Question

I’m trying to store three square, 2d arrays into a buffer so I use one contiguous block of memory. If the array size is rxr, my formula is:

buffer = (double*) malloc((r + r + r) * sizeof(double *) + 
    (r*r + r*r + r*r) * sizeof(double));

  if(buffer == NULL) {
    printf("out of memory\n");
    return 0;
  }

  for(i = 0, j = 0; i < r; i++) {
    a[i] = &buffer[j];
    j+=r;
  }

  for(i = 0; i < r; i++) {
    b[i] = &buffer[j];
    j+=r;
  }

  for(i = 0; i < r; i++) {
    c[i] = &buffer[j];
    j+=r;
  }

  a = buffer[j];
  b = buffer[j + r];
  c = buffer[j + r + r];

As you can see, I’m lost. a, b, c, are declared as double pointers (meant to be pointers to arrays of arrays), so I want each to have an array of size r, with each element pointing to its own separate array of size r. Any ideas…

Answer 1

You’re confusing malloc’ing pointers with malloc’ing doubles themselves. Just allocate the doubles like:

double* all = (double*) malloc( 3*r*r*sizeof(double));

Then where do your actual matrices go?

double *p1 = all;
double *p2 = &all[r*r];
double *p3 = &all[2*r*r];

Please check for off by one errors 🙂 But the point is you don’t need to malloc out both the double* and the double.