[petsc-users] MatCreateShell, MatShellGetContext, MatShellSetContext in fortran

[Samuel Lanthaler]

Dear petsc-/slepc-users,

I have been trying to understand matrix-free/shell matrices in PETSc for 
eventual use in solving a non-linear eigenvalue problem using SLEPC. But 
I seem to be having trouble with calls to MatShellGetContext. As far as 
I understand, this function should initialize a pointer (second 
argument) so that the subroutine output will point to the context 
associated with my shell-matrix (let's say of TYPE(MatCtx))? When 
calling that subroutine, I get the following error message:

[0]PETSC ERROR: --------------------- Error Message 
--------------------------------------------------------------
[0]PETSC ERROR: Null argument, when expecting valid pointer
[0]PETSC ERROR: Null Pointer: Parameter # 2

In my code, the second input argument to the routine is a null-pointer 
of TYPE(MatCtx),POINTER :: arg2. Which the error message appears to be 
unhappy with. I noticed that there is no error message if I instead pass 
an object TYPE(MatCtx) :: arg2 to the routine... which doesn't really 
make sense to me? Could someone maybe explain to me what is going on, here?

Just in case, let me also attach my concrete example code (it is 
supposed to be a Fortran version of the slepc-example in 
slepc-3.8.1/src/nep/examples/tutorials/ex21.c). I have added an extra 
call to MatShellGetContext on line 138, after the function and jacobian 
should supposedly have been set up.

Thanks a lot for your help!

Cheers,

Samuel

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