Fast Fourier Trasform in C

Fast Fourier Transform(2 Radix)

      for(int k=0;k<(data_len/2);k++)  
      {  
           for(int m=0;m<(data_len/2);m++)  
           {  
                even_buf += sin_arr[2*m] * cexp(-2 * I * pinum * 2 * m * k / data_len) ;  
                odd_buf += sin_arr[2*m + 1] = cexp(-2 * I * pinum * ( 2*m + 1) * k / data_len) ;  
           }  
           ph_fac = cexp(-2 * I * pinum * k / data_len) ;  
           fft_test_out[k] = even_buf + (ph_fac * odd_buf) ;  
           fft_test_out[k + (data_len/2)] = even_buf - (ph_fac * odd_buf) ;  
           even_buf = 0 + 0 * I;  
           odd_buf = 0 + 0 * I;  
           ph_fac = 0 + 0 * I;  
      }