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# PETSc for Fortran Users

Make sure the suffix of your Fortran files is .F90, not .f or .f90.

## Basic Fortran API Differences

### Modules and Include Files

You must use both PETSc include files and modules. At the beginning of every function and module definition you need something like

    #include "petsc/finclude/petscXXX.h"
             use petscXXX

The Fortran include files for PETSc are located in the directory `$PETSC_DIR/include/petsc/finclude` and the module files are located in `$PETSC_DIR/$PETSC_ARCH/include`

### Declaring PETSc Object Variables

You can declare PETSc object variables using either of the following:

    XXX variablename

    type(tXXX) variablename

For example,

    #include "petsc/finclude/petscvec.h"
             use petscvec

        Vec b
        type(tVec) x

PETSc types like `PetscInt` and `PetscReal` are simply aliases for basic Fortran types and cannot be written as `type(tPetscInt)`

### Calling Sequences

PETSc Fortran routines have the same names as the corresponding C versions except for a few that use allocatable array arguments and end with `F90()`, see Routines that Return Fortran Allocatable Arrays.

The calling sequences for the Fortran version are in most cases identical to the C version, except for the error checking variable discussed in Error Checking and a few routines listed in Routines with Different Fortran Interfaces.

When passing floating point numbers into PETSc Fortran subroutines, always make sure you have them marked as double precision (e.g., pass in `10.d0` instead of `10.0` or declare them as PETSc variables, e.g. `PetscScalar one = 1.0`). Otherwise, the compiler interprets the input as a single precision number, which can cause crashes or other mysterious problems. We highly recommend using the `implicit none` option at the beginning of each Fortran subroutine and declaring all variables.

### Error Checking

In the Fortran version, each PETSc routine has as its final argument an integer error variable. The error code is nonzero if an error has been detected; otherwise, it is zero. For example, the Fortran and C variants of `KSPSolve()` are given, respectively, below, where `ierr` denotes the `PetscErrorCode` error variable:

    call KSPSolve(ksp, b, x, ierr) ! Fortran
    ierr = KSPSolve(ksp, b, x);    // C

For proper error handling one should not use the above syntax instead one should use

    PetscCall(KSPSolve(ksp, b, x, ierr))   ! Fortran subroutines
    PetscCallA(KSPSolve(ksp, b, x, ierr))  ! Fortran main program
    PetscCall(KSPSolve(ksp, b, x))         // C

### Passing Null Pointers

Many PETSc C functions have the option of passing a `NULL` argument (for example, the fifth argument of `MatCreateSeqAIJ()`). From Fortran, users must pass `PETSC_NULL_XXX` to indicate a null argument (where `XXX` is `INTEGER`, `DOUBLE`, `CHARACTER`, `SCALAR`, `VEC`, `MAT`, etc depending on the argument type); passing a literal 0 from Fortran in this case will crash the code. For example, when no options prefix is desired in the routine `PetscOptionsGetInt()`, one must use the following command in Fortran:

    PetscCall(PetscOptionsGetInt(PETSC_NULL_OPTIONS, PETSC_NULL_CHARACTER, PETSC_NULL_CHARACTER, '-name', N, flg, ierr))

### Matrix, Vector and IS Indices

All matrices, vectors and `IS` in PETSc use zero-based indexing in the PETSc API regardless of whether C or Fortran is being used. For example, `MatSetValues()` and `VecSetValues()` always use zero indexing. See Basic Matrix Operations for further details.

Indexing into Fortran arrays, for example obtained with `VecGetArrayF90()`, uses the Fortran convention and generally begin with 1 except for special routines such as `DMDAVecGetArrayF90()` which uses the ranges provided by `DMDAGetCorners()`.

### Setting Routines and Contexts

Some PETSc functions take as arguments user-functions and contexts for the function. For example

    external func
    SNESSetFunction(snes, r, func, ctx, ierr)
    SNES snes
    Vec r
    PetscErrorCode ierr

where `func` has the calling sequence

    subroutine func(snes, x, f, ctx, ierr)
    SNES snes
    Vec x,f
    PetscErrorCode ierr

and `ctx` can be almost anything (represented as `void *` in C).

