Question

I have what I consider a really strange problem. I have a function with the following prototype:

void generateNodes(const int maxX, const int maxY, node nodes[]);

As one of the first things in this function I define a 2d array of shorts, which i use as boolean values. But when I call this function the value of maxY changes to a large value. The code in question is below:

void generateNodes(const int maxX, const int maxY, node nodes[]){
    int i, currentX, currentY;

    short used[MAX_NODES][MAX_NODES];

    //Generate the nodes
    for(i = 0; i < MAX_NODES; i++){
        currentX = randomNumber(0,maxX);
        currentY = randomNumber(0,maxY);

        nodes[i].color = 0;
        nodes[i].numberOfConnections = 0;
        nodes[i].id = i;
        nodes[i].distanceFromStart = NOT_SET;
        nodes[i].parent = NULL;

        if(!used[currentX][currentY]){
            nodes[i].x = currentX;
            nodes[i].y = currentY;

            used[currentX][currentY] = 1;
        } else {
            i--;
        }
    }

    int numberOfConnections, j, currentNeighbor;

    //Generate the connections
    for(i = 0; i < MAX_NODES; i++){
        numberOfConnections = randomNumber(1,5); //Between one and five outgoing connections

        for(j = 0; j < numberOfConnections; j++){

            currentNeighbor = randomNumber(0,19); //Select the neighbor

            while(currentNeighbor == i){
                currentNeighbor = randomNumber(0,19); //Try again while the selected is self
            }

            nodes[i].canReach[++(nodes[i].numberOfConnections)] = &nodes[currentNeighbor];

            nodes[currentNeighbor].canReach[++(nodes[currentNeighbor].numberOfConnections)] = &nodes[i];
        }
    } 
}

MAX_NODES is defined to 20.

Does anyone know why this might happen?

Answer 1

Since you do not seem to initialize the array used, it may well be that some elements are considered used (!= 0), since an array on stack is not initialized to zero, but takes whatever was in that memory area before.
An if an X,Y pair is considered used, you decrement the loop counter, possibly beyond zero into the negative realm, possibly overwriting – on the next iteration – part of the stack. This may also change the parameters, since they also reside on the same stack, before the local array.

Start with initializing used, and consider rewriting the loop to not change the loop variable except in the for statement.