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725 lines
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725 lines
30 KiB
Text
@node Calling FFTW from Modern Fortran, Calling FFTW from Legacy Fortran, Distributed-memory FFTW with MPI, Top
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@chapter Calling FFTW from Modern Fortran
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@cindex Fortran interface
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Fortran 2003 standardized ways for Fortran code to call C libraries,
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and this allows us to support a direct translation of the FFTW C API
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into Fortran. Compared to the legacy Fortran 77 interface
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(@pxref{Calling FFTW from Legacy Fortran}), this direct interface
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offers many advantages, especially compile-time type-checking and
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aligned memory allocation. As of this writing, support for these C
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interoperability features seems widespread, having been implemented in
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nearly all major Fortran compilers (e.g. GNU, Intel, IBM,
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Oracle/Solaris, Portland Group, NAG).
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@cindex portability
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This chapter documents that interface. For the most part, since this
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interface allows Fortran to call the C interface directly, the usage
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is identical to C translated to Fortran syntax. However, there are a
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few subtle points such as memory allocation, wisdom, and data types
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that deserve closer attention.
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@menu
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* Overview of Fortran interface::
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* Reversing array dimensions::
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* FFTW Fortran type reference::
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* Plan execution in Fortran::
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* Allocating aligned memory in Fortran::
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* Accessing the wisdom API from Fortran::
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* Defining an FFTW module::
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@end menu
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@c -------------------------------------------------------
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@node Overview of Fortran interface, Reversing array dimensions, Calling FFTW from Modern Fortran, Calling FFTW from Modern Fortran
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@section Overview of Fortran interface
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FFTW provides a file @code{fftw3.f03} that defines Fortran 2003
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interfaces for all of its C routines, except for the MPI routines
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described elsewhere, which can be found in the same directory as
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@code{fftw3.h} (the C header file). In any Fortran subroutine where
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you want to use FFTW functions, you should begin with:
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@cindex iso_c_binding
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@example
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use, intrinsic :: iso_c_binding
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include 'fftw3.f03'
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@end example
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This includes the interface definitions and the standard
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@code{iso_c_binding} module (which defines the equivalents of C
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types). You can also put the FFTW functions into a module if you
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prefer (@pxref{Defining an FFTW module}).
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At this point, you can now call anything in the FFTW C interface
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directly, almost exactly as in C other than minor changes in syntax.
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For example:
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@findex fftw_plan_dft_2d
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@findex fftw_execute_dft
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@findex fftw_destroy_plan
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@example
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type(C_PTR) :: plan
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complex(C_DOUBLE_COMPLEX), dimension(1024,1000) :: in, out
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plan = fftw_plan_dft_2d(1000,1024, in,out, FFTW_FORWARD,FFTW_ESTIMATE)
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...
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call fftw_execute_dft(plan, in, out)
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...
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call fftw_destroy_plan(plan)
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@end example
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A few important things to keep in mind are:
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@itemize @bullet
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@item
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@tindex fftw_complex
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@ctindex C_PTR
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@ctindex C_INT
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@ctindex C_DOUBLE
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@ctindex C_DOUBLE_COMPLEX
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FFTW plans are @code{type(C_PTR)}. Other C types are mapped in the
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obvious way via the @code{iso_c_binding} standard: @code{int} turns
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into @code{integer(C_INT)}, @code{fftw_complex} turns into
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@code{complex(C_DOUBLE_COMPLEX)}, @code{double} turns into
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@code{real(C_DOUBLE)}, and so on. @xref{FFTW Fortran type reference}.
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@item
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Functions in C become functions in Fortran if they have a return value,
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and subroutines in Fortran otherwise.
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@item
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The ordering of the Fortran array dimensions must be @emph{reversed}
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when they are passed to the FFTW plan creation, thanks to differences
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in array indexing conventions (@pxref{Multi-dimensional Array
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Format}). This is @emph{unlike} the legacy Fortran interface
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(@pxref{Fortran-interface routines}), which reversed the dimensions
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for you. @xref{Reversing array dimensions}.
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@item
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@cindex alignment
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@cindex SIMD
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Using ordinary Fortran array declarations like this works, but may
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yield suboptimal performance because the data may not be not aligned
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to exploit SIMD instructions on modern proessors (@pxref{SIMD
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alignment and fftw_malloc}). Better performance will often be obtained
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by allocating with @samp{fftw_alloc}. @xref{Allocating aligned memory
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in Fortran}.