It can be a Fortran derived type as in

    subroutine func(snes, x, f, ctx, ierr)
    SNES snes
    Vec x,f
    type (userctx)   ctx
    PetscErrorCode ierr
    ...

    external func
    SNESSetFunction(snes, r, func, ctx, ierr)
    SNES snes
    Vec r
    PetscErrorCode ierr
    type (userctx)   ctx

or a PETSc object

    subroutine func(snes, x, f, ctx, ierr)
    SNES snes
    Vec x,f
    Vec ctx
    PetscErrorCode ierr
    ...

    external func
    SNESSetFunction(snes, r, func, ctx, ierr)
    SNES snes
    Vec r
    PetscErrorCode ierr
    Vec ctx

or nothing

    subroutine func(snes, x, f, dummy, ierr)
    SNES snes
    Vec x,f
    integer dummy(*)
    PetscErrorCode ierr
    ...

    external func
    SNESSetFunction(snes, r, func, 0, ierr)
    SNES snes
    Vec r
    PetscErrorCode ierr

When a function pointer (declared as external in Fortran) is passed as an argument to a PETSc function, it is assumed that this function references a routine written in the same language as the PETSc interface function that was called. For instance, if `SNESSetFunction()` is called from C, the function must be a C function. Likewise, if it is called from Fortran, the function must be (a subroutine) written in Fortran.

### Compiling and Linking Fortran Programs

See Writing C/C++ or Fortran Applications.

### Routines with Different Fortran Interfaces

The following Fortran routines differ slightly from their C counterparts; see the manual pages and previous discussion in this chapter for details:

    PetscInitialize()
    PetscOptionsGetString()

The following functions are not supported in Fortran:

    PetscFClose(), PetscFOpen(), PetscFPrintf(), PetscPrintf(),
    PetscPopErrorHandler(), PetscPushErrorHandler(), PetscInfo(),
    PetscSetDebugger(), VecGetArrays(), VecRestoreArrays(),
    PetscViewerASCIIGetPointer(), PetscViewerBinaryGetDescriptor(),
    PetscViewerStringOpen(), PetscViewerStringSPrintf(),
    PetscOptionsGetStringArray()

### Routines that Return Fortran Allocatable Arrays

Many PETSc functions that return an array of values in C in an argument (such as `ISGetIndices()`) return an allocatable array in Fortran. The Fortran function names for these are suffixed with `F90` as indicated below. A few routines, such as `VecDuplicateVecs()` discussed above, do not return a Fortran allocatable array; a large enough array must be explicitly declared before use and passed into the routine.

C-API
`ISGetIndices()`
`ISRestoreIndices()`
`ISLocalToGlobalMappingGetIndices()`
`ISLocalToGlobalMappingRestoreIndices()`
`VecGetArray()`
`VecRestoreArray()`
`VecGetArrayRead()`
`VecRestoreArrayRead()`
`VecDuplicateVecs()`
`VecDestroyVecs()`
`DMDAVecGetArray()`
`DMDAVecRestoreArray()`
`DMDAVecGetArrayRead()`
`DMDAVecRestoreArrayRead()`
`DMDAVecGetArrayWrite()`
`DMDAVecRestoreArrayWrite()`
`MatGetRowIJ()`
`MatRestoreRowIJ()`
`MatSeqAIJGetArray()`
`MatSeqAIJRestoreArray()`
`MatMPIAIJGetSeqAIJ()`
`MatDenseGetArray()`
`MatDenseRestoreArray()`

The array arguments to these Fortran functions should be declared with forms such as

    PetscScalar, pointer :: x(:)
    PetscInt, pointer :: idx(:)

See the manual pages for details and pointers to example programs.

### Duplicating Multiple Vectors

The Fortran interface to `VecDuplicateVecs()` differs slightly from the C/C++ variant. To create `n` vectors of the same format as an existing vector, the user must declare a vector array, `v_new` of size `n`. Then, after `VecDuplicateVecs()` has been called, `v_new` will contain (pointers to) the new PETSc vector objects. When finished with the vectors, the user should destroy them by calling `VecDestroyVecs()`. For example, the following code fragment duplicates `v_old` to form two new vectors, `v_new(1)` and `v_new(2)`.

