This function registers an implementation of the IMallocSpy interface in OLE, thereafter requiring OLE to call its wrapper methods around every call to the corresponding IMalloc method. IMallocSpy is defined in OLE to allow developers to debug memory allocations.
The CoRegisterMallocSpy function registers the IMallocSpy object, which is used to debug calls to IMalloc methods.
The function calls QueryInterface on the pointer pMallocSpy for the interface IID_IMallocSpy. This is to ensure that pMallocSpy really points to an implementation of IMallocSpy.
By the rules of OLE, it is expected that a successful call to QueryInterface has added a reference (through the AddRef method) to the IMallocSpy object. That is, CoRegisterMallocSpy does not directly call AddRef on pMallocSpy, but fully expects that the QueryInterface call will.
When the IMallocSpy object is registered, whenever there is a call to one of the IMalloc methods, OLE first calls the corresponding IMallocSpy pre-method. Then, after executing the IMalloc method, OLE calls the corresponding IMallocSpy post-method.
For example, whenever there is a call to IMalloc::Alloc, from whatever source, OLE calls IMallocSpy::PreAlloc, calls IMalloc::Alloc, and after that allocation is completed, calls IMallocSpy::PostAlloc.
To determine whether the platform supports this function, see Determining Supported COM APIs.