error Lnk2001: unresolved external symbol "void __cdecl FunctionA() "

Hello,
 I was trying to build a project in VC++ environment. The project includes a .CPP file and rest all .C source files. In the .CPP source file I was trying to call a C function. While building , I was facing the Linker Error:
 
   error Lnk2001: unresolved external symbol "void __cdecl FunctionA() "

Say, FunctionA() is in the C source file.
Now , I tried to have the prototype defined for visibility in the .CPP file as :

extern "C" void FunctionA(void);

Now , Iam facing the following compile error message:

     error C2732: linkage specification contradicts earlier specification

Can , this problem be solved by other means?

Regards,
Vikram