    Vec          v_old, v_new(2)
    PetscInt     ierr
    PetscScalar  alpha
    ....
    PetscCall(VecDuplicateVecs(v_old, 2, v_new, ierr))
    alpha = 4.3
    PetscCall(VecSet(v_new(1), alpha, ierr))
    alpha = 6.0
    PetscCall(VecSet(v_new(2), alpha, ierr))
    ....
    PetscCall(VecDestroyVecs(2, v_new, ierr))

## Sample Fortran Programs

Sample programs that illustrate the PETSc interface for Fortran are given below, corresponding to Vec Test ex19f, Vec Tutorial ex4f, Draw Test ex5f, and SNES Tutorial ex1f, respectively. We also refer Fortran programmers to the C examples listed throughout the manual, since PETSc usage within the two languages differs only slightly.

Listing: `src/vec/vec/tests/ex19f.F90`

    !
    !
          program main
    #include <petsc/finclude/petscvec.h>
          use petscvec
          implicit none
    !
    !  This example demonstrates basic use of the PETSc Fortran interface
    !  to vectors.
    !
           PetscInt  n
           PetscErrorCode ierr
           PetscBool  flg
           PetscScalar      one,two,three,dot
           PetscReal        norm,rdot
           Vec              x,y,w
           PetscOptions     options

           n     = 20
           one   = 1.0
           two   = 2.0
           three = 3.0

           PetscCallA(PetscInitialize(ierr))
           PetscCallA(PetscOptionsCreate(options,ierr))
           PetscCallA(PetscOptionsGetInt(options,PETSC_NULL_CHARACTER,'-n',n,flg,ierr))
           PetscCallA(PetscOptionsDestroy(options,ierr))

    ! Create a vector, then duplicate it
           PetscCallA(VecCreate(PETSC_COMM_WORLD,x,ierr))
           PetscCallA(VecSetSizes(x,PETSC_DECIDE,n,ierr))
           PetscCallA(VecSetFromOptions(x,ierr))
           PetscCallA(VecDuplicate(x,y,ierr))
           PetscCallA(VecDuplicate(x,w,ierr))

           PetscCallA(VecSet(x,one,ierr))
           PetscCallA(VecSet(y,two,ierr))

           PetscCallA(VecDot(x,y,dot,ierr))
           rdot = PetscRealPart(dot)
           write(6,100) rdot
      100  format('Result of inner product ',f10.4)

           PetscCallA(VecScale(x,two,ierr))
           PetscCallA(VecNorm(x,NORM_2,norm,ierr))
           write(6,110) norm
      110  format('Result of scaling ',f10.4)

           PetscCallA(VecCopy(x,w,ierr))
           PetscCallA(VecNorm(w,NORM_2,norm,ierr))
           write(6,120) norm
      120  format('Result of copy ',f10.4)

           PetscCallA(VecAXPY(y,three,x,ierr))
           PetscCallA(VecNorm(y,NORM_2,norm,ierr))
           write(6,130) norm
      130  format('Result of axpy ',f10.4)

           PetscCallA(VecDestroy(x,ierr))
           PetscCallA(VecDestroy(y,ierr))
           PetscCallA(VecDestroy(w,ierr))
           PetscCallA(PetscFinalize(ierr))
           end

Listing: `src/vec/vec/tutorials/ex4f.F90`

    !
    !
    !  Description:  Illustrates the use of VecSetValues() to set
    !  multiple values at once; demonstrates VecGetArrayF90().
    !
    ! -----------------------------------------------------------------------

          program main
    #include <petsc/finclude/petscvec.h>
          use petscvec
          implicit none

    ! - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
    !                 Beginning of program
    ! - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -

           PetscScalar xwork(6)
           PetscScalar, pointer :: xx_v(:),yy_v(:)
           PetscInt     i,n,loc(6),isix
           PetscErrorCode ierr
           Vec         x,y

           PetscCallA(PetscInitialize(ierr))
           n = 6
           isix = 6

    !  Create initial vector and duplicate it

           PetscCallA(VecCreateSeq(PETSC_COMM_SELF,n,x,ierr))
           PetscCallA(VecDuplicate(x,y,ierr))

    !  Fill work arrays with vector entries and locations.  Note that
    !  the vector indices are 0-based in PETSc (for both Fortran and
    !  C vectors)

           do 10 i=1,n
              loc(i) = i-1
              xwork(i) = 10.0*real(i)
      10   continue

    !  Set vector values.  Note that we set multiple entries at once.
    !  Of course, usually one would create a work array that is the
    !  natural size for a particular problem (not one that is as long
    !  as the full vector).

