cannam@95: @node Calling FFTW from Legacy Fortran, Upgrading from FFTW version 2, Calling FFTW from Modern Fortran, Top cannam@95: @chapter Calling FFTW from Legacy Fortran cannam@95: @cindex Fortran interface cannam@95: cannam@95: This chapter describes the interface to FFTW callable by Fortran code cannam@95: in older compilers not supporting the Fortran 2003 C interoperability cannam@95: features (@pxref{Calling FFTW from Modern Fortran}). This interface cannam@95: has the major disadvantage that it is not type-checked, so if you cannam@95: mistake the argument types or ordering then your program will not have cannam@95: any compiler errors, and will likely crash at runtime. So, greater cannam@95: care is needed. Also, technically interfacing older Fortran versions cannam@95: to C is nonstandard, but in practice we have found that the techniques cannam@95: used in this chapter have worked with all known Fortran compilers for cannam@95: many years. cannam@95: cannam@95: The legacy Fortran interface differs from the C interface only in the cannam@95: prefix (@samp{dfftw_} instead of @samp{fftw_} in double precision) and cannam@95: a few other minor details. This Fortran interface is included in the cannam@95: FFTW libraries by default, unless a Fortran compiler isn't found on cannam@95: your system or @code{--disable-fortran} is included in the cannam@95: @code{configure} flags. We assume here that the reader is already cannam@95: familiar with the usage of FFTW in C, as described elsewhere in this cannam@95: manual. cannam@95: cannam@95: The MPI parallel interface to FFTW is @emph{not} currently available cannam@95: to legacy Fortran. cannam@95: cannam@95: @menu cannam@95: * Fortran-interface routines:: cannam@95: * FFTW Constants in Fortran:: cannam@95: * FFTW Execution in Fortran:: cannam@95: * Fortran Examples:: cannam@95: * Wisdom of Fortran?:: cannam@95: @end menu cannam@95: cannam@95: @c ------------------------------------------------------- cannam@95: @node Fortran-interface routines, FFTW Constants in Fortran, Calling FFTW from Legacy Fortran, Calling FFTW from Legacy Fortran cannam@95: @section Fortran-interface routines cannam@95: cannam@95: Nearly all of the FFTW functions have Fortran-callable equivalents. cannam@95: The name of the legacy Fortran routine is the same as that of the cannam@95: corresponding C routine, but with the @samp{fftw_} prefix replaced by cannam@95: @samp{dfftw_}.@footnote{Technically, Fortran 77 identifiers are not cannam@95: allowed to have more than 6 characters, nor may they contain cannam@95: underscores. Any compiler that enforces this limitation doesn't cannam@95: deserve to link to FFTW.} The single and long-double precision cannam@95: versions use @samp{sfftw_} and @samp{lfftw_}, respectively, instead of cannam@95: @samp{fftwf_} and @samp{fftwl_}; quadruple precision (@code{real*16}) cannam@95: is available on some systems as @samp{fftwq_} (@pxref{Precision}). cannam@95: (Note that @code{long double} on x86 hardware is usually at most cannam@95: 80-bit extended precision, @emph{not} quadruple precision.) cannam@95: cannam@95: For the most part, all of the arguments to the functions are the same, cannam@95: with the following exceptions: cannam@95: cannam@95: @itemize @bullet cannam@95: cannam@95: @item cannam@95: @code{plan} variables (what would be of type @code{fftw_plan} in C), cannam@95: must be declared as a type that is at least as big as a pointer cannam@95: (address) on your machine. We recommend using @code{integer*8} everywhere, cannam@95: since this should always be big enough. cannam@95: @cindex portability cannam@95: cannam@95: @item cannam@95: Any function that returns a value (e.g. @code{fftw_plan_dft}) is cannam@95: converted into a @emph{subroutine}. The return value is converted into cannam@95: an additional @emph{first} parameter of this subroutine.