Trying to solve an unknwon segmentation fault in C -

i'm trying learn how use fast fourier transform, , have copied fft algorithm numerical recipies in c called four1. have written small function test fourier transforms simple function. upon execution program returns segmentation fault. can't find fault can me?

#include <stdio.h> #include <math.h> #define swap(a,b) tempr=(a);(a)=(b);(b)=tempr  void four1(float data[], unsigned long nn, int isign) {         unsigned long n,mmax,m,j,istep,i;         double wtemp,wr,wpr,wpi,wi,theta;         float tempr,tempi;          n=nn << 1;         j=1;         (i=1;i<n;i+=2) {                 if (j > i) {                         swap(data[j],data[i]);                         swap(data[j+1],data[i+1]);                 }                 m=n >> 1;                 while (m >= 2 && j > m) {                         j -= m;                         m >>= 1;                 }                 j += m;         }         mmax=2;         while (n > mmax) {                 istep=mmax << 1;                 theta=isign*(6.28318530717959/mmax);                 wtemp=sin(0.5*theta);                 wpr = -2.0*wtemp*wtemp;                 wpi=sin(theta);                 wr=1.0;                 wi=0.0;                 (m=1;m<mmax;m+=2) {                         (i=m;i<=n;i+=istep) {                                 j=i+mmax;                                 tempr=wr*data[j]-wi*data[j+1];                                 tempi=wr*data[j+1]+wi*data[j];                                 data[j]=data[i]-tempr;                                 data[j+1]=data[i+1]-tempi;                                 data[i] += tempr;                                 data[i+1] += tempi;                         }                         wr=(wtemp=wr)*wpr-wi*wpi+wr;                         wi=wi*wpr+wtemp*wpi+wi;                 }                 mmax=istep;         } }  void fourier_transform_test (file* output_file) {         /*          function serves test see whether implementation of         fft working or not.         */         int n = 30;             // number of samples         float x[n];             // array holds values x                      // misc         int = 0;          printf("running fourier transform tests...\n");         fprintf(output_file, "# x t\n");           // fill array x values transformed         (i = 0; <= (n - 1); i++)                  x[i] = cos((2 * 3.1415 * i) / 10);          // according numerical recipies, have decrement pointer data         // 1 compensate zero-offset         four1(x-1, 64, 1);          // loop through transformed array x , print results file         (i = 0; <= (n - 1); i++)                 fprintf(output_file, "%i\t%f\n", i, x[i]);          fclose(output_file);  }  int main (int argc, char *argv[])  {         // open data_file write results         file* file;          if (argc == 1)                 file = fopen("results.dat", "w");         else                 file = fopen(argv[1], "w");          fourier_transform_test(file);          return 0; }  

i'm using latest debian, gcc 4 (pretty sure)

for of want see book yourself: http://www.nrbook.com/a/bookcpdf/c12-2.pdf (its legit)

you have 30 values in array you're telling four1 array 64 floats long.

four1(x-1, n, 1);