           PetscCallA(VecSetValues(x,isix,loc,xwork,INSERT_VALUES,ierr))

    !  Assemble vector

           PetscCallA(VecAssemblyBegin(x,ierr))
           PetscCallA(VecAssemblyEnd(x,ierr))

    !  View vector
           PetscCallA(PetscObjectSetName(x, 'initial vector:',ierr))
           PetscCallA(VecView(x,PETSC_VIEWER_STDOUT_SELF,ierr))
           PetscCallA(VecCopy(x,y,ierr))

    !  Get a pointer to vector data.
    !    - For default PETSc vectors, VecGetArray() returns a pointer to
    !      the data array.  Otherwise, the routine is implementation dependent.
    !    - You MUST call VecRestoreArray() when you no longer need access to
    !      the array.
    !    - Note that the Fortran interface to VecGetArray() differs from the
    !      C version.  See the users manual for details.

           PetscCallA(VecGetArrayF90(x,xx_v,ierr))
           PetscCallA(VecGetArrayF90(y,yy_v,ierr))

    !  Modify vector data

           do 30 i=1,n
              xx_v(i) = 100.0*real(i)
              yy_v(i) = 1000.0*real(i)
      30   continue

    !  Restore vectors

           PetscCallA(VecRestoreArrayF90(x,xx_v,ierr))
           PetscCallA(VecRestoreArrayF90(y,yy_v,ierr))

    !  View vectors
           PetscCallA(PetscObjectSetName(x, 'new vector 1:',ierr))
           PetscCallA(VecView(x,PETSC_VIEWER_STDOUT_SELF,ierr))

           PetscCallA(PetscObjectSetName(y, 'new vector 2:',ierr))
           PetscCallA(VecView(y,PETSC_VIEWER_STDOUT_SELF,ierr))

    !  Free work space.  All PETSc objects should be destroyed when they
    !  are no longer needed.

           PetscCallA(VecDestroy(x,ierr))
           PetscCallA(VecDestroy(y,ierr))
           PetscCallA(PetscFinalize(ierr))
           end

Listing: `src/sys/classes/draw/tests/ex5f.F90`

    !
    !
          program main
    #include <petsc/finclude/petscsys.h>
    #include <petsc/finclude/petscdraw.h>
          use petscsys
          implicit none
    !
    !  This example demonstrates basic use of the Fortran interface for
    !  PetscDraw routines.
    !
          PetscDraw         draw
          PetscDrawLG       lg
          PetscDrawAxis     axis
          PetscErrorCode    ierr
          PetscBool         flg
          integer           x,y,width,height
          PetscScalar       xd,yd
          PetscReal         ten
          PetscInt          i,n,w,h
          PetscInt          one

          n      = 15
          x      = 0
          y      = 0
          w      = 400
          h      = 300
          ten    = 10.0
          one    = 1

          PetscCallA(PetscInitialize(ierr))

    !  GetInt requires a PetscInt so have to do this ugly setting
          PetscCallA(PetscOptionsGetInt(PETSC_NULL_OPTIONS,PETSC_NULL_CHARACTER,'-width',w, flg,ierr))
          width = int(w)
          PetscCallA(PetscOptionsGetInt(PETSC_NULL_OPTIONS,PETSC_NULL_CHARACTER,'-height',h,flg,ierr))
          height = int(h)
          PetscCallA(PetscOptionsGetInt(PETSC_NULL_OPTIONS,PETSC_NULL_CHARACTER,'-n',n,flg,ierr))

          PetscCallA(PetscDrawCreate(PETSC_COMM_WORLD,PETSC_NULL_CHARACTER,PETSC_NULL_CHARACTER,x,y,width,height,draw,ierr))
          PetscCallA(PetscDrawSetFromOptions(draw,ierr))

          PetscCallA(PetscDrawLGCreate(draw,one,lg,ierr))
          PetscCallA(PetscDrawLGGetAxis(lg,axis,ierr))
          PetscCallA(PetscDrawAxisSetColors(axis,PETSC_DRAW_BLACK,PETSC_DRAW_RED,PETSC_DRAW_BLUE,ierr))
          PetscCallA(PetscDrawAxisSetLabels(axis,'toplabel','xlabel','ylabel',ierr))

          do 10, i=0,n-1
            xd = real(i) - 5.0
            yd = xd*xd
            PetscCallA(PetscDrawLGAddPoint(lg,xd,yd,ierr))
     10   continue

