Usually the lib library only is the import library for the DLL. C can't directly and magically call DLL functions. Somehow it needs to know in what DLL they are and how to load them into memory. This can be either done explicitly by calling LoadLibrary() and GetProcAddress() or by linking the import library that came with the DLL. The import library is simply a precompiled lib file that does all the LoadLibrary() and GetProcAddress() stuff already. In LabVIEW you can't link in a lib library but have to use a Call Library Node, which is the LabVIEW interface to LoadLibrary() and GetProcAddress().