Why is volatile not considered useful in multithreaded C or C++ programming?

Question

As demonstrated in this answer I recently posted, I seem to be confused about the utility (or lack thereof) of volatile in multi-threaded programming contexts.<

Answer 1

I don't think you're wrong -- volatile is necessary to guarantee that thread A will see the value change, if the value is changed by something other than thread A. As I understand it, volatile is basically a way to tell the compiler "don't cache this variable in a register, instead be sure to always read/write it from RAM memory on every access".

The confusion is because volatile isn't sufficient for implementing a number of things. In particular, modern systems use multiple levels of caching, modern multi-core CPUs do some fancy optimizations at run-time, and modern compilers do some fancy optimizations at compile time, and these all can result in various side effects showing up in a different order from the order you would expect if you just looked at the source code.

So volatile is fine, as long as you keep in mind that the 'observed' changes in the volatile variable may not occur at the exact time you think they will. Specifically, don't try to use volatile variables as a way to synchronize or order operations across threads, because it won't work reliably.

Personally, my main (only?) use for the volatile flag is as a "pleaseGoAwayNow" boolean. If I have a worker thread that loops continuously, I'll have it check the volatile boolean on each iteration of the loop, and exit if the boolean is ever true. The main thread can then safely clean up the worker thread by setting the boolean to true, and then calling pthread_join() to wait until the worker thread is gone.