[petsc-users] Using PETSc Data Types with Existing F90 Functions

[Kevin Mattalo]

Hello,

I am currently implementing an implicit discretization of a high-order
DG method that uses a pseudo-transient continuation globalization
procedure combined with an inexact Newton method. I am using GMRES for
the linear solves with a preconditioner constructed from the exact
analytic Jacobian.

In order to verify my Jacobian (mostly the linearization of the flux
functions since I have already verified the linearization of the local
volume terms) I was trying to implement the finite difference
approximation (SNESComputeJacobianDefault()). However, since all of my
functions for calculating the residual use intrinsic F90 arrays for
the MPI and the actual computations, I was wondering if there is a way
to extract the values from the PETSc data types (Vec, Mat etc.) and to
input the extracted values into the F90 arrays to use in my F90
functions. The reason I need to do this is because I am assuming the
ComputeJacobianDefault() function perturbs the argument x (the current
point) which is a Vec datatype. Therefore, in order to find the
residual using my existing functions (which don't use PETSc data
types) I have to copy the values in x into my existing F90 arrays in
order to use my existing functions. I tried simply equating the
vectors but I believe that only copied the address and not the actual
values within the vector.

Thanks,

Kevin