To make sure that all parameters are correctly passed to the external routines, you should declare it 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 393.
As might be expected, external variable declarations do not require any calling convention modifier.