Hi Tynan, First off, C is not a multi-threaded language. That doesn't mean you can't do multi-threaded programming in it. It just means that the language does not support multi-threaded programming. (Just as C is not an object-oriented programming language, but you can do object-oriented programming in it just fine -- with a good heaping helping of discipline and diligence.) Adding volatile to the g_counter tells the compiler not to optimize away access to the variable, as the variable can change "at any time". That may sound like just the thing for multi-threaded access to the same variable -- but it's not sufficient. Even though the compiler is no longer going to optimize away access to the variable, the hardware can still effectively isolate the two threads from one another, such that neither thread is aware that the g_counter variable has changed. Why? Because each thread could run with the variable stored in separate caches. That's where a bolt-on library like pthread (POSIX threads) comes into play. Using pthread routines, you can programmatically make sure that all the caches are sync'd so that each core has the same view of the state of the variable. (If C was a multi-threaded language, it would have these kinds of directives as primitives.) Google turned up this nice tutorial: https://computing.llnl.gov/tutorials/pthreads/ Read the sections on Mutex Variables, and on Condition Variables. There are several good books available on PThread programming... I recommend anything from Addison-Wesley or O'Reilly publishers on the topic. (The books I read on PThread programming are 10+ years old, and I can't find them at the moment to recommend them in particular.) HTH, --Eljay