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@item
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@findex fftw_execute
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Similar to the legacy Fortran interface (@pxref{FFTW Execution in
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Fortran}), we currently recommend @emph{not} using @code{fftw_execute}
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but rather using the more specialized functions like
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@code{fftw_execute_dft} (@pxref{New-array Execute Functions}).
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However, you should execute the plan on the @code{same arrays} as the
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ones for which you created the plan, unless you are especially
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careful. @xref{Plan execution in Fortran}. To prevent
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you from using @code{fftw_execute} by mistake, the @code{fftw3.f03}
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file does not provide an @code{fftw_execute} interface declaration.
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@item
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@cindex flags
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Multiple planner flags are combined with @code{ior} (equivalent to @samp{|} in C). e.g. @code{FFTW_MEASURE | FFTW_DESTROY_INPUT} becomes @code{ior(FFTW_MEASURE, FFTW_DESTROY_INPUT)}. (You can also use @samp{+} as long as you don't try to include a given flag more than once.)
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@end itemize
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@menu
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* Extended and quadruple precision in Fortran::
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@end menu
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@node Extended and quadruple precision in Fortran, , Overview of Fortran interface, Overview of Fortran interface
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@subsection Extended and quadruple precision in Fortran
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@cindex precision
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If FFTW is compiled in @code{long double} (extended) precision
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(@pxref{Installation and Customization}), you may be able to call the
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resulting @code{fftwl_} routines (@pxref{Precision}) from Fortran if
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your compiler supports the @code{C_LONG_DOUBLE_COMPLEX} type code.
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Because some Fortran compilers do not support
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@code{C_LONG_DOUBLE_COMPLEX}, the @code{fftwl_} declarations are
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segregated into a separate interface file @code{fftw3l.f03}, which you
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should include @emph{in addition} to @code{fftw3.f03} (which declares
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precision-independent @samp{FFTW_} constants):
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@cindex iso_c_binding
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@example
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use, intrinsic :: iso_c_binding
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include 'fftw3.f03'
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include 'fftw3l.f03'
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@end example
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We also support using the nonstandard @code{__float128}
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quadruple-precision type provided by recent versions of @code{gcc} on
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32- and 64-bit x86 hardware (@pxref{Installation and Customization}),
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using the corresponding @code{real(16)} and @code{complex(16)} types
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supported by @code{gfortran}. The quadruple-precision @samp{fftwq_}
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functions (@pxref{Precision}) are declared in a @code{fftw3q.f03}
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interface file, which should be included in addition to
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@code{fftw3.f03}, as above. You should also link with
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@code{-lfftw3q -lquadmath -lm} as in C.
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@c -------------------------------------------------------
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@node Reversing array dimensions, FFTW Fortran type reference, Overview of Fortran interface, Calling FFTW from Modern Fortran
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@section Reversing array dimensions
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@cindex row-major
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@cindex column-major
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A minor annoyance in calling FFTW from Fortran is that FFTW's array
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dimensions are defined in the C convention (row-major order), while
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Fortran's array dimensions are the opposite convention (column-major
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order). @xref{Multi-dimensional Array Format}. This is just a
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bookkeeping difference, with no effect on performance. The only
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consequence of this is that, whenever you create an FFTW plan for a
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multi-dimensional transform, you must always @emph{reverse the
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ordering of the dimensions}.
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For example, consider the three-dimensional (@threedims{L,M,N}) arrays:
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@example
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complex(C_DOUBLE_COMPLEX), dimension(L,M,N) :: in, out
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@end example
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To plan a DFT for these arrays using @code{fftw_plan_dft_3d}, you could do:
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@findex fftw_plan_dft_3d
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@example
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plan = fftw_plan_dft_3d(N,M,L, in,out, FFTW_FORWARD,FFTW_ESTIMATE)
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@end example
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That is, from FFTW's perspective this is a @threedims{N,M,L} array.
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@emph{No data transposition need occur}, as this is @emph{only
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notation}. Similarly, to use the more generic routine
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@code{fftw_plan_dft} with the same arrays, you could do:
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@example
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integer(C_INT), dimension(3) :: n = [N,M,L]
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plan = fftw_plan_dft_3d(3, n, in,out, FFTW_FORWARD,FFTW_ESTIMATE)
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@end example
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Note, by the way, that this is different from the legacy Fortran
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interface (@pxref{Fortran-interface routines}), which automatically
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reverses the order of the array dimension for you. Here, you are
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calling the C interface directly, so there is no ``translation'' layer.
