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Sane atomic operations for .NET based on the C++11 memory model.

This library is based on the information in this blog post and provides an understandable atomic operations API based on the C++11 memory model (with some simplifications for the Common Language Infrastructure). The goal is that users should be able to write lock-free data structures and algorithms without having to resort to the Thread.VolatileRead and Thread.VolatileWrite methods, or the Interlocked and Volatile classes.


Why the C++11 memory model?

In short: Because it's becoming ubiquitous. C++11, C11, D, Rust, and many other languages, old and new alike, are picking up this memory model. This means that many programmers will be familiar with it and there is plenty of documentation on it.

There is also the empirical evidence that making an atomic operations API not based on the C++11 memory model leads to a complexity disaster, as described in the blog post this library is based on.

Which barrier kind should I use?

I recommend reading this memory_order C++ documentation and applying that to whatever problem you're solving. The answer to the question completely depends on what you're doing, and there's no good, generic answer.

That being said, if you're still in doubt after having read that page, you can simply use sequential consistency barriers and you should be fine in 99% of cases. Your code will end up a bit slower than necessary, but it's not by all that much.

Why are some methods missing for 8-bit and 16-bit types?

The main reason is that there are no methods in the framework for doing atomic add, subtract, or CAS on 8-bit and 16-bit types. Without CAS, add and subtract can't even be implemented with a somewhat slow CAS loop.

These operations also cannot be implemented with locks, because then all other methods for 8-bit and 16-bit data would have to synchronize on that lock, slowing everything down.

It sucks, but consider using 32-bit and 64-bit data instead.

Why are there no AND/OR/XOR methods?

The reason is similar to the lack of add and subtract operations for 8-bit and 16-bit types: The .NET Framework doesn't provide anything out of the box to do these operations.

These could be implemented with a CAS loop, at least for 32-bit and 64-bit types. This may be done in a future version of Atomique.

Why are weak CAS operations not supported?

Three reasons:

  1. There is no evidence to suggest that weak CAS is particularly useful.
  2. There is no mapping to any .NET Framework method that would have different semantics than a regular CAS.
  3. All known C++11 compare_and_exchange_weak implementations at this time just map to the regular compare_and_exchange_strong.

Why do CAS methods only specify one barrier?

In C++11, two barriers can be given to compare_and_exchange_strong. One is the barrier to use for the read-modify-write operation (if the value at the memory location compares equal to the comparand), and the other is the barrier to use for the load operation (in the case where the value at the memory location is not equal to the comparand).

There's no particular reason that the Atomique API couldn't support this, but in practice, it's easier to reason about a single memory order for the whole operation.

Why is there no signal/compiler barrier?

C++11 has the atomic_signal_fence function which does nothing at runtime, but prevents the compiler from reordering memory operations as specified by the barrier kind. The reason Atomique doesn't have this is two-fold.

First, there is no reliable way to insert a compiler barrier in C#/.NET. Some misguided libraries use the [MethodImpl(MethodImplOptions.NoOptimization)] attribute to try to get this effect, but this is not actually guaranteed to have any effect in a given CLI implementation. Worse, with the vague wording of the NoOptimization option, there's no guarantee that it will do what a compiler barrier requires.

Second, signals can only be delivered to a .NET application by using a library like Mono.Posix on POSIX systems. This library only exposes signals as events that can be waited on; signals don't actually interrupt execution at arbitrary points. This makes a compiler barrier unnecessary as asynchronous suspension of a running thread by a signal is the only reason atomic_signal_fence exists in C++11.

Why is there no consume barrier?

This one is easy: Simplicity. This is quite possibly the hardest-to-understand barrier in the C++11 memory model and anecdotal evidence suggests that it's rarely (if ever) used in real code.

Does Atomique support volatile (as in C/C++) variables?

No. This has nothing to do with atomics and therefore is not something Atomique tries to deal with.

There is a common misconception that volatile is related to atomicity. This is likely caused by Java and C# assigning a completely different meaning to the volatile keyword than C and C++ do. The Microsoft C/C++ compiler on x86 and x86-64 also treats volatile the same way that C# does.

To be clear: volatile has nothing to do with with atomics in the standard C/C++ sense. If what you want is actually atomicity and/or acquire and release memory ordering -- i.e. you want C# or Java semantics -- then Atomique fits the bill. But if what you're looking for is the C/C++ volatile which is used to inhibit compiler optimization, Atomique is not the library you need.

Why does Atomique use both Interlocked and Volatile?

As bad as it is to rely on these classes to synchronize on the same lock, it is the only option with the current atomic APIs in the .NET Framework.

The good news is that both Microsoft .NET and Mono do this.

Why are some barriers stronger than specified?

Any given operation is guaranteed to use the specified barrier, or a barrier that is strictly a superset of the specified barrier. In the latter case, the barrier is stronger than strictly necessary, which can cause slight visible slowdowns.

Overly strong barriers are a direct consequence of the limited atomics API surface of the .NET Framework.

Why does Atomique contain unsafe code?

This is necessary for some unsafe pointer casts in e.g. Atomic.Char and Atomic.UInt32. The .NET Framework is missing a lot of overloads, so casting to a different integer type (with the same size) is the only way to implement these operations.


Sane atomic operations for .NET based on the C++11 memory model.




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