          PetscCallA(PetscDrawLGSetUseMarkers(lg,PETSC_TRUE,ierr))
          PetscCallA(PetscDrawLGDraw(lg,ierr))

          PetscCallA(PetscSleep(ten,ierr))

          PetscCallA(PetscDrawLGDestroy(lg,ierr))
          PetscCallA(PetscDrawDestroy(draw,ierr))
          PetscCallA(PetscFinalize(ierr))
          end

Listing: `src/snes/tutorials/ex1f.F90`

    !
    !
    !  Description: Uses the Newton method to solve a two-variable system.
    !

          program main
    #include <petsc/finclude/petsc.h>
          use petsc
          implicit none

    ! - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
    !                   Variable declarations
    ! - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
    !
    !  Variables:
    !     snes        - nonlinear solver
    !     ksp        - linear solver
    !     pc          - preconditioner context
    !     ksp         - Krylov subspace method context
    !     x, r        - solution, residual vectors
    !     J           - Jacobian matrix
    !     its         - iterations for convergence
    !
          SNES     snes
          PC       pc
          KSP      ksp
          Vec      x,r
          Mat      J
          SNESLineSearch linesearch
          PetscErrorCode  ierr
          PetscInt its,i2,i20
          PetscMPIInt size,rank
          PetscScalar   pfive
          PetscReal   tol
          PetscBool   setls
    #if defined(PETSC_USE_LOG)
          PetscViewer viewer
    #endif
          double precision threshold,oldthreshold

    !  Note: Any user-defined Fortran routines (such as FormJacobian)
    !  MUST be declared as external.

          external FormFunction, FormJacobian, MyLineSearch

    ! - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
    !                 Beginning of program
    ! - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -

          PetscCallA(PetscInitialize(ierr))
          PetscCallA(PetscLogNestedBegin(ierr))
          threshold = 1.0
          PetscCallA(PetscLogSetThreshold(threshold,oldthreshold,ierr))
          PetscCallMPIA(MPI_Comm_size(PETSC_COMM_WORLD,size,ierr))
          PetscCallMPIA(MPI_Comm_rank(PETSC_COMM_WORLD,rank,ierr))
          PetscCheckA(size .eq. 1,PETSC_COMM_SELF,PETSC_ERR_WRONG_MPI_SIZE,'Uniprocessor example')

          i2  = 2
          i20 = 20
    ! - - - - - - - - - -- - - - - - - - - - - - - - - - - - - - - - - - - -
    !  Create nonlinear solver context
    ! - - - - - - - - - -- - - - - - - - - - - - - - - - - - - - - - - - - -

          PetscCallA(SNESCreate(PETSC_COMM_WORLD,snes,ierr))

    ! - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
    !  Create matrix and vector data structures; set corresponding routines
    ! - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -

    !  Create vectors for solution and nonlinear function

          PetscCallA(VecCreateSeq(PETSC_COMM_SELF,i2,x,ierr))
          PetscCallA(VecDuplicate(x,r,ierr))

    !  Create Jacobian matrix data structure

          PetscCallA(MatCreate(PETSC_COMM_SELF,J,ierr))
          PetscCallA(MatSetSizes(J,PETSC_DECIDE,PETSC_DECIDE,i2,i2,ierr))
          PetscCallA(MatSetFromOptions(J,ierr))
          PetscCallA(MatSetUp(J,ierr))

    !  Set function evaluation routine and vector

          PetscCallA(SNESSetFunction(snes,r,FormFunction,0,ierr))

    !  Set Jacobian matrix data structure and Jacobian evaluation routine

          PetscCallA(SNESSetJacobian(snes,J,J,FormJacobian,0,ierr))

    ! - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
    !  Customize nonlinear solver; set runtime options
    ! - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -

    !  Set linear solver defaults for this problem. By extracting the
    !  KSP, KSP, and PC contexts from the SNES context, we can then
    !  directly call any KSP, KSP, and PC routines to set various options.

