test.c

#include <math.h>
#include <fftw3.h>
#include <stdlib.h>

int main() {

   int i, n, sign;
   double ignore;

   scanf( "%20i %20i", &n, &sign );

   fftw_complex *in  = fftw_malloc( n * sizeof( fftw_complex ) );
   fftw_complex *out = fftw_malloc( n * sizeof( fftw_complex ) );
   fftw_plan plan = fftw_plan_dft_1d( n, in, out, sign, FFTW_ESTIMATE );

   for( i = 0 ; i < n ; i++ )
      scanf( "%20lf      %20lf      %20lf    %20lf    %20lf",
              &in[i][0], &in[i][1], &ignore, &ignore, &ignore );

   fftw_execute( plan ); // where the magic happens

   sign *= -1;

   printf( "%i %i\n", n, sign );

   for( i = 0 ; i < n ; i++ ) {

      double re  = out[i][0];
      double im  = out[i][1];
      double mod = sqrt( re * re + im * im );
      double arg = atan( im / re );

      if( sign > 0 ) { /* footnote 1 */
         re  /= n;
         im  /= n;
         mod /= n;
      }

      printf( "%.15lf %.15lf %.15lf %.15lf %.15lf\n",
               re, im, mod, arg, 180 * M_1_PI * arg );
   }

   fftw_destroy_plan( plan );
   fftw_free( in  );
   fftw_free( out );
}

/*
 * Footnote 1:
 *
 * This should scale by 1/sqrt(n), not by 1/n; and it should do it in both
 * directions - that is, it should not be conditional on sign > 0.
 *
 */