On Fri, Jan 2, 2026 at 2:48 PM Michael Whitten via petsc-users <
[email protected]> wrote:
> Hi PETSc mailing list users,
>
> I have managed to install PETSc and run some PETSc examples and little
> test codes of my own in Fortran. I am now trying to make PETSc work with my
> existing Fortran code. I have tried to build little test examples of the
> functionality that I can then incorporate into my larger code base.
> However, I am having trouble just passing vectors back and forth between
> PETSc and Fortran.
>
> I have attached a minimum semi-working example that can be compiled with
> the standard 'Makefile.user'. It throws an error when I try to copy the
> PETSc vector back to a Fortran vector using VecGetValues(). I get that it
> can only access values of the array on the local process but how do I fix
> this? Is this even the right approach?
>
No. You should just call VecGetArray(), to get back an F90 pointer to the
values. This is much more convenient.
> In the final implementation I want to be able to assemble my matrix and
> vector, convert them to PETSc data structures, use PETSc to solve, and then
> convert the solution vector back to Fortran and return. I want to be able
> to do this with both the linear and nonlinear solvers. It seems like this
> is what PETSc is, in part, built to do. Is this a reasonable expectation to
> achieve? Is this a reasonable use case for PETSc?
>
Yes, that should work getting the array directly.
Thanks,
Matt
> Thanks in advance for any help you can offer.
>
> best,
> Michael
>
--
What most experimenters take for granted before they begin their
experiments is infinitely more interesting than any results to which their
experiments lead.
-- Norbert Wiener
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