          PetscCallA(SNESGetKSP(snes,ksp,ierr))
          PetscCallA(KSPGetPC(ksp,pc,ierr))
          PetscCallA(PCSetType(pc,PCNONE,ierr))
          tol = 1.e-4
          PetscCallA(KSPSetTolerances(ksp,tol,PETSC_DEFAULT_REAL,PETSC_DEFAULT_REAL,i20,ierr))

    !  Set SNES/KSP/KSP/PC runtime options, e.g.,
    !      -snes_view -snes_monitor -ksp_type <ksp> -pc_type <pc>
    !  These options will override those specified above as long as
    !  SNESSetFromOptions() is called _after_ any other customization
    !  routines.

          PetscCallA(SNESSetFromOptions(snes,ierr))

          PetscCallA(PetscOptionsHasName(PETSC_NULL_OPTIONS,PETSC_NULL_CHARACTER,'-setls',setls,ierr))

          if (setls) then
            PetscCallA(SNESGetLineSearch(snes, linesearch, ierr))
            PetscCallA(SNESLineSearchSetType(linesearch, 'shell', ierr))
            PetscCallA(SNESLineSearchShellSetApply(linesearch, MyLineSearch,0,ierr))
          endif

    ! - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
    !  Evaluate initial guess; then solve nonlinear system
    ! - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -

    !  Note: The user should initialize the vector, x, with the initial guess
    !  for the nonlinear solver prior to calling SNESSolve().  In particular,
    !  to employ an initial guess of zero, the user should explicitly set
    !  this vector to zero by calling VecSet().

          pfive = 0.5
          PetscCallA(VecSet(x,pfive,ierr))
          PetscCallA(SNESSolve(snes,PETSC_NULL_VEC,x,ierr))

    !  View solver converged reason; we could instead use the option -snes_converged_reason
          PetscCallA(SNESConvergedReasonView(snes,PETSC_VIEWER_STDOUT_WORLD,ierr))

          PetscCallA(SNESGetIterationNumber(snes,its,ierr))
          if (rank .eq. 0) then
             write(6,100) its
          endif
      100 format('Number of SNES iterations = ',i5)

    ! - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
    !  Free work space.  All PETSc objects should be destroyed when they
    !  are no longer needed.
    ! - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -

          PetscCallA(VecDestroy(x,ierr))
          PetscCallA(VecDestroy(r,ierr))
          PetscCallA(MatDestroy(J,ierr))
          PetscCallA(SNESDestroy(snes,ierr))
    #if defined(PETSC_USE_LOG)
          PetscCallA(PetscViewerASCIIOpen(PETSC_COMM_WORLD,'filename.xml',viewer,ierr))
          PetscCallA(PetscViewerPushFormat(viewer,PETSC_VIEWER_ASCII_XML,ierr))
          PetscCallA(PetscLogView(viewer,ierr))
          PetscCallA(PetscViewerDestroy(viewer,ierr))
    #endif
          PetscCallA(PetscFinalize(ierr))
          end
    !
    ! ------------------------------------------------------------------------
    !
    !  FormFunction - Evaluates nonlinear function, F(x).
    !
    !  Input Parameters:
    !  snes - the SNES context
    !  x - input vector
    !  dummy - optional user-defined context (not used here)
    !
    !  Output Parameter:
    !  f - function vector
    !
          subroutine FormFunction(snes,x,f,dummy,ierr)
          use petscsnes
          implicit none

          SNES     snes
          Vec      x,f
          PetscErrorCode ierr
          integer dummy(*)

    !  Declarations for use with local arrays
          PetscScalar,pointer :: lx_v(:),lf_v(:)

    !  Get pointers to vector data.
    !    - VecGetArrayF90() returns a pointer to the data array.
    !    - You MUST call VecRestoreArrayF90() when you no longer need access to
    !      the array.

          PetscCall(VecGetArrayReadF90(x,lx_v,ierr))
          PetscCall(VecGetArrayF90(f,lf_v,ierr))

    !  Compute function

          lf_v(1) = lx_v(1)*lx_v(1) + lx_v(1)*lx_v(2) - 3.0
          lf_v(2) = lx_v(1)*lx_v(2) + lx_v(2)*lx_v(2) - 6.0

    !  Restore vectors

          PetscCall(VecRestoreArrayReadF90(x,lx_v,ierr))
          PetscCall(VecRestoreArrayF90(f,lf_v,ierr))

          end

    ! ---------------------------------------------------------------------
    !
    !  FormJacobian - Evaluates Jacobian matrix.
    !
    !  Input Parameters:
    !  snes - the SNES context
    !  x - input vector
    !  dummy - optional user-defined context (not used here)
    !
    !  Output Parameters:
    !  A - Jacobian matrix
    !  B - optionally different preconditioning matrix
    !
          subroutine FormJacobian(snes,X,jac,B,dummy,ierr)
          use petscsnes
          implicit none

