[petsc-users] PETSc MatMFFDSetFunction Function

[Gaetan Kenway]

Hello

I was wondering what the user supplied function is supposed to look like for
setting the function in MatMFFDSetFunction. I am trying to use a Matrix-Free
Matrix for a linear Krylov Solver. The website says:

PetscErrorCode PETSCMAT_DLLEXPORT MatMFFDSetFunction(Mat
mat,PetscErrorCode (*func)(void*,Vec,Vec),void *funcctx)


This indicates the function should have the calling sequence: (void
*,Vec,Vec). Since there are zero examples of actually using this function,
what exactly is the sequence? I gather that the second and third arguments
are  Vec x and Vec y where x is the input and y is the output, but what is
the void * supposed to be.

I'm doing this in Fortran, so I really don't know what argument "void *"
should correspond to?

Currently my code looks like this:

! Setup Matrix-Free dRdw matrix
call MatCreateMFFD(sumb_comm_world,nDimW,nDimW,&
       PETSC_DETERMINE,PETSC_DETERMINE,dRdw,ierr)

call MatMFFDSetFunction(dRdw,FormFunction2,ctx,ierr)
call MatAssemblyBegin(dRdw,MAT_FINAL_ASSEMBLY,ierr)
call MatAssemblyEnd(dRdw,MAT_FINAL_ASSEMBLY,ierr)

The function prototype for FormFunction2 is:

subroutine FormFunction2(mfmat,wVec,rVec,ierr)
  Mat     mfmat
  Vec     wVec, rVec
  PetscInt ierr
end subroutine FormFunction2

When I try to use this in a KSP linear solve I get the following tracsback:

[0]PETSC ERROR:
------------------------------------------------------------------------
[0]PETSC ERROR: Caught signal number 11 SEGV: Segmentation Violation,
probably memory access out of range
[0]PETSC ERROR: Try option -start_in_debugger or -on_error_attach_debugger
[0]PETSC ERROR: or see
http://www.mcs.anl.gov/petsc/petsc-as/documentation/troubleshooting.html#Signal[0]PETSCERROR:
or try
http://valgrind.org on GNU/linux and Apple Mac OS X to find memory
corruption errors
[0]PETSC ERROR: likely location of problem given in stack below
[0]PETSC ERROR: ---------------------  Stack Frames
------------------------------------
[0]PETSC ERROR: Note: The EXACT line numbers in the stack are not available,
[0]PETSC ERROR:       INSTEAD the line number of the start of the function
[0]PETSC ERROR:       is given.
[0]PETSC ERROR: [0] MatMult line 1877 src/mat/interface/matrix.c
[0]PETSC ERROR: [0] PCApplyBAorAB line 540 src/ksp/pc/interface/precon.c
[0]PETSC ERROR: [0] GMREScycle line 132 src/ksp/ksp/impls/gmres/gmres.c
[0]PETSC ERROR: [0] KSPSolve_GMRES line 227 src/ksp/ksp/impls/gmres/gmres.c

If I use KSPPREONLY it works fine.

Thank you,

Gaetan
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