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)
PETSc objects are always automatically initialized when declared so you do not need to (and should not) do
type(tXXX) x = PETSC_NULL_XXX
XXX x = PETSC_NULL_XXX
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:
For proper error handling one should not use the above syntax instead one should use
Passing Arrays#
Many PETSc functions take arrays as arguments; in Fortran they must be passed as arrays even if the “array” is of length one (unlike Fortran 77 where one can pass scalars to functions expecting arrays). When passing a single value one can use the Fortran [] notation to pass the scalar as an array, for example
PetscCall(VecSetValues(v, one, [i], [val], ierr))
This trick can only be used for arrays used to pass data into a PETSc routine, it cannot be used for arrays used to receive data from a PETSc routine. For example,
PetscCall(VecGetValues(v, one, idx, [val], ierr))
is invalid and will not set val with the correct value.
For PETSc routine arguments that return a character string, you should pass a string long enough to hold the result. For example,
character(80) str
PetscCall(KSPGetType(ksp,str,ierr))
The result is copied into str.
For PETSc routine arguments that return an array of PetscInt, PetscScalar, PetscReal or of PETSc objects,
there are two possibilities. In the first, the Fortran routine must pass in an array of sufficient size to hold the result. For example,
PetscInt lx(64)
DMDAGetOwnershipRanges(a, lx, PETSC_NULL_INTEGER_ARRAY, PETSC_NULL_INTEGER_ARRAY);
In the second form one passes in a pointer to an array and the PETSc routine returns an array containing the values.
PetscScalar, pointer :: array(:)
PetscCall(VecGetArrayF90(v, array, ierr))
In this second form the PETSc routine often has a name that ends with F90.
The information for which form to use is documented in the manual page of the routine.
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))
Where the code expects an array, then use PETSC_NULL_XXX_ARRAY. For example:
PetscCall(MatCreateSeqDense(comm, m, n, PETSC_NULL_SCALAR_ARRAY, A))
Finally when a subroutine returns a PetscObject through an argument to check if it is NULL you must use:
if (PetscObjectIsNull(dm)) then
if (.not. PetscObjectIsNull(dm)) then
you cannot use
if (dm .eq. PETSC_NULL_DM) then
Note that
if (PetscObjectIsNull(PETSC_NULL_VEC)) then
will always return true, for any PETSc object.
These specializations are required because of Fortran’s strict type checking system and lack of a concept of NULL.
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.
If you are using Fortran classes that have bound functions (methods) as in
src/snes/tests/ex18f90.F90, the context cannot be passed
to function pointer setting routines, such as SNESSetFunction(). Instead, one must use SNESSetFunctionNoInterface(),
and define the interface directly in the user code, see
ex18f90.F90
for a full demonstration.
Compiling and Linking Fortran Programs#
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:
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 |
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_CURRENT_REAL,PETSC_CURRENT_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 */