Why syncblk is located at -4 and not at 0?

So if you want to look at sync block for an object, under sos you have to look at -4 bytes (on 32 bit machines) before the object address. Does anyone know what is the wisdom for going back 4 bytes? I mean they could have sync block at 0, then type handle at +4 and th开发者_StackOverflow中文版en object fields at +8.

This is an implementation detail, so I can't give you the exact reason for the placement of the syncblock. However, if you look at the shared source CLI, you'll see that the runtime has all sorts of optimizations for how objects are allocated and used, and actually the data associated with a single instance is located in several different places. The syncblock for instance is just an index value for a structure located elsewhere. Similarly the MethodTable and the EEClass are stored elsewhere. These are all implementation details. The important point IMO is understanding how to dig out the information needed during debugging. It is of much less importance to understand why the implementation details are as they are.

I'd say it matches expectations, especially for structs that have been explicitly laid out. As Brian says, it's just an implementation detail though. It's similar to how many implementations of malloc will allocate more space than requested, store the allocation size in the first four (or eight) bytes, and then return a pointer that is offset to point to the next byte beyond that.