LabVIEW
Call library function not finding DLL in directory where my LLBs are
Solved!
By a function dispatch structure do you mean a table of function pointers gotten by GetProcAddress, one such entry for each function?
Yes this is the idea.
I think I understand what the wrapper functions need to do. But I am missing some pieces for the initialize function. It would have to do LoadLibrary and lots of GetProcAddresses -- that much I get. But I still don't understand who decides which DLL. Maybe you mean that the wrapper DLL's initialize function will get the DLL name and path from the registry, load it, try a different path if the load fails, etc. Is that what you mean? That could work very nicely. But from where would it get the registry key? From the VI? This is the part that would clearly be different for each product. I suppose that you had something in mind when you described your solution.
Well somewhere at some point you need to decide for which product and which DLL you want to call this interface. One possibility is that you have an Intialize function that accepts a string parameter that identifies the product to call and it then internally resolves that string to the correct DLL name, looking for that based on some heuristics, registry settings or whatever and then loading it with load library and filling in the function dispatch table. You can even add other information to the structure that contains this function table such as version information, instrument class, magic ID to be able to verify in the other fucntion that you got a valid structure etc.The pointer will be returned as a simple pointer sized integer being an opaque handle to the caller.
Every function then takes that handle casts it to the structure, verifies its correct version, etc and then indexes the correct function pointer checking if it is not NULL (can allow for minimalistic product drivers that may not implement every function) and then calls it.
Rolf Kalbermatter
