Write a function to malloc double pointer

Question

I need to write a function that creates a double pointer using malloc.

This is how I declared my double pointer as I normally would:

double **G; //Create double pointer to hold 2d matrix

*G = malloc(numNodes * sizeof(double*));
for(i = 0; i < numNodes; i++)
{
    G[i] = malloc(numNodes*sizeof(double));
    for (j = 0; j < numNodes; j++)
    {
        G[i][j] = 0;
    }
}

Now I tried replacing it with:

double **G;
mallocDoubleArr(G, numNodes);

With the function being:

void mallocDoubleArr(double **arr, int size)
{
    int i, j;

    *arr = malloc(size * sizeof(double*));

    for(i = 0; i < size; i++)
    {
        arr[i]= malloc(size*sizeof(double));
        for (j = 0; j < size; j++)
        {
            arr[i][j] = 0;
        }
    }
}

Why doesn't this work?

Answer 1

You need one more "indirection", in other words pass G by reference like a pointer to a pointer to a pointer to float:

void mallocDoubleArr(double ***arr, int size);

And then call it as

mallocDoubleArr(&G, numNodes);

Modify mallocDoubleArr accordingly, like for example

(*arr)[i] = malloc(size*sizeof(double));