Are volatile reads and writes atomic on Windows+VisualC?

Question

There are a couple of questions on this site asking whether using a volatile variable for atomic / multithreaded access is possible: See here, here, or here for

Answer 1

under x86, these operations are guaranteed to be atomic without the need for LOCK based instructions such as Interlocked* (see intel's developer manuals 3A section 8.1):

basic memory operations will always be carried out atomically:

• Reading or writing a byte

• Reading or writing a word aligned on a 16-bit boundary

• Reading or writing a doubleword aligned on a 32-bit boundary

The Pentium processor (and newer processors since) guarantees that the following additional memory operations will always be carried out atomically:

• Reading or writing a quadword aligned on a 64-bit boundary

• 16-bit accesses to uncached memory locations that fit within a 32-bit data bus

The P6 family processors (and newer processors since) guarantee that the following additional memory operation will always be carried out atomically:

• Unaligned 16-, 32-, and 64-bit accesses to cached memory that fit within a cache line

This means volatile will only every serve to prevent caching and instruction reordering by the compiler (MSVC won't emit atomic operations for volatile variables, they need to be explicitly used).