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@cindex r2c/c2r multi-dimensional array format
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An important thing to keep in mind is the implication of this for
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multidimensional real-to-complex transforms (@pxref{Multi-Dimensional
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DFTs of Real Data}). In C, a multidimensional real-to-complex DFT
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chops the last dimension roughly in half (@threedims{N,M,L} real input
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goes to @threedims{N,M,L/2+1} complex output). In Fortran, because
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the array dimension notation is reversed, the @emph{first} dimension of
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the complex data is chopped roughly in half. For example consider the
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@samp{r2c} transform of @threedims{L,M,N} real input in Fortran:
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@findex fftw_plan_dft_r2c_3d
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@findex fftw_execute_dft_r2c
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@example
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type(C_PTR) :: plan
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real(C_DOUBLE), dimension(L,M,N) :: in
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complex(C_DOUBLE_COMPLEX), dimension(L/2+1,M,N) :: out
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plan = fftw_plan_dft_r2c_3d(N,M,L, in,out, FFTW_ESTIMATE)
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...
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call fftw_execute_dft_r2c(plan, in, out)
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@end example
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@cindex in-place
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@cindex padding
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Alternatively, for an in-place r2c transform, as described in the C
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documentation we must @emph{pad} the @emph{first} dimension of the
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real input with an extra two entries (which are ignored by FFTW) so as
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to leave enough space for the complex output. The input is
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@emph{allocated} as a @threedims{2[L/2+1],M,N} array, even though only
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@threedims{L,M,N} of it is actually used. In this example, we will
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allocate the array as a pointer type, using @samp{fftw_alloc} to
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ensure aligned memory for maximum performance (@pxref{Allocating
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aligned memory in Fortran}); this also makes it easy to reference the
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same memory as both a real array and a complex array.
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@findex fftw_alloc_complex
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@findex c_f_pointer
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@example
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real(C_DOUBLE), pointer :: in(:,:,:)
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complex(C_DOUBLE_COMPLEX), pointer :: out(:,:,:)
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type(C_PTR) :: plan, data
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data = fftw_alloc_complex(int((L/2+1) * M * N, C_SIZE_T))
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call c_f_pointer(data, in, [2*(L/2+1),M,N])
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call c_f_pointer(data, out, [L/2+1,M,N])
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plan = fftw_plan_dft_r2c_3d(N,M,L, in,out, FFTW_ESTIMATE)
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...
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call fftw_execute_dft_r2c(plan, in, out)
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...
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call fftw_destroy_plan(plan)
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call fftw_free(data)
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@end example
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@c -------------------------------------------------------
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@node FFTW Fortran type reference, Plan execution in Fortran, Reversing array dimensions, Calling FFTW from Modern Fortran
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@section FFTW Fortran type reference
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The following are the most important type correspondences between the
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C interface and Fortran:
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@itemize @bullet
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@item
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@tindex fftw_plan
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Plans (@code{fftw_plan} and variants) are @code{type(C_PTR)} (i.e. an
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opaque pointer).
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@item
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@tindex fftw_complex
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@cindex precision
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@ctindex C_DOUBLE
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@ctindex C_FLOAT
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@ctindex C_LONG_DOUBLE
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@ctindex C_DOUBLE_COMPLEX
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@ctindex C_FLOAT_COMPLEX
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@ctindex C_LONG_DOUBLE_COMPLEX
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The C floating-point types @code{double}, @code{float}, and @code{long
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double} correspond to @code{real(C_DOUBLE)}, @code{real(C_FLOAT)}, and
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@code{real(C_LONG_DOUBLE)}, respectively. The C complex types
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@code{fftw_complex}, @code{fftwf_complex}, and @code{fftwl_complex}
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correspond in Fortran to @code{complex(C_DOUBLE_COMPLEX)},
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@code{complex(C_FLOAT_COMPLEX)}, and
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@code{complex(C_LONG_DOUBLE_COMPLEX)}, respectively.
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Just as in C
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(@pxref{Precision}), the FFTW subroutines and types are prefixed with
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@samp{fftw_}, @code{fftwf_}, and @code{fftwl_} for the different precisions, and link to different libraries (@code{-lfftw3}, @code{-lfftw3f}, and @code{-lfftw3l} on Unix), but use the @emph{same} include file @code{fftw3.f03} and the @emph{same} constants (all of which begin with @samp{FFTW_}). The exception is @code{long double} precision, for which you should @emph{also} include @code{fftw3l.f03} (@pxref{Extended and quadruple precision in Fortran}).
