OT(ish): Strange coding problem (audio related)

[James Stone <jamesmstone@xxxxxxxxx>]

Hi All,

I have been working on the Neil tracker program recently, and hit a 
weird bug that seems to affect only my computer! I get a segfault when 
trying to use fft.h (which is used by the oomek 303 emulator, which has 
incidentally now been released as open source - a really good sound 
IMHO, but I digress). The following test code results in a segfault on 
my computer - an AMD XP2500+ (but no-one elses as far as I can tell).

bt:

Program received signal SIGSEGV, Segmentation fault.
0x080486d7 in IFFT (fftBuffer=0x8049b00, fftFrameSize=2048, sign=1) at 
fft.h:51
51              tr = *p2r * ur - *p2i * ui;

Any thoughts as to what might be happening?

James

test.cpp:

#include <cstdio>
#include <cmath>
#include "fft.h"

float a[4096];

int main ()
{
for(int i=0; i<4096; i++)
{
a[i]=0;
}
IFFT(a,2048,1);
printf("%f",a[1]);
}

fft.h:
#define M_PI 3.14159265358979323846

void IFFT(float *fftBuffer, long fftFrameSize, long sign)
/*
   FFT routine, (C)1996 S.M.Sprenger. Sign = -1 is FFT, 1 is iFFT (inverse)
   Fills fftBuffer[0...2*fftFrameSize-1] with the Fourier transform of the
   time domain data in fftBuffer[0...2*fftFrameSize-1]. The FFT array takes
   and returns the cosine and sine parts in an interleaved manner, ie.
   fftBuffer[0] = cosPart[0], fftBuffer[1] = sinPart[0], asf. fftFrameSize
   must be a power of 2. It expects a complex input signal (see 
footnote 2),
   ie. when working with 'common' audio signals our input signal has to be
   passed as {in[0],0.,in[1],0.,in[2],0.,...} asf. In that case, the 
transform
   of the frequencies of interest is in fftBuffer[0...fftFrameSize].
*/
{
  float wr, wi, arg, *p1, *p2, temp;
  float tr, ti, ur, ui, *p1r, *p1i, *p2r, *p2i;
  long i, bitm, j, le, le2, k;

  for (i = 2; i < 2 * fftFrameSize - 2; i += 2) {
    for (bitm = 2, j = 0; bitm < 2 * fftFrameSize; bitm <<= 1) {
      if (i & bitm) j++;
      j <<= 1;
    }
    if (i < j)
      {
        p1 = fftBuffer + i;
        p2 = fftBuffer + j;
        temp = *p1;
        *(p1++) = *p2;
        *(p2++) = temp;
        temp = *p1;
        *p1 = *p2;
        *p2 = temp;
      }
  }
  for (k = 0, le = 2; k < log(fftFrameSize)/log(2.); k++) {
    le <<= 1;
    le2 = le>>1;
    ur = 1.0;
    ui = 0.0;
    arg = (float)M_PI / (le2>>1);
    wr = (float)cos(arg);
    wi = sign*(float)sin(arg);
    for (j = 0; j < le2; j += 2) {
      p1r = fftBuffer+j;
      p1i = p1r+1;
      p2r = p1r+le2;
      p2i = p2r+1;
      for (i = j; i < 2 * fftFrameSize; i += le) {
        tr = *p2r * ur - *p2i * ui;
        ti = *p2r * ui + *p2i * ur;
        *p2r = *p1r - tr;
        *p2i = *p1i - ti;
        *p1r += tr;
        *p1i += ti;
        p1r += le;
        p1i += le;
        p2r += le;
        p2i += le;
      }
      tr = ur*wr - ui*wi;
      ui = ur*wi + ui*wr;
      ur = tr;
    }
  }
}


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