alt Hacker NewsYes, there is a data race there. The value of a volatile can be changed by something outside the current thread. That’s what volatile means and why it exists.
Edit: thread=thread of execution. I’m not making a point about thread safety within a program.
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Can also represent a register that has an effect reading it. Reading a memory mapped register can have side effects. Like memory mapped io on a UART will fetch the next byte to be read.
Was going to say the same thing until I saw this comment. volatile is defined the way I'd expect, plus it's a strange code example.
Not sure why you're being downvoted. That's completely right. The example is silly. The code is obviously bad, doesn't matter if it's UB or not.
I'm also not convinced (yet) that the example really is UB: I agree reading a volatile is "a side effect" in some sense, and GP cited a paragraph that says just that. But GP doesn't clearly quote that it's a side effect on the object (or how a side effect on an object is defined). Reading an object doesn't mutate it after all.
But whatever language lawyer things, the code is obviously broken, with an obvious fix, so I'm not so interested in what its semantics should be. Here is the fix:
volatile int x;
// ...
int val = x; // volatile read
printf("%x %d\n", val, val);
Not from the standard’s point of view. The traditional (in some circles) use of volatile for atomic variables was not sanctioned by the C11/C++11 thread model; if you want an atomic, write atomic, not volatile, or be aware of your dependency on a compiler (like MSVC) that explicitly amends the language definition so as to allow cross-thread access to volatile variables.