Standard include files declare C run-time library functions as they are implemented in the libraries. If you use the Full Optimization (/Ox) or fastcall Calling Convention (/Gr) option, the compiler assumes that all functions should be called using the register calling convention. The run-time library functions were compiled using the C calling convention, and the declarations in the standard include files tell the compiler to generate correct external references to these functions.

Multithreading with C and Win32