Tokenizing a Linked List help

[elite zero]

Having an error with this program. It runs, however it doesn't tokenize and insert. When it gets to the display function, it crashes due to trying to access a NULL. Any help would be greatly appreciated.

#include <iostream>
#include <fstream>
#include <cstdlib>
#include <stdlib.h>
#include <iomanip>
#include <sstream>
#include <stdio.h>
#include <cstddef>
#include <string.h>

using namespace std;

const int buff_size = 81; // buffer size of 81 characters

ifstream input_File;
ifstream delete_File;
int i = 0;
char *buffer = new char[buff_size], // Tokenizing pointers
     *tokens[3],
	 *ptr;

int pause; // Global for pause use

//-----------------------------------------------------------------------------

struct listNode // Structure named listNode
 {
  string age;       // Values within nodes
  string firstName;
  string lastName;
  listNode* next;
 };

listNode* newNode; // Node pointers

//-----------------------------------------------------------------------------
//-----------------------------------------------------------------------------

void reportDuplicate() // prints duplicates
 {
  cout << "Duplicate Record: " << newNode-> lastName << endl;
 }// end void reportDuplicate()

//-----------------------------------------------------------------------------

void incompleteRecord()
{
 cout << "Record incomplete: " << tokens[0] << endl;
}// end void incompleteRecord()

//-----------------------------------------------------------------------------
//-----------------------------------------------------------------------------
//-----------------------------------------------------------------------------

void insertName(char tokens[0])
{

 // Initializes list on insertion
 // Creates new node and inserts tokenized values

 newNode = new listNode;
 newNode-> lastName = tokens[0];
 newNode-> next = NULL;

 listNode* head;
 listNode* current = head; // set traversal pointer

 if(head == NULL) // If no head node, create one
  {
   head = newNode;
   newNode-> next = NULL;
  }
 else if(head-> lastName > newNode-> lastName)
  {
   newNode-> next = head;
   head = newNode;
  }

 else
  {
   while((current-> next != NULL) && (current-> next-> lastName < newNode-> lastName)) // Traverse the list
   {
	if(current-> lastName == newNode-> lastName)
	 {
	  reportDuplicate();
	 }
	else
	 {
	  current = current-> next;
	 }
   }
	 
	if(current-> lastName < newNode-> lastName)
	 {
	  newNode-> next = current-> next;
	  current-> next = newNode;
	 }

   }//end if(lastName)
}//end void insertName(...)

//-----------------------------------------------------------------------------

void tokenizingFunction()
 {
	 cout << "Tokenizing..." << endl << endl;

    while(input_File.getline(buffer, buff_size))
     {

      // If we found a blank line (newline char in column 1), ignore it and 
      // continue. 

	  cout << buffer << endl;

      if(strlen(buffer) == 0)
	   continue;
      
	  // ** Windows Machines Only **
      // Break the input string into white space separated tokens.
	  // 3rd parameter needs to be a char**_Context
      // Use char* (placeHolder) as a place holder within the string (used in strtok_s)
	  // Format: ptr = (char *)strtok_s(buffer, " ", &placeHolder);

      ptr = (char *)strtok(buffer, " ");

      // If ptr == NULL, then other than whitespace, the input line is
      // empty.
    
      if(!ptr)
	   continue;
      
      // Store the tokens in tokens[]

      i = 0;

      tokens[i] = (char *)_strdup(ptr);

      do
	   {
        ptr = (char *)strtok('\0', " "); 

         if(ptr)
          {
           tokens[++i] = (char *)_strdup(ptr);
          }
		 
       }
      while(ptr); 

	  // char* tokens[0] contains string lastName
	  // char* tokens[1] contains string firstName
	  // char* tokens[2] contains string ages
	   
	  cout << tokens[0] << endl;

      insertName(tokens[0]); // Insert
	   
	  
	 }

  delete(buffer);
 } // end void tokenizingFunction()

//-----------------------------------------------------------------------------

  // Possible node activity //

  // newNode-> lastName;   -value of lastName
  // newNode-> firstName;  -value of firstName
  // newNode-> age;        -value of age
  // newNode-> next;       -pointer to next node

//-----------------------------------------------------------------------------

void displayList()
{

 listNode* head;
 listNode* current;

 cout << "\nDisplaying List" << endl << endl;

 current = head;
 
  while(current != NULL)
   {
    if(current == NULL)
     {
	  cout << "\nEnd of list!" << endl;
     }
    else
     {
	  cout << current-> lastName << endl;
	  current = current-> next;
     }
   
  }// end while
}// end void displayList()

//-----------------------------------------------------------------------------
//-----------------------------------------------------------------------------

int main(int argc, char *argv[])
{

if(argc != 2)	// Usage statement
 {
  cout << "\nUsage: " << argv[0] << " 'Input File Name 1'" <<  endl;
  return(0);
 }

//--------------------------- ifstreams

ifstream input_File(argv[1], ios::in);	// Open input file to read from
 if(input_File.is_open())
  {
   cout << "\nInput File is open..." << endl;
  } 
 else
  {
   cout << "\nInput File did NOT open..." << endl;
   return (1);
  }


//-----------------------------------------------------------------------------

// Functions

listNode* head = NULL; // Initialize head pointer

if(input_File.is_open())
{
 if(input_File.good())
  {
   tokenizingFunction(); // tokenizes and inserts into list
  }
}

displayList();

//-----------------------------------------------------------------------------
 
  cin >> pause; // Non Windows Pause similar to (system("pause");)
  return 0;
}