Example

As an example of this in practice, for the case of Fortran code, consider a thorn A, which wants to gather terms for a calculation from any thorn that wishes to provide them. Thorn A could have the lines in its source code

c Get source code from other thorns
      allterms = 0d0
#include "AllSources.inc"

and would then add to interface.ccl the line

USES INCLUDE SOURCE: AllSources.inc

If thorn B wants to add terms for the calculation, it would create a file, say Bterms.inc with the lines

c Add this to AllSources.inc
      allterms = allterms + 1d0

and would add to its own interface.ccl

INCLUDE SOURCE: Bterms.inc in AllSources.inc

The final file for thorn A which is compiled, will contain the code

c Get source code from other thorns
      allterms = 0d0
      if (CCTK_IsThornActive("B").ne.0) then
c Add this to AllSources.inc
        allterms = allterms + 1d0
      end if

Any Fortran thorn routines which include source code must include the declaration DECLARE_CCTK_FUNCTIONS.