@footnote{The cannam@95: reason for this is that some Fortran implementations seem to have cannam@95: trouble with C function return values, and vice versa.} cannam@95: cannam@95: @item cannam@95: @cindex column-major cannam@95: The Fortran routines expect multi-dimensional arrays to be in cannam@95: @emph{column-major} order, which is the ordinary format of Fortran cannam@95: arrays (@pxref{Multi-dimensional Array Format}). They do this cannam@95: transparently and costlessly simply by reversing the order of the cannam@95: dimensions passed to FFTW, but this has one important consequence for cannam@95: multi-dimensional real-complex transforms, discussed below. cannam@95: cannam@95: @item cannam@95: Wisdom import and export is somewhat more tricky because one cannot cannam@95: easily pass files or strings between C and Fortran; see @ref{Wisdom of cannam@95: Fortran?}. cannam@95: cannam@95: @item cannam@95: Legacy Fortran cannot use the @code{fftw_malloc} dynamic-allocation routine. cannam@95: If you want to exploit the SIMD FFTW (@pxref{SIMD alignment and fftw_malloc}), you'll cannam@95: need to figure out some other way to ensure that your arrays are at cannam@95: least 16-byte aligned. cannam@95: cannam@95: @item cannam@95: @tindex fftw_iodim cannam@95: @cindex guru interface cannam@95: Since Fortran 77 does not have data structures, the @code{fftw_iodim} cannam@95: structure from the guru interface (@pxref{Guru vector and transform cannam@95: sizes}) must be split into separate arguments. In particular, any cannam@95: @code{fftw_iodim} array arguments in the C guru interface become three cannam@95: integer array arguments (@code{n}, @code{is}, and @code{os}) in the cannam@95: Fortran guru interface, all of whose lengths should be equal to the cannam@95: corresponding @code{rank} argument. cannam@95: cannam@95: @item cannam@95: The guru planner interface in Fortran does @emph{not} do any automatic cannam@95: translation between column-major and row-major; you are responsible cannam@95: for setting the strides etcetera to correspond to your Fortran arrays. cannam@95: However, as a slight bug that we are preserving for backwards cannam@95: compatibility, the @samp{plan_guru_r2r} in Fortran @emph{does} reverse the cannam@95: order of its @code{kind} array parameter, so the @code{kind} array cannam@95: of that routine should be in the reverse of the order of the iodim cannam@95: arrays (see above). cannam@95: cannam@95: @end itemize cannam@95: cannam@95: In general, you should take care to use Fortran data types that cannam@95: correspond to (i.e. are the same size as) the C types used by FFTW. cannam@95: In practice, this correspondence is usually straightforward cannam@95: (i.e. @code{integer} corresponds to @code{int}, @code{real} cannam@95: corresponds to @code{float}, etcetera). The native Fortran cannam@95: double/single-precision complex type should be compatible with cannam@95: @code{fftw_complex}/@code{fftwf_complex}. Such simple correspondences cannam@95: are assumed in the examples below. cannam@95: @cindex portability cannam@95: cannam@95: @c ------------------------------------------------------- cannam@95: @node FFTW Constants in Fortran, FFTW Execution in Fortran, Fortran-interface routines, Calling FFTW from Legacy Fortran cannam@95: @section FFTW Constants in Fortran cannam@95: cannam@95: When creating plans in FFTW, a number of constants are used to specify cannam@95: options, such as @code{FFTW_MEASURE} or @code{FFTW_ESTIMATE}. The cannam@95: same constants must be used with the wrapper routines, but of course the cannam@95: C header files where the constants are defined can't be