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cannam@127:A multi-dimensional array whose size is declared at compile time in C cannam@127: is already in row-major order. You don’t have to do anything cannam@127: special to transform it. For example: cannam@127:
cannam@127:{
cannam@127: fftw_complex data[N0][N1][N2];
cannam@127: fftw_plan plan;
cannam@127: ...
cannam@127: plan = fftw_plan_dft_3d(N0, N1, N2, &data[0][0][0], &data[0][0][0],
cannam@127: FFTW_FORWARD, FFTW_ESTIMATE);
cannam@127: ...
cannam@127: }
cannam@127: This will plan a 3d in-place transform of size N0 x N1 x N2.
cannam@127: Notice how we took the address of the zero-th element to pass to the
cannam@127: planner (we could also have used a typecast).
cannam@127:
However, we tend to discourage users from declaring their
cannam@127: arrays in this way, for two reasons. First, this allocates the array
cannam@127: on the stack (“automatic” storage), which has a very limited size on
cannam@127: most operating systems (declaring an array with more than a few
cannam@127: thousand elements will often cause a crash). (You can get around this
cannam@127: limitation on many systems by declaring the array as
cannam@127: static and/or global, but that has its own drawbacks.)
cannam@127: Second, it may not optimally align the array for use with a SIMD
cannam@127: FFTW (see SIMD alignment and fftw_malloc). Instead, we recommend
cannam@127: using fftw_malloc, as described below.
cannam@127: