[petsc-users] PCMGSetResidual and fortran

[Steven Dargaville]

Hi all

I have a quick question regarding the use of PCMGSetResidual within fortran
code. I'm calling PCMGSetResidual from within fortran:

call PCMGSetResidual(pc_mg, petsc_level, mg_residual, coarse_matrix ierr)

and just for testing purposes I've written a trivial residual evaluation
routine:

    subroutine mg_residual(mat, b, x, r, ierr)
      !!< Compute the residual

      ! ~~~~~~
      type(tMat) :: mat
      type(tVec) :: b, x, r
      PetscErrorCode :: ierr
      ! ~~~~~~

      print *, "inside residual evaluation"
      call MatResidual(mat, b, x, r, ierr)

   end subroutine

The problem I am having is that this segfaults when the residual routine is
called. Valgrind shows that it is failing in the fortran interface in
ftn-custom/zmgfuncf.c, with the message:

==24742== Invalid read of size 8
==24742==    at 0x5B7CBC0: ourresidualfunction (in
/home/projects/dependencies/petsc_main/arch-linux-c-opt/lib/libpetsc.so.3.015.0)
==24742==    by 0x5B6D804: PCMGMCycle_Private (in
/home/projects/dependencies/petsc_main/arch-linux-c-opt/lib/libpetsc.so.3.015.0)

==24742== Process terminating with default action of signal 11 (SIGSEGV)
==24742==  Access not within mapped region at address 0x0
==24742==    at 0x5B7CBC0: ourresidualfunction (in
/home/sdargavi/projects/dependencies/petsc_main/arch-linux-c-opt/lib/libpetsc.so.3.015.0)
==24742==    by 0x5B6D804: PCMGMCycle_Private (in
/home/sdargavi/projects/dependencies/petsc_main/arch-linux-c-opt/lib/libpetsc.so.3.015.0)

I'm guessing this is because the fortran_func_pointers isn't pointing to
the mg_residual routine, but I am not sure why. I noticed that in the C
code of PCMGSetResidual that it calls MatDestroy on the A matrix in
mg_levels and replaces it with the mat passed in:

if (mat) PetscObjectReference((PetscObject)mat);
MatDestroy(&mglevels[l]->A);
mglevels[l]->A = mat

so I modified my code to call PCMGSetResidual either before the operators
are set, or after but passing in an extra copy, but this doesn't seem to
help.

I'm guessing I'm doing something silly, but just wondering if anyone had
any ideas? Thanks for your help
Steven
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