The system and the app C runtimes are one and the same in Win10+.
As far as cross-DLL interop: the usual solution was to avoid the C stdlib altogether, and just use the underlying Win32 API functions to manage memory that has to cross the boundary. Or, in the COM land, every object manages its own heap memory, exposing it via the ABI-standard IUnknown::Release.
As far as cross-DLL interop: the usual solution was to avoid the C stdlib altogether, and just use the underlying Win32 API functions to manage memory that has to cross the boundary. Or, in the COM land, every object manages its own heap memory, exposing it via the ABI-standard IUnknown::Release.