How can I free this 2D calloc'd array in C?

Question

I'm doing an assignment for my C algorithms class and I have a memory allocation as follows:

Graph* new_graph = (Graph*)malloc(sizeof(Graph));
new_graph->adjacency_matrix = malloc(num_nodes*sizeof(int*));
for(int i = 0; i < num_nodes; i++) {
        for(int j = 0; j < num_nodes; j++) {
            new_graph->adjacency_matrix[i] = calloc(sizeof(int), num_nodes);
        }
    }

When I'm trying to free the memory I'm doing it as follows, but valgrind keeps saying I have lost 1,176 bytes in 42 blocks due to my calloc.

void delete_graph(Graph *g) {
    for(int i = 0; i < g->num_vertices; i++) {
        free(g->adjacency_matrix[i]);
    }
    free(g->adjacency_matrix);
    free(g);
}

The size of num_nodes is 7, so I'm assuming only the first row is being freed or something, but I've tried freeing it with another for loop etc., but i keep getting the same errors. Would really appreciate any help with this thanks!

Answer 1

How can I free this 2D calloc'd array in C?

You can't.... because your allocation is wrong.

In the allocation this line

for(int j = 0; j < num_nodes; j++) {

seems wrong. You keep overwriting the same pointer as you only use i as index and therefore generates memory leaks.

EDIT

Let's say that num_nodes is 2

Then your code is doing:

new_graph->adjacency_matrix[0] = calloc(sizeof(int), num_nodes);
new_graph->adjacency_matrix[0] = calloc(sizeof(int), num_nodes); // LEAK
new_graph->adjacency_matrix[1] = calloc(sizeof(int), num_nodes);
new_graph->adjacency_matrix[1] = calloc(sizeof(int), num_nodes); // LEAK

So you are allocating memory and saving the pointer... fine, but the you overwrite the same pointer with a new allocation. That's a memory leak that can't be recovered.