Generic Linked List in C - c

Question

I have a linked_list struct:
typedef struct linked_list{
void *data;
struct linked_list *next;
struct linked_list *previous;
} linked_list;
And some linked list operations:
linked_list *init_linked_list() {
linked_list *ll;
ll = (linked_list *) malloc(sizeof(linked_list));
ll->next = ll;
ll->previous = ll;
return ll;
}
void add_element( linked_list *list, void *element) {
linked_list *list_element;
list_element = malloc(sizeof(linked_list));
list_element->data = element;
list_element->next = list->next;
list->next->previous = list_element ;
list->next = list_element ;
list_element->previous = list;
}
I have a graph struct:
typedef struct graph {
int number_vertices;
vertex **vertices;
} graph;
I have a vertex struct:
typedef struct vertex {
int time;
char *name;
linked_list *edges;
} vertex;
I have a edge struct:
typedef struct edge{
int weight;
int change;
vertex *to;
} edge;
And an "add-edge" function:
void add_edge_to_vertex(vertex *v, int weight, int change, vertex *to) {
edge *pEdge = malloc(sizeof(edge));
pEdge->weight = weight;
pEdge->change = change;
pEdge->to = to;
// add edge to vertex linked list
add_element(v->edges, pEdge);
}
Now to my problem. I setup my graph:
int aSize = 30;
int bSize = 30;
pGraph = malloc(sizeof(graph));
pGraph->vertices = malloc(sizeof(vertex*) * aSize);
pGraph->vertices[0] = malloc(sizeof(vertex) * bSize);
I setup my a vertex and init the linked_list:
vertex *pVertex ;
pVertex = malloc(sizeof(vertex));
pVertex->edges = init_linked_list();
And I add the vertex to my graph:
pGraph->vertices[a][b] = *pVertex;
Last I add an edge between two vertices:
add_edge_to_vertex(&pGraph->vertices[a][i], 100, 0, &pGraph->vertices[a][i+1]);
When I try to fetch the edge weight, I get an segment fault: 11
vertex *v = &pGraph->vertices[0][0];
linked_list *ll = v->edges;
int s = linked_list_size(ll);
printf("%d\n", s); // outputs 1 - works so far!
edge *e = (edge *) ll->data;
int weight = e->weight; // segment fault: 11 ..
I have also tried to add an int and a char to the linked_list struct, and fetched that value, instead of fetching (and casting) edge from the "void *data". This works.
My problem now is, that I don't know if my fault is when I fetch the data, or when I store the data.

Answer 1

Your linked list has one extra node at the start that does not have its data member initialized (the node created by the init_linked_list() function).
When you do :
edge *e = (edge *) ll->data;
you get that first node's data member, which is uninitialized. That causes the segmentation fault when you try to dereference e.
Try this instead :
edge *e = (edge *) ll->next->data;
which will get the data member for the node that was inserted by the last add_element function call. Obviously, this is only safe if there has been at least one element added into the linked list.