How to reallocate memory for a 2D float array?

Question

I tried the following to reallocate a 2D float array whose size chages from 2X2 to 3X3. The code throws a segfault while trying to realloc memory for weights[2].

num_vertices = 2;
float **weights = malloc(num_vertices*sizeof(float *));      // weight array matrix
for(i = 0; i < num_vertices; i++){
    weights[i] = malloc(num_vertices*sizeof(float));
}

num_vertices = 3;
weights = realloc(weights, num_vertices*sizeof(float *));      // weight array matrix
for(i = 0; i < num_vertices; i++){       
    weights[i] = realloc(weights[i], num_vertices*sizeof(float));
}

Of course, I can free the 2D array and malloc again, but I was looking for a more elegant solution. Any ideas?

Answer 1

The problem is that weights[2] contains garbage after you realloc weights.

You probably want to do something like this:

new_vertices = 3;
weights = realloc(weights, new_vertices*sizeof(float *));
for(i = 0; i < new_vertices; i++)
{
    if (i >= num_vertices)
        weights[i] = NULL;
    weights[i] = realloc(weights[i], new_vertices*sizeof(float));
}
num_vertices = new_vertices;

Note that you have a potential memory leak if realloc ever fails. Since you have no error checking yet though this probably doesn't matter for now.