[petsc-users] Checking for NULL Objects in Fortran

[Smith, Barry F.]

   Sorry for your difficulties

   The recent branch barry/change-fortran-null was intended to simplify the handling of NULL slightly in Fortran and make it more consistent with the handling in C. It essentially changed  %v to be equal to 0 for NULL objects rather than -1. This allows easier passing objects directly to C code also.

    We did not intend to change the user API; we intended that comparisons with PETSC_NULL_DM would behave exactly as before.

    Unfortunately it looks like some places in the code had -1 hardwired, for example,

/* Definitions of Fortran Wrapper routines */
PETSC_EXTERN void PETSC_STDCALL dmplexdistribute_(DM *dm, PetscInt *overlap, PetscSF *sf, DM *dmParallel, int *ierr)
{
  CHKFORTRANNULLOBJECT(sf);
  CHKFORTRANNULLOBJECT(dmParallel);
  *ierr = DMPlexDistribute(*dm, *overlap, sf, dmParallel);
  if (dmParallel && !*dmParallel) *dmParallel = (DM)-1;
}

I did not know about this hardwiring and thus did not update them while making the new branch. 

The branch barry/change-fortran-null-fixup fixes the two bad uses of -1 that you pointed out. Please try it and let us know other problems that come up since we are planning a release very shortly.




> On Sep 10, 2018, at 4:02 PM, Fabian.Jakub <Fabian.Jakub at physik.uni-muenchen.de> wrote:
> 
> Greetings my dear PETSc list.
> 
> In my Fortran Code, I usually checked against PETSC_NULL_DM or
> PETSC_NULL_IS after calling methods such as DMPlexDistribute or
> DMGetStratumIS.
> 
> Recently this fails me.
> 
> I saw the DMPlexDistribute example in
> dm/label/examples/tutorials/ex1f90.F90
> 
> where the value of dm_distributed%v is compared against -1

    We do not recommend the usage of dm_distributed%v /= 0 and instead recommend comparing with the appropriate PETSC_NULL_XXX as you have done. (the %v business will work, we just don't recommend it).

> 
> Now, after the recent change of PETSC_NULL_DM (tDM(1) -> tDM(0) in
> 3d1df95b1) I am not 100% percent sure how to deal with it.
> 
> Could you please clarify if testing against ``-1`` is the intended way
> to do it or may this change again soonish?

    Definitely not. Your code should remain the same and we simply need to fix the stray -1 to change them to 0.

   Barry

> 
> Many Thanks,
> 
> Fabian
> 
> 
> Here some code snippets to clarify what lead me to put up this question...
> 
> This used to validate:
> 
> call DMGetStratumIS(dm, 'DomainBoundary', SIDEFACE, bc_idx, ierr)
> if (bc_ids.eq.PETSC_NULL_IS) then ! dont have domain boundary points
> ...
> endif
> 
> or
> 
> call DMPlexDistribute(dm, i0, PETSC_NULL_SF, dmdist, ierr)
> if (dmdist.eq.PETSC_NULL_DM) then ! DM was not distributed
> endif
> 
> 
> Now I could do something like:
> 
> if(bc_ids%v.eq.-1)
> 
> of (dmdist%v.eq.-1)
> 
> ...
>