When compiled in 64-bit mode on a 64-bit architecture (where addresses
are 64 bits wide), FFTW uses 64-bit quantities internally for all
transform sizes, strides, and so on—you don't have to do anything
special to exploit this. However, in the ordinary FFTW interfaces,
you specify the transform size by an `int`

quantity, which is
normally only 32 bits wide. This means that, even though FFTW is
using 64-bit sizes internally, you cannot specify a single transform
dimension larger than
2^{31}−1numbers.

We expect that few users will require transforms larger than this, but,
for those who do, we provide a 64-bit version of the guru interface in
which all sizes are specified as integers of type `ptrdiff_t`

instead of `int`

. (`ptrdiff_t`

is a signed integer type
defined by the C standard to be wide enough to represent address
differences, and thus must be at least 64 bits wide on a 64-bit
machine.) We stress that there is *no performance advantage* to
using this interface—the same internal FFTW code is employed
regardless—and it is only necessary if you want to specify very
large transform sizes.

In particular, the 64-bit guru interface is a set of planner routines
that are exactly the same as the guru planner routines, except that
they are named with ‘`guru64`’ instead of ‘`guru`’ and they take
arguments of type `fftw_iodim64`

instead of `fftw_iodim`

.
For example, instead of `fftw_plan_guru_dft`

, we have
`fftw_plan_guru64_dft`

.

fftw_plan fftw_plan_guru64_dft( int rank, const fftw_iodim64 *dims, int howmany_rank, const fftw_iodim64 *howmany_dims, fftw_complex *in, fftw_complex *out, int sign, unsigned flags);

The `fftw_iodim64`

type is similar to `fftw_iodim`

, with the
same interpretation, except that it uses type `ptrdiff_t`

instead
of type `int`

.

typedef struct { ptrdiff_t n; ptrdiff_t is; ptrdiff_t os; } fftw_iodim64;

Every other ‘`fftw_plan_guru`’ function also has a
‘`fftw_plan_guru64`’ equivalent, but we do not repeat their
documentation here since they are identical to the 32-bit versions
except as noted above.