incorporated cannam@95: directly into Fortran code. cannam@95: cannam@95: Instead, we have placed Fortran equivalents of the FFTW constant cannam@95: definitions in the file @code{fftw3.f}, which can be found in the same cannam@95: directory as @code{fftw3.h}. If your Fortran compiler supports a cannam@95: preprocessor of some sort, you should be able to @code{include} or cannam@95: @code{#include} this file; otherwise, you can paste it directly into cannam@95: your code. cannam@95: cannam@95: @cindex flags cannam@95: In C, you combine different flags (like @code{FFTW_PRESERVE_INPUT} and cannam@95: @code{FFTW_MEASURE}) using the @samp{@code{|}} operator; in Fortran cannam@95: you should just use @samp{@code{+}}. (Take care not to add in the cannam@95: same flag more than once, though. Alternatively, you can use the cannam@95: @code{ior} intrinsic function standardized in Fortran 95.) cannam@95: cannam@95: @c ------------------------------------------------------- cannam@95: @node FFTW Execution in Fortran, Fortran Examples, FFTW Constants in Fortran, Calling FFTW from Legacy Fortran cannam@95: @section FFTW Execution in Fortran cannam@95: cannam@95: In C, in order to use a plan, one normally calls @code{fftw_execute}, cannam@95: which executes the plan to perform the transform on the input/output cannam@95: arrays passed when the plan was created (@pxref{Using Plans}). The cannam@95: corresponding subroutine call in legacy Fortran is: cannam@95: @example cannam@95: call dfftw_execute(plan) cannam@95: @end example cannam@95: @findex dfftw_execute cannam@95: cannam@95: However, we have had reports that this causes problems with some cannam@95: recent optimizing Fortran compilers. The problem is, because the cannam@95: input/output arrays are not passed as explicit arguments to cannam@95: @code{dfftw_execute}, the semantics of Fortran (unlike C) allow the cannam@95: compiler to assume that the input/output arrays are not changed by cannam@95: @code{dfftw_execute}. As a consequence, certain compilers end up cannam@95: optimizing out or repositioning the call to @code{dfftw_execute}, cannam@95: assuming incorrectly that it does nothing. cannam@95: cannam@95: There are various workarounds to this, but the safest and simplest cannam@95: thing is to not use @code{dfftw_execute} in Fortran. Instead, use the cannam@95: functions described in @ref{New-array Execute Functions}, which take cannam@95: the input/output arrays as explicit arguments. For example, if the cannam@95: plan is for a complex-data DFT and was created for the arrays cannam@95: @code{in} and @code{out}, you would do: cannam@95: @example cannam@95: call dfftw_execute_dft(plan, in, out) cannam@95: @end example cannam@95: @findex dfftw_execute_dft cannam@95: cannam@95: There are a few things to be careful of, however: cannam@95: cannam@95: @itemize @bullet cannam@95: cannam@95: @item cannam@95: You must use the correct type of execute function, matching the way cannam@95: the plan was created. Complex DFT plans should use cannam@95: @code{dfftw_execute_dft}, Real-input (r2c) DFT plans should use use cannam@95: @code{dfftw_execute_dft_r2c}, and real-output (c2r) DFT plans should cannam@95: use @code{dfftw_execute_dft_c2r}. The various r2r plans should use cannam@95: @code{dfftw_execute_r2r}. cannam@95: cannam@95: @item cannam@95: You should normally pass the same input/output arrays that were used when cannam@95: creating the plan. This is always safe. cannam@95: cannam@95: @item cannam@95: @emph{If} you pass @emph{different} input/output arrays compared to cannam@95: those used when creating the plan, you must abide by all the cannam@95: restrictions of the new-array execute functions (@pxref{New-array cannam@95: Execute Functions}). The most difficult of