          SNES         snes
          Vec          X
          Mat          jac,B
          PetscScalar  A(4)
          PetscErrorCode ierr
          PetscInt idx(2),i2
          integer dummy(*)

    !  Declarations for use with local arrays

          PetscScalar,pointer :: lx_v(:)

    !  Get pointer to vector data

          i2 = 2
          PetscCall(VecGetArrayReadF90(x,lx_v,ierr))

    !  Compute Jacobian entries and insert into matrix.
    !   - Since this is such a small problem, we set all entries for
    !     the matrix at once.
    !   - Note that MatSetValues() uses 0-based row and column numbers
    !     in Fortran as well as in C (as set here in the array idx).

          idx(1) = 0
          idx(2) = 1
          A(1) = 2.0*lx_v(1) + lx_v(2)
          A(2) = lx_v(1)
          A(3) = lx_v(2)
          A(4) = lx_v(1) + 2.0*lx_v(2)
          PetscCall(MatSetValues(B,i2,idx,i2,idx,A,INSERT_VALUES,ierr))

    !  Restore vector

          PetscCall(VecRestoreArrayReadF90(x,lx_v,ierr))

    !  Assemble matrix

          PetscCall(MatAssemblyBegin(B,MAT_FINAL_ASSEMBLY,ierr))
          PetscCall(MatAssemblyEnd(B,MAT_FINAL_ASSEMBLY,ierr))
          if (B .ne. jac) then
            PetscCall(MatAssemblyBegin(jac,MAT_FINAL_ASSEMBLY,ierr))
            PetscCall(MatAssemblyEnd(jac,MAT_FINAL_ASSEMBLY,ierr))
          endif

          end

          subroutine MyLineSearch(linesearch, lctx, ierr)
          use petscsnes
          implicit none

          SNESLineSearch    linesearch
          SNES              snes
          integer           lctx
          Vec               x, f,g, y, w
          PetscReal         ynorm,gnorm,xnorm
          PetscErrorCode    ierr

          PetscScalar       mone

          mone = -1.0
          PetscCall(SNESLineSearchGetSNES(linesearch, snes, ierr))
          PetscCall(SNESLineSearchGetVecs(linesearch, x, f, y, w, g, ierr))
          PetscCall(VecNorm(y,NORM_2,ynorm,ierr))
          PetscCall(VecAXPY(x,mone,y,ierr))
          PetscCall(SNESComputeFunction(snes,x,f,ierr))
          PetscCall(VecNorm(f,NORM_2,gnorm,ierr))
          PetscCall(VecNorm(x,NORM_2,xnorm,ierr))
          PetscCall(VecNorm(y,NORM_2,ynorm,ierr))
          PetscCall(SNESLineSearchSetNorms(linesearch, xnorm, gnorm, ynorm,ierr))
          end

### Calling Fortran Routines from C (and C Routines from Fortran)

The information here applies only if you plan to call your own C functions from Fortran or Fortran functions from C. Different compilers have different methods of naming Fortran routines called from C (or C routines called from Fortran). Most Fortran compilers change the capital letters in Fortran routines to all lowercase. With some compilers, the Fortran compiler appends an underscore to the end of each Fortran routine name; for example, the Fortran routine `Dabsc()` would be called from C with `dabsc_()`. Other compilers change all the letters in Fortran routine names to capitals.

PETSc provides two macros (defined in C/C++) to help write portable code that mixes C/C++ and Fortran. They are `PETSC_HAVE_FORTRAN_UNDERSCORE` and `PETSC_HAVE_FORTRAN_CAPS` , which will be defined in the file `$PETSC_DIR/$PETSC_ARCH/include/petscconf.h` based on the compilers conventions. The macros are used, for example, as follows:

    #if defined(PETSC_HAVE_FORTRAN_CAPS)
    #define dabsc_ DABSC
    #elif !defined(PETSC_HAVE_FORTRAN_UNDERSCORE)
    #define dabsc_ dabsc
    #endif
    .....
    dabsc_( &n,x,y); /* call the Fortran function */