I hope that I am replying in the correct manner to respond to the "Having
trouble with basic Fortran-PETSc interoperability" thread I previously started.
If not, please correct me.
Thank you everyone for your replies, they were very helpful.
I have what appears to be a working example code using the suggested updates,
specifically the VecGetArray() and VecRestoreArray() and getting the sequential
vector back from PETSc using the information from the FAQ. <<Example code
attached>>
I have a question about this example code to make sure I am writing reasonably
efficient code. It seems like I have to create an additional PETSc vector
'out_seq' which will essentially be a copy of the PETSc vector 'v1p' which is
not the most efficient use of memory. It also seems to me like there isn't a
way around this additional 'out_seq' vector because there needs a place to
aggregate the data from the various processes. Is this a reasonable use of
PETSc or is there a more efficient way? Note, I am trying to interface my
existing code base with PETSc to use the solvers and this may be the
performance trade-off for not developing my program fully within the PETSc
ecosystem.
I have another question only tangentially related to this topic. Should I ask
it as part of this thread or create a new topic?
Michael
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To: [email protected] <[email protected]>
Subject: petsc-users Digest, Vol 205, Issue 2
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Today's Topics:
1. Re: Having trouble with basic Fortran-PETSc interoperability
(Barry Smith)
2. Re: Having trouble with basic Fortran-PETSc interoperability
(Matthew Knepley)
----------------------------------------------------------------------
Message: 1
Date: Fri, 2 Jan 2026 16:33:27 -0500
From: Barry Smith <[email protected]>
To: Michael Whitten <[email protected]>
Cc: "[email protected]" <[email protected]>
Subject: Re: [petsc-users] Having trouble with basic Fortran-PETSc
interoperability
Message-ID: <[email protected]>
Content-Type: text/plain; charset="utf-8"
VecGetValues() uses 0 based indexing in both Fortran and C.
You don't want to use VecGetValues() and VecSetValues() usually since they
result in two copies of the arrays and copying entire arrays back and forth.
You can avoid copying between PETSc vectors and your arrays by using
VecGetArray(), VecGetArrayWrite(), and VecGetArrayRead(). You can also use
VecCreateMPIWithArray() to create a PETSc vector using your array; for example
for input to the right hand side of a KSP. These arrays start their indexing
with the Fortran default of 1.
Barry
> On Jan 2, 2026, at 2:42?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?
>
> 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?
>
> Thanks in advance for any help you can offer.
>
> best,
> Michael
> <test.F90>
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------------------------------
Message: 2
Date: Fri, 2 Jan 2026 17:49:30 -0500
From: Matthew Knepley <[email protected]>
To: Michael Whitten <[email protected]>
Cc: "[email protected]" <[email protected]>
Subject: Re: [petsc-users] Having trouble with basic Fortran-PETSc
interoperability
Message-ID:
<CAMYG4GmKRM1=cok6jwbo6itwvpkksepeoh1q1cllkgr2qhk...@mail.gmail.com>
Content-Type: text/plain; charset="utf-8"
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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*******************************************
program main
#include <petsc/finclude/petscksp.h>
use petscvec
implicit none
PetscErrorCode ierr
PetscInt i
Vec v1p
PetscScalar, pointer :: v1ptr(:), v2ptr(:)
Vec out_seq
VecScatter ctx
integer :: n
real(8), allocatable, target :: v1(:), v2(:)
n = 10
allocate(v1(n),v2(n),source=0.0d0)
v1 = 1.0d0
PetscCallA(PetscInitialize(ierr))
!Create PETSc vector
PetscCallA(VecCreate(PETSC_COMM_WORLD,v1p,ierr))
PetscCallA(VecSetSizes(v1p,PETSC_DECIDE,n,ierr))
PetscCallA(VecSetFromOptions(v1p,ierr))
!Pass Fortran Vector to PETSc using VecGetArray
!Saves memory, only passing a pointer instead of a copy
!of the whole array
PetscCallA(VecGetArray(v1p,v1ptr,ierr))
v1ptr = v1
PetscCallA(VecRestoreArray(v1p,v1ptr,ierr))
!Send PETSc vector to each processor
PetscCallA(VecAssemblyBegin(v1p,ierr))
PetscCallA(VecAssemblyEnd(v1p,ierr))
!Call PETSc operations, e.g. solver, etc.
!Print PETSc vector, debugging only
PetscCallA(VecView(v1p,PETSC_VIEWER_STDOUT_WORLD,ierr))
!Create sequential PETSc vector from parallel processes
PetscCallA(VecScatterCreateToAll(v1p,ctx,out_seq,ierr))
PetscCallA(VecScatterBegin(ctx,v1p,out_seq,INSERT_VALUES,SCATTER_FORWARD,ierr))
PetscCallA(VecScatterEnd(ctx,v1p,out_seq,INSERT_VALUES,SCATTER_FORWARD,ierr))
PetscCallA(VecScatterDestroy(ctx,ierr))
!Get sequential vector back from PETSc
!Using separate array v2 and pointer v2ptr
!here to insure data is being moved from
!PETSc vector to Fortran array
PetscCallA(VecGetArray(out_seq,v2ptr,ierr))
v2 = v2ptr
PetscCallA(VecRestoreArray(out_seq,v2ptr,ierr))
!Print Fortran vector
print *, "v2: ", v2
!PETSc clean up
!PetscCallA(VecDestroy(v1p,ierr))
!PetscCallA(VecDestroy(out_seq,ierr))
PetscCallA(PetscFinalize(ierr))
end program main
