[petsc-users] Fortran external procedure in ctx()

[Gaetan Kenway]

Hello

I'm wondering if it is possible to put an external procedure reference in a
ctx() in fortran.

 I'm in the process of writing a Newton--Krylov solver for an
aero-structural system. My two different codes are wrapped with python and
so each code is called through python for residual and preconditioning
operations. Nominally this would be a good use of petsc4py but it doesn't
allow for PCShell so it is no use to me.

I then wrote up the solver in Fortran and attempted to use callbacks to
python for computing the required information. Using f2py, I can pass my two
call back functions cb1 and cb2 fortran. A schematic of the code is below:

subroutine solver(cb1, cb2)

   ! cb1 and cb2 are python callbacks set using f2py
   external cb1,cb2
   petscFortranAddress ctx(2)

   ! I would like to do the following, but this doesn't compile
   ! ctx(1) = cb1
   ! ctx(2) = cb2

  call SNESCreate(comm,snes,ierr)
  call SNESSetFunction(snes,resVec,FormFunction,ctx,ierr)

  call KSPGetPC(ksp,pc,ierr)
  call PCSetType(pc,PCSHELL,ierr)

  call PCShellSetContext(pc,ctx,ierr)
  call PCShellSetApply(pc,applyaspc_fortran,ierr)

end subroutine solver

subroutine applyaspc_fortran(pc,inputVec,outputVec,ierr)

  PC      pc
  Vec     inputVec,outputVec
  PetscFortranAddress ctx(2)
  external func

  call PCShellGetContext(pc,ctx,ierr)
  func = ctx(2)

  call VecGetArrayF90(inputVec,states_in,ierr)
  call VecGetArrayF90(outputVec,states_out,ierr)

  ! Call the callback to python
  call func(states_in,states_out,shape(states_in))

  call VecRestoreArrayF90(inputVec,states_in,ierr)
  call VecRestoreArrayF90(outputVec,states_out,ierr)
end subroutine applyaspc_fortran


 In general, in Fortran, is it possible to put an external function
reference in a module such that I wouldn't have to try to pass it through
the application ctx? I realize this may be impossible to do in Fortran.
Would such a procedure be possible in C? I'm only using Fortran since I'm
much more familiar with it then with C.

Sorry there isn't much to go on, but any suggestions would be greatly
appreciated.

Gaetan
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