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@item
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@tindex ptrdiff_t
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@ctindex C_INT
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@ctindex C_INTPTR_T
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@ctindex C_SIZE_T
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@findex fftw_malloc
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The C integer types @code{int} and @code{unsigned} (used for planner
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flags) become @code{integer(C_INT)}. The C integer type @code{ptrdiff_t} (e.g. in the @ref{64-bit Guru Interface}) becomes @code{integer(C_INTPTR_T)}, and @code{size_t} (in @code{fftw_malloc} etc.) becomes @code{integer(C_SIZE_T)}.
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@item
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@tindex fftw_r2r_kind
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@ctindex C_FFTW_R2R_KIND
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The @code{fftw_r2r_kind} type (@pxref{Real-to-Real Transform Kinds})
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becomes @code{integer(C_FFTW_R2R_KIND)}. The various constant values
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of the C enumerated type (@code{FFTW_R2HC} etc.) become simply integer
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constants of the same names in Fortran.
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@item
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@ctindex FFTW_DESTROY_INPUT
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@cindex in-place
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@findex fftw_flops
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Numeric array pointer arguments (e.g. @code{double *})
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become @code{dimension(*), intent(out)} arrays of the same type, or
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@code{dimension(*), intent(in)} if they are pointers to constant data
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(e.g. @code{const int *}). There are a few exceptions where numeric
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pointers refer to scalar outputs (e.g. for @code{fftw_flops}), in which
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case they are @code{intent(out)} scalar arguments in Fortran too.
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For the new-array execute functions (@pxref{New-array Execute Functions}),
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the input arrays are declared @code{dimension(*), intent(inout)}, since
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they can be modified in the case of in-place or @code{FFTW_DESTROY_INPUT}
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transforms.
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@item
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@findex fftw_alloc_real
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@findex c_f_pointer
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Pointer @emph{return} values (e.g @code{double *}) become
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@code{type(C_PTR)}. (If they are pointers to arrays, as for
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@code{fftw_alloc_real}, you can convert them back to Fortran array
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pointers with the standard intrinsic function @code{c_f_pointer}.)
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@item
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@cindex guru interface
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@tindex fftw_iodim
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@tindex fftw_iodim64
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@cindex 64-bit architecture
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The @code{fftw_iodim} type in the guru interface (@pxref{Guru vector
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and transform sizes}) becomes @code{type(fftw_iodim)} in Fortran, a
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derived data type (the Fortran analogue of C's @code{struct}) with
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three @code{integer(C_INT)} components: @code{n}, @code{is}, and
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@code{os}, with the same meanings as in C. The @code{fftw_iodim64} type in the 64-bit guru interface (@pxref{64-bit Guru Interface}) is the same, except that its components are of type @code{integer(C_INTPTR_T)}.
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@item
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@ctindex C_FUNPTR
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Using the wisdom import/export functions from Fortran is a bit tricky,
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and is discussed in @ref{Accessing the wisdom API from Fortran}. In
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brief, the @code{FILE *} arguments map to @code{type(C_PTR)}, @code{const char *} to @code{character(C_CHAR), dimension(*), intent(in)} (null-terminated!), and the generic read-char/write-char functions map to @code{type(C_FUNPTR)}.
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@end itemize
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@cindex portability
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You may be wondering if you need to search-and-replace
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@code{real(kind(0.0d0))} (or whatever your favorite Fortran spelling
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of ``double precision'' is) with @code{real(C_DOUBLE)} everywhere in
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your program, and similarly for @code{complex} and @code{integer}
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types. The answer is no; you can still use your existing types. As
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long as these types match their C counterparts, things should work
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without a hitch. The worst that can happen, e.g. in the (unlikely)
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event of a system where @code{real(kind(0.0d0))} is different from
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@code{real(C_DOUBLE)}, is that the compiler will give you a
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type-mismatch error. That is, if you don't use the
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@code{iso_c_binding} kinds you need to accept at least the theoretical
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possibility of having to change your code in response to compiler
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errors on some future machine, but you don't need to worry about
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silently compiling incorrect code that yields runtime errors.
