7.1.3 Declaring the calling convention modifier

To make sure that all parameters are correctly passed to the external routines, you should declare them with the correct calling convention modifier. When linking with code blocks compiled with standard C compilers (such as GCC), the cdecl modifier should be used so as to indicate that the external routine uses C type calling conventions. For more information on the supported calling conventions, see section 6.3, page 406.

As might be expected, external variable declarations do not require any calling convention modifier.