Question

I’ve writen a Bread-First Search algorithm in C that searches through a graph structure (in this case representing a grid of streets) and returns ALL possible routes from node A to node B.

What I’ve found is that the function works very quickly for small graphs (around 24 nodes), but will crash with anything larger than this. I thought it was a problem with too many mallocs, so I added free() into my function to remove space while running through the queue. This does not, unfortunately, fix the problem. Also note that I never get the error message “Out of memory,” either, so I’m not sure what’s happening…

void BFS_search(struct map *m, int start, int end){
int n = m->nb_vertice+1;
int i=0;
int num=0;

//BFS requires a queue (pile) to maintain a list of nodes to visit 
struct queue {
    int current_node;
    int visited[n]; //cannot be a pointer! Otherwise the pointer may influence other queue structures
    struct queue *suivant;
};

//Function to add a node at the end of the queue.
void addqueue (int value, struct queue *old, int * old_seen) {
    int i;
    if (old->suivant==NULL){
        struct queue *nouveau;
        nouveau = (struct queue *)malloc(sizeof(struct queue));
        if (nouveau == NULL){
            printf("\n\nSnap! Out of memory, exiting...\n");
            exit(1);
        }
        nouveau->current_node = value;
        for (i = 0; i <= n; ++i){ 
            if (old_seen[i]==1)
                nouveau->visited[i]=1;
            else nouveau->visited[i]=0;
        }
        nouveau->suivant = NULL;
        old->suivant=nouveau;
        return;
    }
    else addqueue(value,old->suivant,old_seen);
}

struct queue * dequeue (struct queue *old){
    struct queue *nouveau;
    nouveau = (struct queue *)malloc(sizeof(struct queue));
    if (nouveau == NULL){
        printf("\n\nSnap! Out of memory, exiting...\n");
        exit(1);
    }
    nouveau = old->suivant;
    free(old);
    return(nouveau);
}

//the actual Breadth First Search Algorithm
int BFS(struct map *m, struct queue *q, int num, int end){
    int k;
    q->visited[q->current_node]=1; //mark current node as visited

    while(q!=NULL){
        //if we reached the destination, add +1 to the counter
        if (q->current_node==end){
            num+=1;
        }
        //if not the destination, look at adjacent nodes
        else {
            for (k=1;k<n;++k)
                if (m->dist[q->current_node][k]!=0 && q->visited[k]!=1){
                    addqueue(k,q,q->visited);
                }
            }
        //if queue is empty, stop and return the number 
        if (q->suivant==NULL){
            return(num);
        }
        //if queue is not empty, then move to next in queue
        else
            return(BFS(m,dequeue(q),num,end));
    }
}

//create and initialize start structure
struct queue *debut;
debut = (struct queue *)malloc(sizeof(struct queue));
for (i = 0; i <= n; ++i)
    debut->visited[i]=0;            
debut->current_node=start;
debut->visited[start]=1;
debut->suivant = NULL;

num=BFS(m,debut,0,end);
printf("\nIl existe %d routes possibles! \n",num);
}

Note that I’m using a struct map, which stores all the edges and nodes for my graph, including nb_vertices (number of nodes), and a distance matrix dist[i][j], which is the distance from node i to j, or 0 if not connected.

Any help would be greatly appreciated! I assume it’s an error with the amount of memory available. I’d at least like to have a way to output a specific error message if I can’t avoid the memory problems…

Answer 1

Your dequeue operation is leaking memory. You malloc some memory and store the pointer in nouveau, but then you say nouveau = old->suivant, losing the malloc‘d buffer. There’s no need to malloc at all when popping from the front of a linked list:

struct queue *dequeue(struct queue *q)
{
    struct queue *next = q->suivant;
    free(q);
    return next;
}

As for why you don’t get an “out of memory” error, I’m guessing you’re on Linux and you’re experiencing the sad effects of overcommit.