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@c -------------------------------------------------------
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@node Plan execution in Fortran, Allocating aligned memory in Fortran, FFTW Fortran type reference, Calling FFTW from Modern Fortran
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@section Plan execution in Fortran
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In C, in order to use a plan, one normally calls @code{fftw_execute},
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which executes the plan to perform the transform on the input/output
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arrays passed when the plan was created (@pxref{Using Plans}). The
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corresponding subroutine call in modern Fortran is:
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@example
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call fftw_execute(plan)
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@end example
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@findex fftw_execute
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However, we have had reports that this causes problems with some
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recent optimizing Fortran compilers. The problem is, because the
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input/output arrays are not passed as explicit arguments to
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@code{fftw_execute}, the semantics of Fortran (unlike C) allow the
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compiler to assume that the input/output arrays are not changed by
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@code{fftw_execute}. As a consequence, certain compilers end up
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repositioning the call to @code{fftw_execute}, assuming incorrectly
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that it does nothing to the arrays.
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There are various workarounds to this, but the safest and simplest
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thing is to not use @code{fftw_execute} in Fortran. Instead, use the
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functions described in @ref{New-array Execute Functions}, which take
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the input/output arrays as explicit arguments. For example, if the
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plan is for a complex-data DFT and was created for the arrays
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@code{in} and @code{out}, you would do:
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@example
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call fftw_execute_dft(plan, in, out)
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@end example
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@findex fftw_execute_dft
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There are a few things to be careful of, however:
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@itemize @bullet
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@item
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@findex fftw_execute_dft_r2c
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@findex fftw_execute_dft_c2r
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@findex fftw_execute_r2r
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You must use the correct type of execute function, matching the way
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the plan was created. Complex DFT plans should use
|
|
@code{fftw_execute_dft}, Real-input (r2c) DFT plans should use use
|
|
@code{fftw_execute_dft_r2c}, and real-output (c2r) DFT plans should
|
|
use @code{fftw_execute_dft_c2r}. The various r2r plans should use
|
|
@code{fftw_execute_r2r}. Fortunately, if you use the wrong one you
|
|
will get a compile-time type-mismatch error (unlike legacy Fortran).
|
|
|
|
@item
|
|
You should normally pass the same input/output arrays that were used when
|
|
creating the plan. This is always safe.
|
|
|
|
@item
|
|
@emph{If} you pass @emph{different} input/output arrays compared to
|
|
those used when creating the plan, you must abide by all the
|
|
restrictions of the new-array execute functions (@pxref{New-array
|
|
Execute Functions}). The most tricky of these is the
|
|
requirement that the new arrays have the same alignment as the
|
|
original arrays; the best (and possibly only) way to guarantee this
|
|
is to use the @samp{fftw_alloc} functions to allocate your arrays (@pxref{Allocating aligned memory in Fortran}). Alternatively, you can
|
|
use the @code{FFTW_UNALIGNED} flag when creating the
|
|
plan, in which case the plan does not depend on the alignment, but
|
|
this may sacrifice substantial performance on architectures (like x86)
|
|
with SIMD instructions (@pxref{SIMD alignment and fftw_malloc}).
|
|
@ctindex FFTW_UNALIGNED
|
|
|
|
@end itemize
|
|
|
|
@c -------------------------------------------------------
|
|
@node Allocating aligned memory in Fortran, Accessing the wisdom API from Fortran, Plan execution in Fortran, Calling FFTW from Modern Fortran
|
|
@section Allocating aligned memory in Fortran
|
|
|
|
@cindex alignment
|
|
@findex fftw_alloc_real
|
|
@findex fftw_alloc_complex
|
|
In order to obtain maximum performance in FFTW, you should store your
|
|
data in arrays that have been specially aligned in memory (@pxref{SIMD
|
|
alignment and fftw_malloc}). Enforcing alignment also permits you to
|
|
safely use the new-array execute functions (@pxref{New-array Execute
|
|
Functions}) to apply a given plan to more than one pair of in/out
|
|
arrays. Unfortunately, standard Fortran arrays do @emph{not} provide
|
|
any alignment guarantees. The @emph{only} way to allocate aligned
|
|
memory in standard Fortran is to allocate it with an external C
|
|
function, like the @code{fftw_alloc_real} and
|
|
@code{fftw_alloc_complex} functions. Fortunately, Fortran 2003 provides
|
|
a simple way to associate such allocated memory with a standard Fortran
|
|
array pointer that you can then use normally.
