Question

What is the memory model for concurrency in C++03?

(And, does C++11 change the memory model to support concurrency better?)

Answer 1

The C++ memory model is the specification of when and why physical memory is read/written with respect to C++ code.

Until the next C++ standard, the C++ memory model is the same as C. In the C++0x standard, a proper memory model for multithreading is expected to be included (see here), and it will be part possibly of the next revision of the C standard, C1X. The current one is rudimentary:

• it only specifies the behavior of memory operations observable by the current program.
• it doesn’t say anything about concurrent memory accesses when multiple processes access the same memory (there is no notion of shared memory or processes).
• it doesn’t say anything about concurrent memory accesses when multiple threads access the same memory (there is no notion of threads).
• it offers no way to specify an ordering for memory accesses (compiler optimizations include code motion and recent processors reorder accesses, both can break patterns such as double checked initialization).

So, the current state is: C++ memory operations are only specified when you have 1 process, with its main thread and don’t write code which depends on a specific ordering of variable read/writes and that’s it. In essence, this means that aside from the traditional hello world program you’re screwed.

Of course, you’ll be prompt to add that ‘it works today on my machine, you can’t possibly be right’. The correct sentence would be ‘it works today on my machine with this specific combination of hardware, operating system (thread library) and compiler who know enough of each other to implement something which is somewhat working but will probably break at some point’.

Ok ok, this is a bit harsh but hell, even Herb Sutter acknowledges that (just read the intro) and he is talking about all pre 2007 versions of one of the most ubiquitous C/C++ toolchain…

The C++ standard committee attempts to come up with something which will address all those issues while still being less constraining (and thus better performing) than Java’s memory model.

Hans Boehm has collected here some pointers to papers on the issue, both academic, and from the C++ committee.