.. _chapter_fortran: PETSc for Fortran Users ----------------------- Most of the functionality of PETSc can be obtained by people who program purely in Fortran. C vs. Fortran Interfaces ~~~~~~~~~~~~~~~~~~~~~~~~ Only a few differences exist between the C and Fortran PETSc interfaces, are due to Fortran syntax differences. All Fortran routines have the same names as the corresponding C versions, and PETSc command line options are fully supported. The routine arguments follow the usual Fortran conventions; the user need not worry about passing pointers or values. The calling sequences for the Fortran version are in most cases identical to the C version, except for the error checking variable discussed in :any:`sec_fortran_errors` and a few routines listed in :any:`sec_fortran_exceptions`. .. _sec_fortran_includes: Fortran Include Files ^^^^^^^^^^^^^^^^^^^^^ The Fortran include files for PETSc are located in the directory ``${PETSC_DIR}/include/petsc/finclude`` and should be used via statements such as the following: .. code-block:: fortran #include for example, .. code-block:: fortran #include You must also use the appropriate Fortran module which is done with .. code-block:: fortran use petscXXX for example, .. code-block:: fortran use petscksp .. _sec_fortran_errors: Error Checking ^^^^^^^^^^^^^^ In the Fortran version, each PETSc routine has as its final argument an integer error variable, in contrast to the C convention of providing the error variable as the routine’s return value. The error code is set to be 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 error variable: .. code-block:: fortran call KSPSolve(ksp,b,x,ierr) ! Fortran ierr = KSPSolve(ksp,b,x); /* C */ Fortran programmers can check these error codes with ``CHKERRQ(ierr)``, which terminates all processes when an error is encountered. Likewise, one can set error codes within Fortran programs by using ``SETERRQ(comm,p,' ',ierr)``, which again terminates all processes upon detection of an error. Note that complete error tracebacks with ``CHKERRQ()`` and ``SETERRQ()``, as described in :any:`sec_simple` for C routines, are *not* directly supported for Fortran routines; however, Fortran programmers can easily use the error codes in writing their own tracebacks. For example, one could use code such as the following: .. code-block:: fortran call KSPSolve(ksp,b,x,ierr) if (ierr .ne. 0) then print*, 'Error in routine ...' return end if Calling Fortran Routines from C (and C Routines from Fortran) ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Different machines have different methods of naming Fortran routines called from C (or C routines called from Fortran). Most Fortran compilers change all the capital letters in Fortran routines to lowercase. On some machines, 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 machines 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 are defined in the file ``${PETSC_DIR}/${PETSC_ARCH}/include/petscconf.h``. The macros are used, for example, as follows: .. code-block:: fortran #if defined(PETSC_HAVE_FORTRAN_CAPS) #define dabsc_ DMDABSC #elif !defined(PETSC_HAVE_FORTRAN_UNDERSCORE) #define dabsc_ dabsc #endif ..... dabsc_( &n,x,y); /* call the Fortran function */ Passing Null Pointers ^^^^^^^^^^^^^^^^^^^^^ In several PETSc C functions, one has the option of passing a NULL (0) 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``, or ``SCALAR`` depending on the type of argument required); passing 0 from Fortran will crash the code. Note that the C convention of passing NULL (or 0) *cannot* be used. For example, when no options prefix is desired in the routine ``PetscOptionsGetInt()``, one must use the following command in Fortran: .. code-block:: fortran call PetscOptionsGetInt(PETSC_NULL_OPTIONS,PETSC_NULL_CHARACTER,PETSC_NULL_CHARACTER,'-name',N,flg,ierr) This Fortran requirement is inconsistent with C, where the user can employ ``NULL`` for all null arguments. .. _sec_fortvecd: Duplicating Multiple Vectors ^^^^^^^^^^^^^^^^^^^^^^^^^^^^ The Fortran interface to ``VecDuplicateVecs()`` differs slightly from the C/C++ variant because Fortran does not allow conventional arrays to be returned in routine arguments. 