|
|
|
|
We therefore recommend allocating all your input/output arrays using
|
|
the following technique:
|
|
|
|
@enumerate
|
|
|
|
@item
|
|
Declare a @code{pointer}, @code{arr}, to your array of the desired type
|
|
and dimensions. For example, @code{real(C_DOUBLE), pointer :: a(:,:)}
|
|
for a 2d real array, or @code{complex(C_DOUBLE_COMPLEX), pointer ::
|
|
a(:,:,:)} for a 3d complex array.
|
|
|
|
@item
|
|
The number of elements to allocate must be an
|
|
@code{integer(C_SIZE_T)}. You can either declare a variable of this
|
|
type, e.g. @code{integer(C_SIZE_T) :: sz}, to store the number of
|
|
elements to allocate, or you can use the @code{int(..., C_SIZE_T)}
|
|
intrinsic function. e.g. set @code{sz = L * M * N} or use
|
|
@code{int(L * M * N, C_SIZE_T)} for an @threedims{L,M,N} array.
|
|
|
|
@item
|
|
Declare a @code{type(C_PTR) :: p} to hold the return value from
|
|
FFTW's allocation routine. Set @code{p = fftw_alloc_real(sz)} for a real array, or @code{p = fftw_alloc_complex(sz)} for a complex array.
|
|
|
|
@item
|
|
@findex c_f_pointer
|
|
Associate your pointer @code{arr} with the allocated memory @code{p}
|
|
using the standard @code{c_f_pointer} subroutine: @code{call
|
|
c_f_pointer(p, arr, [...dimensions...])}, where
|
|
@code{[...dimensions...])} are an array of the dimensions of the array
|
|
(in the usual Fortran order). e.g. @code{call c_f_pointer(p, arr,
|
|
[L,M,N])} for an @threedims{L,M,N} array. (Alternatively, you can
|
|
omit the dimensions argument if you specified the shape explicitly
|
|
when declaring @code{arr}.) You can now use @code{arr} as a usual
|
|
multidimensional array.
|
|
|
|
@item
|
|
When you are done using the array, deallocate the memory by @code{call
|
|
fftw_free(p)} on @code{p}.
|
|
|
|
@end enumerate
|
|
|
|
For example, here is how we would allocate an @twodims{L,M} 2d real array:
|
|
|
|
@example
|
|
real(C_DOUBLE), pointer :: arr(:,:)
|
|
type(C_PTR) :: p
|
|
p = fftw_alloc_real(int(L * M, C_SIZE_T))
|
|
call c_f_pointer(p, arr, [L,M])
|
|
@emph{...use arr and arr(i,j) as usual...}
|
|
call fftw_free(p)
|
|
@end example
|
|
|
|
and here is an @threedims{L,M,N} 3d complex array:
|
|
|
|
@example
|
|
complex(C_DOUBLE_COMPLEX), pointer :: arr(:,:,:)
|
|
type(C_PTR) :: p
|
|
p = fftw_alloc_complex(int(L * M * N, C_SIZE_T))
|
|
call c_f_pointer(p, arr, [L,M,N])
|
|
@emph{...use arr and arr(i,j,k) as usual...}
|
|
call fftw_free(p)
|
|
@end example
|
|
|
|
See @ref{Reversing array dimensions} for an example allocating a
|
|
single array and associating both real and complex array pointers with
|
|
it, for in-place real-to-complex transforms.
|
|
|
|
@c -------------------------------------------------------
|
|
@node Accessing the wisdom API from Fortran, Defining an FFTW module, Allocating aligned memory in Fortran, Calling FFTW from Modern Fortran
|
|
@section Accessing the wisdom API from Fortran
|
|
@cindex wisdom
|
|
@cindex saving plans to disk
|
|
|
|
As explained in @ref{Words of Wisdom-Saving Plans}, FFTW provides a
|
|
``wisdom'' API for saving plans to disk so that they can be recreated
|
|
quickly. The C API for exporting (@pxref{Wisdom Export}) and
|
|
importing (@pxref{Wisdom Import}) wisdom is somewhat tricky to use
|
|
from Fortran, however, because of differences in file I/O and string
|
|
types between C and Fortran.
