Question

I’m trying to write a simple FFTW routine (version 2), and I think I’ve just about got the bones down, but I’m running into a persistent segfault when I call the fftwnd_one function (I’m doing a one dimensional transform, but I’m using the n-dimensional code for modularity’s sake). However, I believe my problem to be in the plan creation; could anyone offer some insight as to what’s wrong with this code? I’d greatly appreciate it if so – thank you!

For reference, the function that I’m working with here is sin(x). I realize that not all of the mathematical operations have been implemented, I’m just trying to get the FFTW2 library working first, then I can make the data useful.

#include <stdio.h>
#include <fftw.h>
#include <math.h>
#include <complex.h>

int main(){
  int i;
  fftw_complex in[8];
  fftwnd_plan p;
  const int *n;
  int temp = (int)pow(2, 2)*pow(3,2)*pow(5,1)*pow(7,1)*pow(11,1)*pow(13,0);
  n = &temp;

  in[0].re = 0;
  in[1].re = (sqrt(2)/2);
  in[2].re = 1;
  in[3].re = (sqrt(2)/2);
  in[4].re = 0;
  in[5].re = -(sqrt(2)/2);
  in[6].re = -1;
  in[7].re = -(sqrt(2)/2);

  for(i = 0; i < 8; i++){
    (in[i]).im = 0;
  }

  p = fftwnd_create_plan(8, n, FFTW_FORWARD, FFTW_ESIMATE | FFTW_IN_PLACE);
  fftwnd_one(p, &in[0], NULL);
  fftwnd_destroy_plan(p);

  printf("Sin\n");
  for(i = 0; i < 8; i++){
    printf("%d\n", n[i]);
  }
  return 0;
}

Answer 1

The first two parameters to fftwnd_create_plan look completely wrong. It seems you just want to create a plan for 1D FFT with size 8 (which raises the question as to why you’re using fftwnd_create_plan if you only need a 1D FFT ?). The call should be something like:

const int n = 8;
p = fftwnd_create_plan(1,   // rank = 1, i.e. 1D
                       &n,  // size of first (and only) dimension = n = 8
                       FFTW_FORWARD,
                       FFTW_ESIMATE | FFTW_IN_PLACE);