I`m having two projects (x64). A. written in C (wxWidgets) B. written in c++ A compiles fine. B uses funcions of A. When I try to compile, I`m getting several errors. I suggest that the reason for the errors is the same for all, so I mention only the first: It says: strlen: identifier not found. But I have included the headers: #include <stdio.h> #include <string.h> So, currently, I have just no idea where to start to search for the reason of the errors. May it be that I have c and c++ files? May it be that I compile for x64? Thanks for help!