these, in Fortran, is the cannam@95: requirement that the new arrays have the same alignment as the cannam@95: original arrays, because there seems to be no way in legacy Fortran to obtain cannam@95: guaranteed-aligned arrays (analogous to @code{fftw_malloc} in C). You cannam@95: can, of course, use the @code{FFTW_UNALIGNED} flag when creating the cannam@95: plan, in which case the plan does not depend on the alignment, but cannam@95: this may sacrifice substantial performance on architectures (like x86) cannam@95: with SIMD instructions (@pxref{SIMD alignment and fftw_malloc}). cannam@95: @ctindex FFTW_UNALIGNED cannam@95: cannam@95: @end itemize cannam@95: cannam@95: @c ------------------------------------------------------- cannam@95: @node Fortran Examples, Wisdom of Fortran?, FFTW Execution in Fortran, Calling FFTW from Legacy Fortran cannam@95: @section Fortran Examples cannam@95: cannam@95: In C, you might have something like the following to transform a cannam@95: one-dimensional complex array: cannam@95: cannam@95: @example cannam@95: fftw_complex in[N], out[N]; cannam@95: fftw_plan plan; cannam@95: cannam@95: plan = fftw_plan_dft_1d(N,in,out,FFTW_FORWARD,FFTW_ESTIMATE); cannam@95: fftw_execute(plan); cannam@95: fftw_destroy_plan(plan); cannam@95: @end example cannam@95: cannam@95: In Fortran, you would use the following to accomplish the same thing: cannam@95: cannam@95: @example cannam@95: double complex in, out cannam@95: dimension in(N), out(N) cannam@95: integer*8 plan cannam@95: cannam@95: call dfftw_plan_dft_1d(plan,N,in,out,FFTW_FORWARD,FFTW_ESTIMATE) cannam@95: call dfftw_execute_dft(plan, in, out) cannam@95: call dfftw_destroy_plan(plan) cannam@95: @end example cannam@95: @findex dfftw_plan_dft_1d cannam@95: @findex dfftw_execute_dft cannam@95: @findex dfftw_destroy_plan cannam@95: cannam@95: Notice how all routines are called as Fortran subroutines, and the cannam@95: plan is returned via the first argument to @code{dfftw_plan_dft_1d}. cannam@95: Notice also that we changed @code{fftw_execute} to cannam@95: @code{dfftw_execute_dft} (@pxref{FFTW Execution in Fortran}). To do cannam@95: the same thing, but using 8 threads in parallel (@pxref{Multi-threaded cannam@95: FFTW}), you would simply prefix these calls with: cannam@95: cannam@95: @example cannam@95: integer iret cannam@95: call dfftw_init_threads(iret) cannam@95: call dfftw_plan_with_nthreads(8) cannam@95: @end example cannam@95: @findex dfftw_init_threads cannam@95: @findex dfftw_plan_with_nthreads cannam@95: cannam@95: (You might want to check the value of @code{iret}: if it is zero, it cannam@95: indicates an unlikely error during thread initialization.) cannam@95: cannam@95: To transform a three-dimensional array in-place with C, you might do: cannam@95: cannam@95: @example cannam@95: fftw_complex arr[L][M][N]; cannam@95: fftw_plan plan; cannam@95: cannam@95: plan = fftw_plan_dft_3d(L,M,N, arr,arr, cannam@95: FFTW_FORWARD, FFTW_ESTIMATE); cannam@95: fftw_execute(plan); cannam@95: fftw_destroy_plan(plan); cannam@95: @end example cannam@95: cannam@95: In Fortran, you would use this instead: cannam@95: cannam@95: @example cannam@95: double complex arr cannam@95: dimension arr(L,M,N) cannam@95: integer*8 plan cannam@95: cannam@95: call dfftw_plan_dft_3d(plan, L,M,N, arr,arr, cannam@95: & FFTW_FORWARD, FFTW_ESTIMATE) cannam@95: call dfftw_execute_dft(plan, arr, arr) cannam@95: call dfftw_destroy_plan(plan) cannam@95: @end example cannam@95: @findex dfftw_plan_dft_3d cannam@95: cannam@95: Note that we pass the array dimensions in the ``natural'' order in both C cannam@95: and Fortran. cannam@95: cannam@95: To transform a one-dimensional real array in Fortran, you might do: cannam@95: cannam@95: @example cannam@95: double precision in