1) For the code
v1 = 1.0d0
....
PetscCallA(VecCreate(PETSC_COMM_WORLD,v1p,ierr))
...
PetscCallA(VecGetArray(v1p,v1ptr,ierr))
v1ptr = v1
PetscCallA(VecRestoreArray(v1p,v1ptr,ierr))
This produces two copies of the array plus time to copy values over. Instead
drop the v1 completely and just use
PetscCallA(VecGetArray(v1p,v1ptr,ierr))
v1ptr = 1.0d0 ! or fill it up with a loop etc
PetscCallA(VecRestoreArray(v1p,v1ptr,ierr))
2) If you want/need PETSc's entire parallel vector on each process then you
have no choice but to use the code you wrote with VecScatter. Normally when
writing a new MPI code from scratch one designs it so the entire vector is
never needed together on a single process (because that is not scalable) but if
you have a current code you need to work with this is an ok way to start.
Barry
> On Jan 16, 2026, at 8:40 AM, Michael Whitten via petsc-users
> <[email protected]> wrote:
>
> 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
> From: petsc-users <[email protected]
> <mailto:[email protected]>> on behalf of
> [email protected] <mailto:[email protected]>
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> Sent: Saturday, January 3, 2026 1:00 PM
> To: [email protected] <mailto:[email protected]>
> <[email protected] <mailto:[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] <mailto:[email protected]>>
> To: Michael Whitten <[email protected] <mailto:[email protected]>>
> Cc: "[email protected] <mailto:[email protected]>"
> <[email protected] <mailto:[email protected]>>
> Subject: Re: [petsc-users] Having trouble with basic Fortran-PETSc
> interoperability
> Message-ID: <[email protected]
> <mailto:[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] <mailto:[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] <mailto:[email protected]>>
> To: Michael Whitten <[email protected] <mailto:[email protected]>>
> Cc: "[email protected] <mailto:[email protected]>"
> <[email protected] <mailto:[email protected]>>
> Subject: Re: [petsc-users] Having trouble with basic Fortran-PETSc
> interoperability
> Message-ID:
> <CAMYG4GmKRM1=cok6jwbo6itwvpkksepeoh1q1cllkgr2qhk...@mail.gmail.com
> <mailto: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] <mailto:[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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