|
|
|
|
@menu
|
|
* Wisdom File Export/Import from Fortran::
|
|
* Wisdom String Export/Import from Fortran::
|
|
* Wisdom Generic Export/Import from Fortran::
|
|
@end menu
|
|
|
|
@c =========>
|
|
@node Wisdom File Export/Import from Fortran, Wisdom String Export/Import from Fortran, Accessing the wisdom API from Fortran, Accessing the wisdom API from Fortran
|
|
@subsection Wisdom File Export/Import from Fortran
|
|
|
|
@findex fftw_import wisdom_from_filename
|
|
@findex fftw_export_wisdom_to_filename
|
|
The easiest way to export and import wisdom is to do so using
|
|
@code{fftw_export_wisdom_to_filename} and
|
|
@code{fftw_wisdom_from_filename}. The only trick is that these
|
|
require you to pass a C string, which is an array of type
|
|
@code{CHARACTER(C_CHAR)} that is terminated by @code{C_NULL_CHAR}.
|
|
You can call them like this:
|
|
|
|
@example
|
|
integer(C_INT) :: ret
|
|
ret = fftw_export_wisdom_to_filename(C_CHAR_'my_wisdom.dat' // C_NULL_CHAR)
|
|
if (ret .eq. 0) stop 'error exporting wisdom to file'
|
|
ret = fftw_import_wisdom_from_filename(C_CHAR_'my_wisdom.dat' // C_NULL_CHAR)
|
|
if (ret .eq. 0) stop 'error importing wisdom from file'
|
|
@end example
|
|
|
|
Note that prepending @samp{C_CHAR_} is needed to specify that the
|
|
literal string is of kind @code{C_CHAR}, and we null-terminate the
|
|
string by appending @samp{// C_NULL_CHAR}. These functions return an
|
|
@code{integer(C_INT)} (@code{ret}) which is @code{0} if an error
|
|
occurred during export/import and nonzero otherwise.
|
|
|
|
It is also possible to use the lower-level routines
|
|
@code{fftw_export_wisdom_to_file} and
|
|
@code{fftw_import_wisdom_from_file}, which accept parameters of the C
|
|
type @code{FILE*}, expressed in Fortran as @code{type(C_PTR)}.
|
|
However, you are then responsible for creating the @code{FILE*}
|
|
yourself. You can do this by using @code{iso_c_binding} to define
|
|
Fortran intefaces for the C library functions @code{fopen} and
|
|
@code{fclose}, which is a bit strange in Fortran but workable.
|
|
|
|
@c =========>
|
|
@node Wisdom String Export/Import from Fortran, Wisdom Generic Export/Import from Fortran, Wisdom File Export/Import from Fortran, Accessing the wisdom API from Fortran
|
|
@subsection Wisdom String Export/Import from Fortran
|
|
|
|
@findex fftw_export_wisdom_to_string
|
|
Dealing with FFTW's C string export/import is a bit more painful. In
|
|
particular, the @code{fftw_export_wisdom_to_string} function requires
|
|
you to deal with a dynamically allocated C string. To get its length,
|
|
you must define an interface to the C @code{strlen} function, and to
|
|
deallocate it you must define an interface to C @code{free}:
|
|
|
|
@example
|
|
use, intrinsic :: iso_c_binding
|
|
interface
|
|
integer(C_INT) function strlen(s) bind(C, name='strlen')
|
|
import
|
|
type(C_PTR), value :: s
|
|
end function strlen
|
|
subroutine free(p) bind(C, name='free')
|
|
import
|
|
type(C_PTR), value :: p
|
|
end subroutine free
|
|
end interface
|
|
@end example
|
|
|
|
Given these definitions, you can then export wisdom to a Fortran
|
|
character array:
|
|
|
|
@example
|
|
character(C_CHAR), pointer :: s(:)
|
|
integer(C_SIZE_T) :: slen
|
|
type(C_PTR) :: p
|
|
p = fftw_export_wisdom_to_string()
|
|
if (.not. c_associated(p)) stop 'error exporting wisdom'
|
|
slen = strlen(p)
|
|
call c_f_pointer(p, s, [slen+1])
|
|
...
|
|
call free(p)
|
|
@end example
|
|
@findex c_associated
|
|
@findex c_f_pointer
|
|
|
|
Note that @code{slen} is the length of the C string, but the length of
|
|
the array is @code{slen+1} because it includes the terminating null
|
|
character. (You can omit the @samp{+1} if you don't want Fortran to
|
|
know about the null character.) The standard @code{c_associated} function
|
|
checks whether @code{p} is a null pointer, which is returned by
|
|
@code{fftw_export_wisdom_to_string} if there was an error.
|
|
|
|
@findex fftw_import_wisdom_from_string
|
|
To import wisdom from a string, use
|
|
@code{fftw_import_wisdom_from_string} as usual; note that the argument
|
|
of this function must be a @code{character(C_CHAR)} that is terminated
|
|
by the @code{C_NULL_CHAR} character, like the @code{s} array above.
