyone still in confustion about the volatile keyword - volatile guarantees the compiler will not generate code that explicitly caches data in registers, but this is NOT the same thing as dealing with hardware that transparently caches / reorders reads and writes. Read e.g. this or this, or this Dr Dobbs article, or the answer to this SO question, or just pick your favourite compiler that targets a weakly consistent memory architecture like Cell, write some test code and compare what the compiler generates to what you'd need in order to ensure writes are visible to other processes.