Volatile variables

Question

I recently had an interview with a software company who asked me the following question:

Can you describe to me what adding volatile in front of variables does? Can you explain to me why it's important?

Most of my programming knowledge comes from C, yet the job position is for C# (I thought I might add this bit of info if necessary for the question specifically)

I answered by saying it just lets the compiler know that the variable can be used across processes or threads, and that it should not use optimization on that variable; as optimizing it can deteriorate behavior. In a nutshell, it's a warning to the compiler.

According to the interviewer, however, it's the other way around, and the volatile keyword warns the OS, not the compiler.

I was a bit befuddled by this, so I did some research and actually found conflicting answers! Some sources say it's for the compiler, and others for the OS.

Which is it? Does it differ by language?

Answer 1

I answered by saying it just lets the compiler know that the variable can be used across processes or threads, and that it should not use optimization on that variable; as optimizing it can deteriorate behavior. In a nutshell, it's a warning to the compiler.

This is going in the right direction for C# but misses some important aspects.

First off, delete "processes" entirely. Variables are not shared across processes in C#.

Second, don't concentrate on optimizations. Instead concentrate on permissible semantics. A compiler is not required to generate optimal code; a compiler is required to generate specification-compliant code. A re-ordering need not be for performance reasons and need not be faster / smaller / whatever. A volatile declaration adds an additional restriction on the permissible semantics of a multithreaded program.

Third, don't think of it as a warning to the compiler. It's a directive to the compiler: to generate code that is guaranteed to be compliant with the specification for volatile variables. How the compiler does so is up to it.

The actual answer to the question

Can you describe to me what adding volatile in front of variables does?

is: A C# compiler and runtime environment have great latitude to re-order variable reads and writes for any reason they see fit. They are restricted to only those re-orderings which preserve the meaning of programs on a single thread. So "x = y; a = b;" could move the read of b to before the read to y; that's legal because the outcome is unchanged. (This is not the only restriction on re-ordering, but it is in some sense the most fundamental one.) However, re-orderings are permitted to be noticeable on multiple threads; it is possible that another thread observes that b is read before y. This can cause problems.

The C# compiler and runtime have additional restrictions on how volatile reads and writes may be re-ordered with respect to each other, and furthermore how they may be ordered with respect to other events such as threads starting and stopping, locks, exceptions being thrown, and so on.

Consult the C# specification for a detailed list of the restrictions on observed orderings of reads, writes and other effects.

Note in particular that even with volatile variables, there is not required to be a consistent total ordering of all variable accesses as seen from all threads. And specifically, the notion that volatile "reads the latest value of the variable" is simply false; that phrasing suggests that there is such a thing as "the latest value", which implies a total consistent ordering.

If that sounds confusing, it is. Don't write multithreaded programs that share data across threads. If you must, use the highest level abstractions at your disposal. Hardly anyone should be writing code that uses volatile; use the TPL and let it manage your threads.

Now let's consider your answer in the context of C.

The question is ill-posed with respect to C. In C# volatile is a modifier on member variable declarations; in C, it's part of a type. So saying "before a variable" is ambiguous; where before the variable? There's a difference between a volatile int * x and an int * volatile x. (Can you see the difference?)

But more importantly: the C specification does not guarantee that volatile will have any particular behaviour with respect to threads. If your C compiler does, that's an extension of the language by your compiler vendor. Volatile in C is guaranteed to have certain behaviour with respect to memory mapped IO, long jumps, and signals, and that's all; if you rely on it to have certain behaviour with respect to threads then you are writing non-portable code.

According to the interviewer: it's the other way around, and the volatile keyword warns the OS, not the compiler.

That's nonsense from start to finish. Interviewers should not ask questions that they don't understand the answers to.