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)``. .. code-block:: fortran Vec v_old, v_new(2) PetscInt ierr PetscScalar alpha .... call VecDuplicateVecs(v_old,2,v_new,ierr) alpha = 4.3 call VecSet(v_new(1),alpha,ierr) alpha = 6.0 call VecSet(v_new(2),alpha,ierr) .... call VecDestroyVecs(2, &v_new,ierr) Matrix, Vector and IS Indices ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ All matrices, vectors and ``IS`` in PETSc use zero-based indexing, regardless of whether C or Fortran is being used. The interface routines, such as ``MatSetValues()`` and ``VecSetValues()``, always use zero indexing. See :any:`sec_matoptions` for further details. Setting Routines ^^^^^^^^^^^^^^^^ When a function pointer is passed as an argument to a PETSc function, such as the test in ``KSPSetConvergenceTest()``, it is assumed that this pointer references a routine written in the same language as the PETSc interface function that was called. For instance, if ``KSPSetConvergenceTest()`` is called from C, the test argument is assumed to be a C function. Likewise, if it is called from Fortran, the test is assumed to be written in Fortran. .. _sec_fortcompile: Compiling and Linking Fortran Programs ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ See :any:`sec_writing_application_codes`. .. _sec_fortran_exceptions: 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: .. code-block:: fortran PetscInitialize(char *filename,int ierr) PetscError(MPI_COMM,int err,char *message,int ierr) VecGetArray(), MatDenseGetArray() ISGetIndices(), VecDuplicateVecs(), VecDestroyVecs() PetscOptionsGetString() The following functions are not supported in Fortran: .. code-block:: fortran PetscFClose(), PetscFOpen(), PetscFPrintf(), PetscPrintf() PetscPopErrorHandler(), PetscPushErrorHandler() PetscInfo() PetscSetDebugger() VecGetArrays(), VecRestoreArrays() PetscViewerASCIIGetPointer(), PetscViewerBinaryGetDescriptor() PetscViewerStringOpen(), PetscViewerStringSPrintf() PetscOptionsGetStringArray() PETSc includes some support for direct use of Fortran90 pointers. Current routines include: .. code-block:: fortran VecGetArrayF90(), VecRestoreArrayF90() VecGetArrayReadF90(), VecRestoreArrayReadF90() VecDuplicateVecsF90(), VecDestroyVecsF90() DMDAVecGetArrayF90(), DMDAVecGetArrayReadF90(), ISLocalToGlobalMappingGetIndicesF90() MatDenseGetArrayF90(), MatDenseRestoreArrayF90() ISGetIndicesF90(), ISRestoreIndicesF90() See the manual pages for details and pointers to example programs. .. _sec_fortran-examples: 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. .. admonition:: Listing: ``src/vec/vec/tests/ex19f.F`` :name: vec-test-ex19f .. literalinclude:: ../../../vec/vec/tests/ex19f.F :language: fortran .. _listing_vec_ex4f: .. admonition:: Listing: ``src/vec/vec/tutorials/ex4f.F`` :name: vec-ex4f .. literalinclude:: ../../../vec/vec/tutorials/ex4f.F :language: fortran .. admonition:: Listing: ``src/sys/classes/draw/tests/ex5f.F`` :name: draw-test-ex5f .. literalinclude:: ../../../sys/classes/draw/tests/ex5f.F :language: fortran .. admonition:: Listing: ``src/snes/tutorials/ex1f.F90`` :name: snes-ex1f .. literalinclude:: ../../../snes/tutorials/ex1f.F90 :language: fortran .. _sec_fortranarrays: Array Arguments ^^^^^^^^^^^^^^^ This material is no longer relevent since one should use ``VecGetArrayF90()`` and the other routines that utilize Fortran pointers, instead of the code below, but it is included for historical reasons and because many of the Fortran examples still utilize the old approach. Since Fortran 77 does not allow arrays to be returned in routine arguments, all PETSc routines that return arrays, such as ``VecGetArray()``, ``MatDenseGetArray()``, and ``ISGetIndices()``, are defined slightly differently in Fortran than in C. Instead of returning the array itself, these routines accept as input a user-specified array of dimension one and return an integer index to the actual array used for data storage within PETSc. The Fortran interface for several routines is as follows: .. code-block:: fortran PetscScalar xx_v(1), aa_v(1) PetscErrorCode ierr PetscInt ss_v(1), dd_v(1), nloc PetscOffset ss_i, xx_i, aa_i, dd_i Vec x Mat A IS s DM d call VecGetArray(x,xx_v,xx_i,ierr) call MatDenseGetArray(A,aa_v,aa_i,ierr) call ISGetIndices(s,ss_v,ss_i,ierr) To access array elements directly, both the user-specified array and the integer index *must* then be used together. For example, the following Fortran program fragment illustrates directly setting the values of a vector array instead of using ``VecSetValues()``. Note the (optional) use of the preprocessor ``#define`` statement to enable array manipulations in the conventional Fortran manner. .. code-block:: fortran #define xx_a(ib) xx_v(xx_i + (ib)) double precision xx_v(1) PetscOffset xx_i PetscErrorCode ierr PetscInt i, n Vec x call VecGetArray(x,xx_v,xx_i,ierr) call VecGetLocalSize(x,n,ierr) do 10, i=1,n xx_a(i) = 3*i + 1 10 continue call VecRestoreArray(x,xx_v,xx_i,ierr) :ref:`The Vec ex4f Tutorial listed above ` contains an example of using ``VecGetArray()`` within a Fortran routine. Since in this case the array is accessed directly from Fortran, indexing begins with 1, not 0 (unless the array is declared as ``xx_v(0:1)``). This is different from the use of ``VecSetValues()`` where, indexing always starts with 0. *Note*: If using ``VecGetArray()``, ``MatDenseGetArray()``, or ``ISGetIndices()``, from Fortran, the user *must not* compile the Fortran code with options to check for “array entries out of bounds” (e.g., on the IBM RS/6000 this is done with the ``-C`` compiler option, so never use the ``-C`` option with this).