Question

I want to move my caching library to a DLL and allow multiple applications to share a single pointer allocated within the DLL using GlobalAlloc(). How could I accomplish this, and would it result in a significant performance decrease?

Answer 1

You could certainly do this and there won’t be any performance implication for a single pointer.

Rather than use GlobalAlloc, a legacy API, you should opt for a different shared heap. For example the simplest to use is the COM allocator, CoTaskMemAlloc. Or you can use HeapAlloc passing the process heap obtained by GetProcessHeap.

For example, and neglecting to show error checking:

void *mem = HeapAlloc(GetProcessHeap(), HEAP_ZERO_MEMORY, size);

Note that you only need to worry about heap sharing if you expect the memory to be deallocated in a different module from where it was created. If your DLL both creates and destroys the memory then you can use plain old malloc. Because all modules live in the same process address space, memory allocated by any module in that process, can be used by any other module.

Update

I failed on first reading of the question to pick up on the possibility that you may be wanting multiple process to have access to the same memory. If that’s what you need then it is only possible with memory mapped files, or perhaps with some form of IPC.