Fortran standard 2003 is used, mostly the iso_c_binding intrinsics, but also some other features.

Ghost is based on a collection of C structs with data and function members, and some global wrapper functions. The Fortran interface is designed in two stages.

• STAGE 1 1:1 mapping from C struct to Fortran TYPE and from global C functions to Fortran functions or subroutines. The pointer members (data and function) cannot be
  used directly in Fortran. S1 types have the exact same name as in C, e.g. ghost_vec_t. S1 types can be passed to global ghost functions.

• STAGE 2 only exists in Fortran, has Fortran pointers instead of C pointers so it gives full access
  to all functionality. Stage 2 types and the
  subroutines c2f have to be implemented only for objects for which more access should
  be available. An S2 object has a name like 'fghost_dvec_t', i.e. it casts the void pointers to the data into the correct types directly. While an S2 type does not correspond directly to a C struct, it should only consist of pointers to an actual ghost object, so that manipulations of the fghost object affect the C object directly. Each S2 type should have a member c_object pointing to the actual ghost object.

S1 interfaces are provided in ghost_types.F90 and ghost_fun.F90. they should be available for all structs relevant to the user, and have to be kept up-to-date. The compiler/linker do NOT tell you when the C structs or global functions have changed and the Fortran interface has not been adjusted! This is the task of the ghost developers.

A Stage 2 type implementation requires an interface specification for the function pointers that should be available. A start has been made in fghost.F90. It is up to the developers, which functions they want to make available through the S2 interfaces. The compiler and linker should be able to detect if an S2 interface is out-of-date, at least to some extent.

Strategy

• The basic structs ghost_vec_t, ghost_context_t etc. should be available in the S1 interface and always be kept up-to-date by the developers

• Important functionality such as ghost_createVector(...), ghost_spMVM(...) etc. should be available in the S1 interface and kept up-to-date.

• Important and stable functionality that is available through function pointers in ghost should be available in the S2 interface, for example context%gnnz(ctx)

Questions about the Fortran interface? Contact jonas.thies@dlr.de