cannam@95: dimension in(N) cannam@95: double complex out cannam@95: dimension out(N/2 + 1) cannam@95: integer*8 plan cannam@95: cannam@95: call dfftw_plan_dft_r2c_1d(plan,N,in,out,FFTW_ESTIMATE) cannam@95: call dfftw_execute_dft_r2c(plan, in, out) cannam@95: call dfftw_destroy_plan(plan) cannam@95: @end example cannam@95: @findex dfftw_plan_dft_r2c_1d cannam@95: @findex dfftw_execute_dft_r2c cannam@95: cannam@95: To transform a two-dimensional real array, out of place, you might use cannam@95: the following: cannam@95: cannam@95: @example cannam@95: double precision in cannam@95: dimension in(M,N) cannam@95: double complex out cannam@95: dimension out(M/2 + 1, N) cannam@95: integer*8 plan cannam@95: cannam@95: call dfftw_plan_dft_r2c_2d(plan,M,N,in,out,FFTW_ESTIMATE) cannam@95: call dfftw_execute_dft_r2c(plan, in, out) cannam@95: call dfftw_destroy_plan(plan) cannam@95: @end example cannam@95: @findex dfftw_plan_dft_r2c_2d cannam@95: cannam@95: @strong{Important:} Notice that it is the @emph{first} dimension of the cannam@95: complex output array that is cut in half in Fortran, rather than the cannam@95: last dimension as in C. This is a consequence of the interface routines cannam@95: reversing the order of the array dimensions passed to FFTW so that the cannam@95: Fortran program can use its ordinary column-major order. cannam@95: @cindex column-major cannam@95: @cindex r2c/c2r multi-dimensional array format cannam@95: cannam@95: @c ------------------------------------------------------- cannam@95: @node Wisdom of Fortran?, , Fortran Examples, Calling FFTW from Legacy Fortran cannam@95: @section Wisdom of Fortran? cannam@95: cannam@95: In this section, we discuss how one can import/export FFTW wisdom cannam@95: (saved plans) to/from a Fortran program; we assume that the reader is cannam@95: already familiar with wisdom, as described in @ref{Words of cannam@95: Wisdom-Saving Plans}. cannam@95: cannam@95: @cindex portability cannam@95: The basic problem is that is difficult to (portably) pass files and cannam@95: strings between Fortran and C, so we cannot provide a direct Fortran cannam@95: equivalent to the @code{fftw_export_wisdom_to_file}, etcetera, cannam@95: functions. Fortran interfaces @emph{are} provided for the functions cannam@95: that do not take file/string arguments, however: cannam@95: @code{dfftw_import_system_wisdom}, @code{dfftw_import_wisdom}, cannam@95: @code{dfftw_export_wisdom}, and @code{dfftw_forget_wisdom}. cannam@95: @findex dfftw_import_system_wisdom cannam@95: @findex dfftw_import_wisdom cannam@95: @findex dfftw_export_wisdom cannam@95: @findex dfftw_forget_wisdom cannam@95: cannam@95: cannam@95: So, for example, to import the system-wide wisdom, you would do: cannam@95: cannam@95: @example cannam@95: integer isuccess cannam@95: call dfftw_import_system_wisdom(isuccess) cannam@95: @end example cannam@95: cannam@95: As usual, the C return value is turned into a first parameter; cannam@95: @code{isuccess} is non-zero on success and zero on failure (e.g. if cannam@95: there is no system wisdom installed). cannam@95: cannam@95: If you want to import/export wisdom from/to an arbitrary file or cannam@95: elsewhere, you can employ the generic @code{dfftw_import_wisdom} and cannam@95: @code{dfftw_export_wisdom} functions, for which you must supply a cannam@95: subroutine to read/write one character at a time. The FFTW package cannam@95: contains an example file @code{doc/f77_wisdom.f} demonstrating how to cannam@95: implement @code{import_wisdom_from_file} and cannam@95: @code{export_wisdom_to_file} subroutines in this way. (These routines cannam@95: cannot be compiled into the FFTW library itself, lest all FFTW-using cannam@95: programs be required to link with the Fortran I/O library.)