Can't allocate complex 2D array

Question

I'm trying to make a "union-find".

Here is my code :

UnionFind uf_create(){
    UnionFind uf= malloc(sizeof(UnionFind));
    uf->vt=malloc(11*sizeof(VertexTree*));
    uf->nbElems=VERTEX_MAX;
    uf->nbGroups=VERTEX_MAX;
    int i;
    for(i=0;i<uf->nbElems;i++){
        printf("%d\n", i);
        uf->vt[i]->vtx=i+1;
        uf->vt[i]->parent=uf->vt[i];
    }
    return uf;
}

The UnionFind is defined by :

typedef struct unionfind{
    unsigned int nbElems;
    unsigned int nbGroups;
    VertexTree **vt;
}*UnionFind;

And here is the definition of the Tree :

typedef struct sTree{
    GraphVertex vtx;
    struct sTree* parent;
}VertexTree;

I know the segfault is because the tree is not allocated correctly. Can someone please tell me how to allocate memory correctly for the tree of vertices?

Thanks

Answer 1

UnionFind uf= malloc(sizeof(UnionFind));

I think this is your problem line..

UnionFind is a pointer type so sizeof will only return the size of a pointer for your machine.

Try:

UnionFind uf= malloc(sizeof(struct unionfind));

This will return the actual size of the structure.

Answer 2

I found the problem!

I had to allocate the "pointer of pointers" (**vt) and then allocate the pointers for each tree in a for loop.

So the final code is :

UnionFind uf_create(){ UnionFind uf= malloc(sizeof(UnionFind)); uf->nbElems=VERTEX_MAX; uf->nbGroups=VERTEX_MAX; uf->vt=malloc(VERTEX_MAX*sizeof(VertexTree*));//uf->vt is now defined int i; for(i=0;i<uf->nbElems;i++){ uf->vt[i]=malloc(sizeof(VertexTree));//I can now allocate the trees one by one uf->vt[i]->vtx=i+1; uf->vt[i]->parent=uf->vt[i]; } return uf; }