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Nearly all of the FFTW functions have Fortran-callable equivalents.
The name of the legacy Fortran routine is the same as that of the
corresponding C routine, but with the ‘fftw_’ prefix replaced by
‘dfftw_’.^{9} The single and long-double precision
versions use ‘sfftw_’ and ‘lfftw_’, respectively, instead of
‘fftwf_’ and ‘fftwl_’; quadruple precision (real*16
)
is available on some systems as ‘fftwq_’ (see Precision).
(Note that long double
on x86 hardware is usually at most
80-bit extended precision, not quadruple precision.)
For the most part, all of the arguments to the functions are the same, with the following exceptions:
plan
variables (what would be of type fftw_plan
in C),
must be declared as a type that is at least as big as a pointer
(address) on your machine. We recommend using integer*8
everywhere,
since this should always be big enough.
fftw_plan_dft
) is
converted into a subroutine. The return value is converted into
an additional first parameter of this subroutine.^{10}
fftw_malloc
dynamic-allocation routine.
If you want to exploit the SIMD FFTW (see SIMD alignment and fftw_malloc), you’ll
need to figure out some other way to ensure that your arrays are at
least 16-byte aligned.
fftw_iodim
structure from the guru interface (see Guru vector and transform sizes) must be split into separate arguments. In particular, any
fftw_iodim
array arguments in the C guru interface become three
integer array arguments (n
, is
, and os
) in the
Fortran guru interface, all of whose lengths should be equal to the
corresponding rank
argument.
kind
array parameter, so the kind
array
of that routine should be in the reverse of the order of the iodim
arrays (see above).
In general, you should take care to use Fortran data types that
correspond to (i.e. are the same size as) the C types used by FFTW.
In practice, this correspondence is usually straightforward
(i.e. integer
corresponds to int
, real
corresponds to float
, etcetera). The native Fortran
double/single-precision complex type should be compatible with
fftw_complex
/fftwf_complex
. Such simple correspondences
are assumed in the examples below.
Technically, Fortran 77 identifiers are not allowed to have more than 6 characters, nor may they contain underscores. Any compiler that enforces this limitation doesn’t deserve to link to FFTW.
The reason for this is that some Fortran implementations seem to have trouble with C function return values, and vice versa.
Next: FFTW Constants in Fortran, Previous: Calling FFTW from Legacy Fortran, Up: Calling FFTW from Legacy Fortran [Contents][Index]