Using C/Pthreads: do shared variables need to be volatile?

Question

In the C programming language and Pthreads as the threading library; do variables/structures that are shared between threads need to be declared as volatile? Assuming that t

Answer 1

The underlying reason is that the C language semantic is based upon a single-threaded abstract machine. And the compiler is within its own right to transform the program as long as the program's 'observable behaviors' on the abstract machine stay unchanged. It can merge adjacent or overlapping memory accesses, redo a memory access multiple times (upon register spilling for example), or simply discard a memory access, if it thinks the program's behaviors, when executed in a single thread, doesn't change. Therefore as you may suspect, the behaviors do change if the program is actually supposed to be executing in a multi-threaded way.

As Paul Mckenney pointed out in a famous Linux kernel document:

It _must_not_ be assumed that the compiler will do what you want with memory references that are not protected by READ_ONCE() and WRITE_ONCE(). Without them, the compiler is within its rights to do all sorts of "creative" transformations, which are covered in the COMPILER BARRIER section.

READ_ONCE() and WRITE_ONCE() are defined as volatile casts on referenced variables. Thus:

int y;
int x = READ_ONCE(y);

is equivalent to:

int y;
int x = *(volatile int *)&y;

So, unless you make a 'volatile' access, you are not assured that the access happens exactly once, no matter what synchronization mechanism you are using. Calling an external function (pthread_mutex_lock for example) may force the compiler do memory accesses to global variables. But this happens only when the compiler fails to figure out whether the external function changes these global variables or not. Modern compilers employing sophisticated inter-procedure analysis and link-time optimization make this trick simply useless.

In summary, you should mark variables shared by multiple threads volatile or access them using volatile casts.

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As Paul McKenney has also pointed out:

I have seen the glint in their eyes when they discuss optimization techniques that you would not want your children to know about!

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But see what happens to C11/C++11.