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fftw3: Fixed-size Arrays in C

 
 3.2.3 Fixed-size Arrays in C
 ----------------------------
 
 A multi-dimensional array whose size is declared at compile time in C is
 _already_ in row-major order.  You don't have to do anything special to
 transform it.  For example:
 
      {
           fftw_complex data[N0][N1][N2];
           fftw_plan plan;
           ...
           plan = fftw_plan_dft_3d(N0, N1, N2, &data[0][0][0], &data[0][0][0],
                                   FFTW_FORWARD, FFTW_ESTIMATE);
           ...
      }
 
    This will plan a 3d in-place transform of size 'N0 x N1 x N2'.
 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 in
 this way, for two reasons.  First, this allocates the array on the stack
 ("automatic" storage), 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).  (You can get around this limitation on many
 systems by declaring the array as 'static' and/or global, but that has
 its own drawbacks.)  Second, it may not optimally align the array for
 use with a SIMD FFTW (SIMD alignment and fftw_malloc).  Instead,
 we recommend using 'fftw_malloc', as described below.
 

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