How is this causing segmentation fault (short code) (C-language)

Question

This is causing me segmentation fault, whenever i comment out breadthFirst = queue_enqueue(breadthFirst,n) or breadthFirst = queue_dequeue(breadthFirst) i no longer get seg fault but it obviously can't solve my task anymore.

I suppose it might be very hard to help me with this unless you know how the datatypes are implemented but i thought i might give it a shot and see if it is something obvious im missing.

bool find_path(graph *g,node *src, node *dest) {

 
  
  queue *breadthFirst = queue_empty(NULL);

  breadthFirst = queue_enqueue(breadthFirst,src);

  while (!queue_is_empty(breadthFirst))` {
    void *v = queue_front(breadthFirst);

    g = graph_node_set_seen(g,v,1);

    dlist *neighbours = graph_neighbours(g,v);
    dlist_pos pos = dlist_first(neighbours);

    while (!(dlist_is_end(neighbours,pos))) {
        void *n = dlist_inspect(neighbours,pos);
        if (graph_node_is_seen(g,n)) {

        }
        else {
        breadthFirst = queue_enqueue(breadthFirst,n);
        }
        pos = dlist_next(neighbours,pos);
    }

   breadthFirst = queue_dequeue(breadthFirst);
  }
}




struct queue {
    list *elements;
};

queue *queue_empty(free_function free_func)
{
    // Allocate the queue head.
    queue *q=calloc(1, sizeof(*q));
    // Create an empty list.
    q->elements=list_empty(free_func);

    return q;
}

queue *queue_enqueue(queue *q, void *v)
{
    list_insert(q->elements, v, list_end(q->elements));
    return q;
}

queue *queue_dequeue(queue *q)
{
    list_remove(q->elements, list_first(q->elements));
    return q;
}

void *queue_front(const queue *q)
{
    return list_inspect(q->elements, list_first(q->elements));
}

As per request i added the queue implementation

Answer 1

Solved it! Thx for the tips tho, the information i presented was not enough for anyone to help me really. I did some debugging and found that i had misunderstood my implementations basically.

*n is a value in a struc

the *src i send in, is a node with a value and list

so it works fine on the first iteration, but then im trying to find a list from *n which is only just a void *value and is not a struc or a list. Then my program attempts to use first on whatever that list is and that caused segmentation fault.

The fix is to with the n value to use a function i have, to find the struct corresponding to that value.

Answer 2

Try modifying your queue_empty function like this :

queue *queue_empty(free_function free_func)
{
    // Allocate the queue head.
    queue *q=calloc(1, sizeof(queue));
    // Create an empty list.
    q->elements=list_empty(free_func);

    return q;
}