passing a vector to a function, getting negative length and size, aka segault

Hello,

I am writing a raytracer, and currently I'm working on creating a bounding volume hierarchy to accelerate the process. To do this, I am first creating a vector that holds each of the objects in the scene, and passing this vector to the constructor for my BVH.

Code:

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//in header
BVH_Node* bvh;
//in main raytrace function
bvh = new BVH_Node(objList,0);

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The BVH_Node constructor:

Code:

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//here's how the beginning of it looks
BVH_Node::BVH_Node(vector<GeomObj*> list, int axis){
  int size = list.size(); //number of elements in list
  this->isObj = false;
  if(size == 1){
    this->left = makeLeaf(list[0]);
    this->right = makeLeaf(NULL);
    this->bb = list[0]->bb;
    return;
  }
  if(size == 2){
    this->left = makeLeaf(list[0]);
    this->right = makeLeaf(list[1]);
    this->bb = combineBoxes(list[0]->bb,list[1]->bb);
    return;
  }
//the rest of it
}

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I am testing a scene that has only 2 objects, and so it goes to the size == 2 check. The first time it hits makeLeaf(), I segfault. I've used both gdb and valgrind, and of course it's a memory mapping error. gdb's backtrace tells me that the length of the vector I've passed in is -805305610 and the capacity is -21, and that it is inside my makeLeaf() function that the error occurs. Here's the function:

Code:

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BVH_Node* BVH_Node::makeLeaf(GeomObj* v){
  BVH_Node* node;
  node->obj = v;
  node->isObj = true;
  return node;
}

---

The segfault happens at

Code:

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node->obj = v;

---

Could someone help me understand why this is happening? If I run my raytracer without a BVH, the objList works perfectly.

Thanks

its hard for me to say given the limited code, and my knowledge - but dont you want to pass a reference to a vector into your constructor

If you would like, I could post more code. Just let me know what you need to see. Also, I don't believe I'm passing a reference of my vector into my constructor, but rather the object itself. Is this an incorrect understanding of what I'm doing?

Quote:

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I don't believe I'm passing a reference of my vector into my constructor, but rather the object itself.

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O_o

You misunderstood; that was the way rogster001 chose to tell you to pass the `std::vector' by reference.

Also, if it crashes where you say, the problem isn't with this code; look into the function that populates the `std::vector' object.

Soma

Ok, I think I got it. Would it be beneficial to change the prototype of the constructor to take a const vector<GeomObj*>& rather than the way I've done it?

Also, I just use the given std::vector.push_back() to populate the vector. As you can guess, I'm pushing back GeomObj* objects. Is there some way that the pointers or my data could have changed while being passed through the constructor or to makeLeaf() (I realize this is unlikely)? Like I said before, until I started to try and implement this BVH, my object list vector was giving me no problems at all.

Sorry, I'm not the greatest with all this memory stuff. I appreciate the help.

Reanalyzing my code, I've realized that I'm not properly allocating memory for my left and right nodes. Here's the class definition of a BVH_Node:

Code:

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class BVH_Node{
  public:
    BVH_Node();
    BVH_Node(const vector<GeomObj*>&,int);
    ~BVH_Node();
    BVH_Node* left; //left child
    BVH_Node* right; //right child
    BBox bb; //bounding box
    bool isObj; //leaf of tree or not
    GeomObj* obj; //the object itself

    //functions
    BBox combineBoxes(BBox,BBox);
    BVH_Node* makeLeaf(GeomObj*);
    bool intersect(vec3,vec3,BVH_Node*);
    static bool compareByX(GeomObj*,GeomObj*);
    static bool compareByY(GeomObj*,GeomObj*);
    static bool compareByZ(GeomObj*,GeomObj*);
};

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I malloc'd some space in my constructor, and everything is dandy. Don't know why this didn't occur to me earlier. Now I just need to work on the rest of the bugs with the tree itself :)

Do not use malloc (it will not call constructors)! Use new and delete, but even better is to not use it at all! Why do you need new/delete/malloc/free anyway?
For good measurement, some good advice:
- Do not remove parameter names - cpwiki

Questions:
- Do you need to implement your own tree? Won't std::set or std::map suffice?

Quote:

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Don't use malloc (it will not call constructors)!

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[Edit]
O_o
[/Edit]

Indeed.

Quote:

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Do you need to implement your own tree? Won't std::set or std::map suffice?

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The tree is specialized for scene/object relationships.

The algorithms depends on navigating the tree not simply having access to the nodes.

You could probably force `std::set' to do the job by using keys related to how the scene is arranged, but that would probably be more work than just implementing the tree.

This may be one of those times when implementing a thing is the right to go.

Soma

Quote:

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Originally Posted by synhyborex

Code:

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BVH_Node* BVH_Node::makeLeaf(GeomObj* v){
  BVH_Node* node;
  node->obj = v;
  node->isObj = true;
  return node;
}

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The segfault happens at

Code:

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node->obj = v;

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Could someone help me understand why this is happening?

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I would have thought there would be no mystery here.
You're declaring a pointer, leaving it uninitialised, and then immediately dereferencing it. Of course that wont work.