Where is InterlockedRead?

Question

Win32 api has a set of InterlockedXXX functions to atomically and synchronously manipulate simple variables, however there doesn't seem to be any InterlockedRead function, to simply retrive the value of the variable. How come?

MSDN says that "Simple reads and writes to properly-aligned 32-bit variables are atomic operations", but adds that "However, access is not guaranteed to be synchronized", which means, as I understand it, that a simple read operation of a variable can take place while another, say, InterlockedAdd operation is in place. So why isn't there an interlocked function to read a variable?

I guess the value can be read as the result InterlockedAdd-ing zero, but that doesn't seem the right way to go.

Answer 1

The normal way of implementing this is to use a compare-exchange operation (e.g. InterlockedCompareExchange64) where both values are the same. I have a sneaking suspicion this can be performed more efficiently than an add of 0 for some reason, but I have no evidence to back this up.

Interestingly, .NET's Interlocked class didn't gain a Read method until .NET 2.0. I believe that Interlocked.Read is implemented using Interlocked.CompareExchange. (Note that the documentation for Interlocked.Read strikes me as somewhat misleading - it talks about atomicity, but not volatility, which means something very specific on .NET. I'm not sure what the Win32 memory model guarantees about visibility of newly written values from a different thread, if anything.)

Answer 2

I think that your interpretation of "not synchronized" is wrong. Simple reads are atomic, but you have to take care of reordering and memory visibility issues yourself. The former is handled by using fence instructions at appropriate places, the latter is a non-issue with read (but a potential concurrent write has to ensure proper visibility, which Interlocked functions should do if they map to LOCKED asm instructions).

Answer 3

The crux of this whole discussion is proper alignment, which is devined in Partition I of xxx, in section '12.6.2 Alignment':

Built-in datatypes shall be properly aligned, which is defined as follows:
• 1-byte, 2-byte, and 4-byte data is properly aligned when it is stored at
  a 1-byte, 2-byte, or 4-byte boundary, respectively.
• 8-byte data is properly aligned when it is stored on the same boundary
  required by the underlying hardware for atomic access to a native int.

Basically, all 32-bit values have the required alignment, and on a 64-bit platform, 64-bit values also have the required alignment.

Note though: There are attributes to explicitly alter the layout of classes in memory, which may cause you to lose this alignment. These are attributes specificially for this purpose though, so unless you have set out to alter the layout, this should not apply to you.

With that out of the way, the purpose of the Interlocked class is to provide operations that (to paraphrase) can only be observed in their 'before' or 'after' state. Interlocked operations are normally only of concern when modifying memory (typically in some non-trivial compare-exchange type way). As the MSDN article you found indicates, read operations (when properly aligned) can be considered atomic at all times without further precautions.

There are however other considerations when dealing with read operations:

• On modern CPUs, although the read may be atomic, it also may return the wrong value from a stale cache somewhere... this is where you may need to make the field 'volatile' to get the behaviour you expect
• If you are dealing with a 64-bit value on 32-bit hardware, you may need to use the Interlocked.Read operation to guarantee the whole 64-bit value is read in a single atomic operation (otherwise it may be performed as 2 separate 32-bit reads which can be from either side of a memory update)
• Re-ordering of your reads / writes may cause you to not get the value you expected; in which case some memory barrier may be needed (either explicit, or through the use of the Interlocked class operations)

Short summary; as far as atomicity goes, it is very likely that what you are doing does not need any special instruction for the read... there may however be other things you need to be careful of, depending on what exactly you are doing.