Do I need to protect this variable with a lock?

Question

so I have a boolean type in C++ on a multiprocessor machine. The variable starts out life as true, and then there are a few threads, any one or more of which might write it to b

Answer 1

On most commodity hardware single word read and writes are not atomic operations; remember you have virtual memory machines here and either of those operations might cause a page fault.

In more general terms you'll probably find it easier and quicker to use mutexes as a matter of routine than scratch your head wondering if you are going to get away without one this time. Adding one where it isn't necessary doesn't cause a bug; leaving one out where it is might.