[petsc-users] Problem with KSPGetPC within FieldSplit context

[Matthew Knepley]

On Tue, Mar 24, 2026 at 2:31 PM Lucia Barandiaran via petsc-users <
petsc-users at mcs.anl.gov> wrote:

> Dear PETSc community,
>
> We recently updated PETSc libraries and came across with a problem with
> FieldSplit preconditioning (our code is implemented in Fortran 90).
>
Hi Lucia,

It looks like no one has answered, so I will try.

I found this example doing what you want:

  https://urldefense.us/v3/__https://petsc.org/main/src/ksp/ksp/tutorials/ex7f.F90.html__;!!G_uCfscf7eWS!ffhItiXPRGqL12Cm8zfgA3cJ-lq0wTR-Ps_Yfj0WcTtLylJ7w1bKSGfj7gfbUC-DkI-SOf_Sn1OyZD0ylKex$ 

It seems that you need to initialize the pointer to null, and also the
array starts at 1.

Can you run the example?

  Thanks,

     Matt


> In the older version, when calling PCFieldSplitGetSubKSP we declared the
> subksp array as KSP and it worked perfectly. However, now we must declare
> it as a Fortran pointer and allocate it before
> calling PCFieldSplitGetSubKSP otherwise the code won't compile...
>
> With the new modification, that part of the code is the following:
>
> ****Variable declarations
>
>       PetscInt              ::  n_splits_retrieved
>       IS                    ::  IS_field(max_field)
>       KSP, pointer :: subksp_array(:)
>       PC                    ::  subpc(max_field)
>       INTEGER(iwp)          ::  nfield, ifield
>
> **** FieldSplit Part ****
>
>    !    Allocate Fortran array to hold the KSP contexts
>         ALLOCATE(subksp_array(n_splits_retrieved))
>
>    !    Second call to get the actual KSP contexts
>         CALL PCFieldSplitGetSubKSP(solver%pc, n_splits_retrieved, &
>                                    subksp_array, ierr)
>
>         IF (n_splits_retrieved >= 2) THEN
>           DO ifield = 1, n_splits_retrieved
>
>           ! Configure the KSP for the velocity block (field 0)
>             CALL KSPGetPC(subksp_array(ifield), subpc(ifield), ierr)
>             CALL KSPSetType(subksp_array(ifield), KSPGMRES, ierr)
>             CALL PCSetType(subpc(ifield), PCJACOBI, ierr)
>           ! Allows runtime options for sub-KSP
>             CALL KSPSetFromOptions(subksp_array(ifield), ierr)
>           ! Choose a viewer (stdout for example)
> !           viewer = PETSC_VIEWER_STDOUT_WORLD
> !           CALL PCView(subpc(ifield),viewer,ierr)
>           END DO
>         END IF
>
> When debugging the fortran code we observed that it stops with an error
> when calling KSPGetPC  and we can't figure out what are we doing wrong...
> Maybe we need an interface when calling that function?
>
> Thanks very much for your time,
>
> Sincerely,
>
> Lucía Barandiarán
>
> Scientific software developer  - Dracsys
>
> Collaborator at MECMAT group - Universitat Politècnica de Catalunya (UPC)
>


-- 
What most experimenters take for granted before they begin their
experiments is infinitely more interesting than any results to which their
experiments lead.
-- Norbert Wiener

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