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3.2.3 Static Arrays in C

Multi-dimensional arrays declared statically (that is, at compile time, not necessarily with the static keyword) in C are already in row-major order. You don't have to do anything special to transform them. For example:

     fftw_complex data[NX][NY][NZ];
     fftw_plan plan;
     plan = fftw_plan_dft_3d(NX, NY, NZ, &data[0][0][0], &data[0][0][0],
                             FFTW_FORWARD, FFTW_ESTIMATE);

This will plan a 3d in-place transform of size NX x NY x NZ. Notice how we took the address of the zero-th element to pass to the planner (we could also have used a typecast).

However, we tend to discourage users from declaring their arrays statically in this way, for two reasons. First, this allocates the array on the stack, which has a very limited size on most operating systems (declaring an array with more than a few thousand elements will often cause a crash). Second, it may not optimally align the array if you link with a SIMD FFTW (see SIMD alignment and fftw_malloc). Instead, we recommend using fftw_malloc, as described below.