FFTW plan segmentation fault - c

Question

I am using FFTW3 to perform an fft on multiple columns of data (i.e multi channel audio, where I desire the transform of each channel). This is working fine on OSX but porting the code over to linux gives me a seg fault.
const int fftwFlags = FFTW_PRESERVE_INPUT|FFTW_PATIENT;
struct fft {
fftw_complex **complexSig;
double **realSig;
fftw_plan forwardR2C;
int fftLen;
int numChan;
}
void create FFT(struct fft *fft) {
int bufLen = 1024;
int numChan = 4;
fft->fftLen = bufLen;
fft->numChan = numChan;
fft->realSig = fftw_malloc(sizeof(double *) * numChan);
for(int i = 0; i < numChan; i++) {
fft->realSig[i] = fftw_malloc(sizeof(double) * bufLen);
}
fft->complexSig = fftw_malloc(sizeof(fftw_complex *) * numChan);
for(int i = 0; i < numChan; i++) {
fft->complexSig[i] = fftw_malloc(sizeof(fftw_complex) * bufLen);
}
fft->forwardR2C = fftw_plan_many_dft_r2c(1, &fft->fftLen, fft->numChan, *fft->realSig, &fft->fftLen, 1, fft->fftLen, *fft->complexSig, &fft->fftLen, 1, fft->fftLen, fftwFlags);
}
valgrind is showing that the fftw planner is attempting to access past the end of this array (by 8 bytes, one sample), resulting in a segmentation fault. When increasing the amount of memory allocated to realSig to bufLen * 2 this error is absent.
I am sure this is an error in how I am telling FFTW to read my data, but I can not spot it!

Answer 1

You seem to be assuming that successive malloc calls will be contiguous, which of course they are unlikely to be (you probably just "got lucky" on OS X). You can fix this quite easily though by making one large allocation, e.g.
void createFFT(struct fft *fft)
{
const int bufLen = 1024;
const int numChan = 4;
fft->fftLen = bufLen;
fft->numChan = numChan;
fft->realSig = fftw_malloc(sizeof(double *) * numChan);
// array of numChan pointers
fft->realSig[0] = fftw_malloc(sizeof(double) * numChan * bufLen);
// one large contiguous block of size `numChan * bufLen`
for(int i = 1; i < numChan; i++) // init pointers
{
fft->realSig[i] = fft->realSig[i - 1] + bufLen;
}
// ...
}
Note: when you're done you just need to:
fftw_free(fft->realSig[0]);