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I see nothing in your code you posted that is NOT valid C code; but, you still need to include the needed header files!

Tim S.

Look at the header file that should be used with the DLL; but, based on you code you are not using any header files.
Edit: If his code example used extern "C" then likely it is a C DLL; otherwise...

Was the DLL written in C or in C++?
If it was written in C++; calling it from C may be impossible to very hard to do.

Tim S.