Segmentation Fault, cannot find where any pointers are NULL

Question

#include <iostream>
#include <cstring>
#include <fstream>
using namespace std;

Standard #includes, prototypes are defined.

struct housingNode * readIn(housingNode * name); 
void displayAll(housingNode * name);

I think the structure is built correctly;

struct housingNode
{
  int index; 
  int rent;
  int size;
  int room;
  char currentLocation[101];
  char tFeat1[101];
  char tFeat2[101];
  char tFeat3[101];
  char * location;
  char * Feat1;
  char * Feat2;
  char * Feat3;
  housingNode * next;
};


int main()
{   
  housingNode * head;
  head = NULL;
  readIn(head);
  return 0; 
}

Now, I know it has to be in one of these two functions, and since the second one isn't ever called yet, I'm betting it's this one, but I can't find any where to cuase my pointers to be null, or to reference any of them in that way.

struct housingNode* readIn(housingNode * name)
{
  housingNode * current;
  housingNode * temp;
  housingNode * head;   

  head = current;
  temp = current; 


  //file with housing information
  ifstream file_in;
  file_in.open("housingFile.txt");

  for(int i = 0; i < 12; ++i)
  { 

    //starts the building process for the LLL
    current = new housingNode;

    //reads in the information for the LLL
    file_in >> current -> rent;
    file_in.ignore(100,';');
    file_in >> current -> size;
    file_in.ignore(100,';');
    file_in >> current -> room;
    file_in.ignore(100,';');
    file_in.get(current -> currentLocation,100,';');
    file_in.ignore(100,';');

    //Allocates the needed memory for storing the information into a LLL. 
    current -> location = new char[strlen(current -> currentLocation)+1]; 
    strcpy(current -> location, current -> currentLocation);        
    file_in.get(current -> tFeat1,100,';');
    file_in.ignore(100,';');
    current -> Feat1 = new char[strlen(current -> tFeat1)+1];
    strcpy(current -> Feat1, current -> tFeat1);
    file_in.get(current -> tFeat2,100,';');
    file_in.ignore(100,';');
    current -> Feat2 = new char[strlen(current -> tFeat2)+1];
    strcpy(current -> Feat2, current -> tFeat2);
    file_in.get(current -> tFeat3,100,';');
    file_in.ignore(100,'\n');
    current -> Feat3 = new char[strlen(current -> tFeat3)+1];
    strcpy(current -> Feat3, current -> tFeat3);

    temp -> next = current;
    temp = temp -> next; 
  }

  //resets the node to null
  current -> next = NULL;

  //Closes file after reading in information.
  file_in.close();

  //returns the head pointer
  return head;
}   

void  displayAll(housingNode * name)
{

  housingNode * traverse;
  housingNode * head;
  traverse = head;

  cout << "Listing housing information: " << '\n';

  for(int i = 0; i < 12; ++i)
  { 
    cout <<"Cost" << traverse -> rent <<'\n';
    cout <<"Size: " << traverse -> size <<'\n';
    cout <<"Number of Rooms: " << traverse -> room <<'\n';
    cout <<"Location: " << traverse -> currentLocation <<'\n';
    cout << "Features: " << '\n';
    cout <<"#1: " << traverse -> Feat1 <<'\n';
    cout <<"#2: " << traverse -> Feat2 <<'\n';
    cout <<"#3: " << traverse -> Feat3 << endl; 

    traverse = traverse -> next;
    //Full completed function for displaying all. 
  }
}

It's hard to diagnose this kind of problem, just use a debugger to help you. — Yu Hao
– Yu Hao, Commented Mar 21, 2014 at 8:17
You don't need to dereference a null pointer to get a segmentation fault. You could simply be dereferencing a pointer to already deleted (freed memory), reading/writing beyond the bounds of an array/C-string... Other than that: what Yu Hao said. — heinrichj
– heinrichj, Commented Mar 21, 2014 at 8:20
You should read ericlippert.com/2014/03/05/how-to-debug-small-programs — hivert
– hivert, Commented Mar 21, 2014 at 8:21
If you are going to be writing code like that you should learn to use a debugger. — juanchopanza
– juanchopanza, Commented Mar 21, 2014 at 8:23
Another one trying to find a problem to a solution. Why are you not using std::string instead of char pointers and C coding? And no, using std::string does not take away from your requirements of coding your own linked list class. You're leaking memory all over the place by going overboard with the new char [] stuff. — PaulMcKenzie
– PaulMcKenzie, Commented Mar 21, 2014 at 9:47

marcinj · Accepted Answer · 2014-03-21 08:21:30Z

2

One problem is here:

struct housingNode* readIn(housingNode * name)
{
housingNode * current;
housingNode * temp;
housingNode * head; 

head = current;  // <-----

current is not initialized here, later on you use temp that is also not correctly initialized

answered Mar 21, 2014 at 8:21

marcinj

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Comments

Ferenc Deak · Accepted Answer · 2014-03-21 08:23:05Z

1

Here is one of the problems (in the function):

housingNode * current;  // and you do not initialize it
housingNode * temp;
housingNode * head;

I think you want to use name instead of current ...

but you also should initialize that before calling the function

head = current;
temp = current; 

current = new housingNode; // here the old value went away
...
temp -> next = current; // but tmp still points to an invalid address

answered Mar 21, 2014 at 8:23

Ferenc Deak

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