Go4Expert - Connecting link lists

Hello, I want to make a linked list program consisting of one main link, and each main links have sub link like

Code:

Main 1 -> Main 2 -> Main 3 -> ... -> Main N -> NULL

   |        |          |                        

Sub 1-1    ...       ...

   |                

Sub 1-2    

   |              

  ...

I have two structs for these:

Code:

struct NodeSmall

{

    int documentNum;

    NodeSmall *nextNum;

    NodeSmall()

    {

        documentNum = 0;

        nextNum = NULL;

    }

};



struct NodeBig

{

    string keyword;

    NodeBig *nxtkyw; //Link to next keyword

    NodeSmall *docNum; //Link to document number



    NodeBig()

    {

        keyword = "";

        nxtkyw = NULL;

    }

    

};

And I have a test main which reads first line from text file:

Code:

int main()

{



    string line, key;

    int number;

    ifstream dbfile;

    dbfile.open("docdb.txt");



    dbfile >> key >> number;



    NodeBig *head = new NodeBig;

    NodeSmall *docNum = new NodeSmall;

    

    head->keyword = key;

    head->nxtkyw = NULL;

    head->docNum->documentNum = number;



    cout << "keyword is " << head->keyword << " and it is on " << head->docNum->documentNum;

It does not give compiler error (vs2008) but it gives stopped working error after running the program. I think I've made a mistake while connecting link lists together. But I cannot find similar probles on internet. What should I do for fixing this error? Thank you.