|
|
|
|
@c =========>
|
|
@node Wisdom Generic Export/Import from Fortran, , Wisdom String Export/Import from Fortran, Accessing the wisdom API from Fortran
|
|
@subsection Wisdom Generic Export/Import from Fortran
|
|
|
|
The most generic wisdom export/import functions allow you to provide
|
|
an arbitrary callback function to read/write one character at a time
|
|
in any way you want. However, your callback function must be written
|
|
in a special way, using the @code{bind(C)} attribute to be passed to a
|
|
C interface.
|
|
|
|
@findex fftw_export_wisdom
|
|
In particular, to call the generic wisdom export function
|
|
@code{fftw_export_wisdom}, you would write a callback subroutine of the form:
|
|
|
|
@example
|
|
subroutine my_write_char(c, p) bind(C)
|
|
use, intrinsic :: iso_c_binding
|
|
character(C_CHAR), value :: c
|
|
type(C_PTR), value :: p
|
|
@emph{...write c...}
|
|
end subroutine my_write_char
|
|
@end example
|
|
|
|
Given such a subroutine (along with the corresponding interface definition), you could then export wisdom using:
|
|
|
|
@findex c_funloc
|
|
@example
|
|
call fftw_export_wisdom(c_funloc(my_write_char), p)
|
|
@end example
|
|
|
|
@findex c_loc
|
|
@findex c_f_pointer
|
|
The standard @code{c_funloc} intrinsic converts a Fortran
|
|
@code{bind(C)} subroutine into a C function pointer. The parameter
|
|
@code{p} is a @code{type(C_PTR)} to any arbitrary data that you want
|
|
to pass to @code{my_write_char} (or @code{C_NULL_PTR} if none). (Note
|
|
that you can get a C pointer to Fortran data using the intrinsic
|
|
@code{c_loc}, and convert it back to a Fortran pointer in
|
|
@code{my_write_char} using @code{c_f_pointer}.)
|
|
|
|
Similarly, to use the generic @code{fftw_import_wisdom}, you would
|
|
define a callback function of the form:
|
|
|
|
@findex fftw_import_wisdom
|
|
@example
|
|
integer(C_INT) function my_read_char(p) bind(C)
|
|
use, intrinsic :: iso_c_binding
|
|
type(C_PTR), value :: p
|
|
character :: c
|
|
@emph{...read a character c...}
|
|
my_read_char = ichar(c, C_INT)
|
|
end function my_read_char
|
|
|
|
....
|
|
|
|
integer(C_INT) :: ret
|
|
ret = fftw_import_wisdom(c_funloc(my_read_char), p)
|
|
if (ret .eq. 0) stop 'error importing wisdom'
|
|
@end example
|
|
|
|
Your function can return @code{-1} if the end of the input is reached.
|
|
Again, @code{p} is an arbitrary @code{type(C_PTR} that is passed
|
|
through to your function. @code{fftw_import_wisdom} returns @code{0}
|
|
if an error occurred and nonzero otherwise.
|
|
|
|
@c -------------------------------------------------------
|
|
@node Defining an FFTW module, , Accessing the wisdom API from Fortran, Calling FFTW from Modern Fortran
|
|
@section Defining an FFTW module
|
|
|
|
Rather than using the @code{include} statement to include the
|
|
@code{fftw3.f03} interface file in any subroutine where you want to
|
|
use FFTW, you might prefer to define an FFTW Fortran module. FFTW
|
|
does not install itself as a module, primarily because
|
|
@code{fftw3.f03} can be shared between different Fortran compilers while
|
|
modules (in general) cannot. However, it is trivial to define your
|
|
own FFTW module if you want. Just create a file containing:
|
|
|
|
@example
|
|
module FFTW3
|
|
use, intrinsic :: iso_c_binding
|
|
include 'fftw3.f03'
|
|
end module
|
|
@end example
|
|
|
|
Compile this file into a module as usual for your compiler (e.g. with
|
|
@code{gfortran -c} you will get a file @code{fftw3.mod}). Now,
|
|
instead of @code{include 'fftw3.f03'}, whenever you want to use FFTW
|
|
routines you can just do:
|
|
|
|
@example
|
|
use FFTW3
|
|
@end example
|
|
|
|
as usual for Fortran modules. (You still need to link to the FFTW
|
|
library, of course.)
|