[petsc-users] Having trouble with basic Fortran-PETSc interoperability

Fri Jan 16 11:18:00 CST 2026

I realized the subject line was incorrect so I fixed it.

I want use SNES for sure and possibly TS and Tao. I would also like to be able to use the matrix-vector multiplication in a similar way. Is there similar guidance for SNES (TS and Tao) as there is for KSP somewhere?

I had started reading the PETSc user manual while simultaneously experimenting with some code and hadn't made it to that section. It's a large ecosystem and I am trying to get oriented. Your help is much appreciated.

Michael

________________________________
From: Barry Smith <bsmith at petsc.dev>
Sent: Friday, January 16, 2026 11:38 AM
To: Michael Whitten <mwhitte6 at jhu.edu>
Cc: petsc-users at mcs.anl.gov <petsc-users at mcs.anl.gov>
Subject: Re: [petsc-users] petsc-users Digest, Vol 205, Issue 12

      External Email - Use Caution

  Michael,

     Yes, I misunderstood the question.

     Are you specifically wanting to use PETSc's KSP linear solver in parallel while your code runs sequentially (and you do not plan to use SNES or TS or Tao)? If so then we have code just for that purpose, explained in the section

Using PETSc's MPI parallel linear solvers from a non-MPI program

in the users manual available at petsc.org<https://urldefense.us/v3/__http://petsc.org/__;!!G_uCfscf7eWS!fz7gu-ydCmGQL5TnY6D1UYsCtPN06PH7qvMAmLG7pfOYZPc2OyKY_uIne_Yau1PHu2cOo9BZ_hNO2c3-h6blFkFi$ >

The PETSc code manages all the parallelism and spreading out of the matrix and vectors to the multiple processes. You just write a sequential program that uses KSP and calls KSPSolve(), make sure it works correctly and then add the options listed in the users manual to run the linear solve in parallel.

If you want to use SNES or TS it will require you to manage moving the vectors and matrices in your code from sequential to parallel using PETSc calls. But again you would just get your code running correctly with PETSc sequentially and then add some extra boilerplate to use the PETSc parallel solvers with it.

  Barry

On Jan 16, 2026, at 10:03 AM, Michael Whitten via petsc-users <petsc-users at mcs.anl.gov> wrote:

Both of your points elude to my other question so I am going to ask it in this thread.

Is it possible/recommended/reasonable to run my current code sequentially, call a function that interfaces with the PETSc solver which runs parallel, and return a sequential vector to my code?

  1.
I understand your point about the additional memory usage. However, I am imagining v1 as my Fortran vector passed into a function. It is possible that my current plan is quite inefficient as I am becoming more aware of how much of a novice I am in parallel computing.
  2.
I don't need PETSc's entire parallel vector on each process. This was not my understanding of that part of the code. I need the entire vector returned to my sequential code.

I think I am trying to do something intermediate between completely sequential and completely parallel.

Michael
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Today's Topics:

   1. Re:  petsc-users Digest, Vol 205, Issue 2 (Barry Smith)

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Message: 1
Date: Fri, 16 Jan 2026 08:51:29 -0600
From: Barry Smith <bsmith at petsc.dev<mailto:bsmith at petsc.dev>>
To: Michael Whitten <mwhitte6 at jhu.edu<mailto:mwhitte6 at jhu.edu>>
Cc: "petsc-users at mcs.anl.gov<mailto:petsc-users at mcs.anl.gov>" <petsc-users at mcs.anl.gov<mailto:petsc-users at mcs.anl.gov>>
Subject: Re: [petsc-users] petsc-users Digest, Vol 205, Issue 2
Message-ID: <B9AF4754-FD0B-49BC-8A34-4D3F51F5C7FC at petsc.dev<mailto:B9AF4754-FD0B-49BC-8A34-4D3F51F5C7FC at petsc.dev>>
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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 <petsc-users at mcs.anl.gov<mailto:petsc-users at mcs.anl.gov>> 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 <petsc-users-bounces at mcs.anl.gov<mailto:petsc-users-bounces at mcs.anl.gov> <mailto:petsc-users-bounces at mcs.anl.gov>> on behalf of petsc-users-request at mcs.anl.gov<mailto:petsc-users-request at mcs.anl.gov><mailto:petsc-users-request at mcs.anl.gov> <petsc-users-request at mcs.anl.gov<mailto:petsc-users-request at mcs.anl.gov> <mailto:petsc-users-request at mcs.anl.gov>>
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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 <bsmith at petsc.dev<mailto:bsmith at petsc.dev> <mailto:bsmith at petsc.dev>>
> To: Michael Whitten <mwhitte6 at jhu.edu<mailto:mwhitte6 at jhu.edu> <mailto:mwhitte6 at jhu.edu>>
> Cc: "petsc-users at mcs.anl.gov<mailto:petsc-users at mcs.anl.gov> <mailto:petsc-users at mcs.anl.gov>" <petsc-users at mcs.anl.gov<mailto:petsc-users at mcs.anl.gov> <mailto:petsc-users at mcs.anl.gov>>
> Subject: Re: [petsc-users] Having trouble with basic Fortran-PETSc
>         interoperability
> Message-ID: <3B11AFD2-F767-4D6A-9CB6-D8D153BEA27E at petsc.dev<mailto:3B11AFD2-F767-4D6A-9CB6-D8D153BEA27E at petsc.dev> <mailto:3B11AFD2-F767-4D6A-9CB6-D8D153BEA27E at petsc.dev>>
> 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 <petsc-users at mcs.anl.gov<mailto:petsc-users at mcs.anl.gov> <mailto:petsc-users at mcs.anl.gov>> 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 <knepley at gmail.com<mailto:knepley at gmail.com> <mailto:knepley at gmail.com>>
> To: Michael Whitten <mwhitte6 at jhu.edu<mailto:mwhitte6 at jhu.edu> <mailto:mwhitte6 at jhu.edu>>
> Cc: "petsc-users at mcs.anl.gov<mailto:petsc-users at mcs.anl.gov> <mailto:petsc-users at mcs.anl.gov>" <petsc-users at mcs.anl.gov<mailto:petsc-users at mcs.anl.gov> <mailto:petsc-users at mcs.anl.gov>>
> Subject: Re: [petsc-users] Having trouble with basic Fortran-PETSc
>         interoperability
> Message-ID:
>         <CAMYG4GmKRM1=CoK6JWbo6iTWVPKksepEOH1q1CLLKgr2QhKKDw at mail.gmail.com<mailto:CAMYG4GmKRM1=CoK6JWbo6iTWVPKksepEOH1q1CLLKgr2QhKKDw at mail.gmail.com><mailto:CAMYG4GmKRM1=CoK6JWbo6iTWVPKksepEOH1q1CLLKgr2QhKKDw at mail.gmail.com>>
> Content-Type: text/plain; charset="utf-8"
>
> On Fri, Jan 2, 2026 at 2:48?PM Michael Whitten via petsc-users <
> petsc-users at mcs.anl.gov<mailto:petsc-users at mcs.anl.gov> <mailto:petsc-users at mcs.anl.gov>> 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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> ------------------------------
>
> End of petsc-users Digest, Vol 205, Issue 2
> *******************************************
> <test2.F90>

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Subject: Digest Footer

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End of petsc-users Digest, Vol 205, Issue 12
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