The calling convention depends on how the dll was built and the correct convention for various functions should be specified in the documentation of the dll, and it often varies based on what language the dll/function was compiled in. C will normally correspond to C function calls and stdcall will normally correspond to Windows API function calls. See the link below for more information on calling conventions.
x86 Calling Conventions
Ryan K.
Product Manager, ATCA and BEEcube
National Instruments
