The standard form of C name mangling (for the __cdecl ("C Calling Convention") calling convention, which is usually the default, unless you changed your project settings) is simply to prepend an underscore.

Interoperability between C and C++ means that the names sometimes get a bit messed up, so nobody knows what works and what doesn't. Basically, just use whichever one works - it'll resolve to the same code in the library.