Segmentation fault when running C program

Question

I have been working on a C program for an assignment in my CS class, which compiles successfully but gets a segmentation fault error upon running. The assignment deals with linkedLists; we had to take methods described and declared in a header file called linkedlist.h and implement them a file called linkedlist.c. A file called listtest.c is provided and used to test the method.

My Code (linkedlist.c, the comments are from the header file we were given describing how the methods should work):

#include "linkedlist.h"
#include <stddef.h>
#include <stdio.h>
#include <stdlib.h>


/* Alloc a new node with given data. */
struct ListNode* createNode(int inputData)
{

    struct ListNode *newNodeCN;
    newNodeCN->data = inputData;
    return newNodeCN;
}

/* Insert data at appropriate place in a sorted list, return new list head. */
struct ListNode* insertSorted(struct ListNode* head, int inputData)
{
    printf("insertsorted started \n");

    struct ListNode * nextIS = NULL;
    struct ListNode * newNodeIS = NULL;
    struct ListNode * currIS = head;
    struct ListNode * listHeadIS = currIS;
    if (currIS == NULL)
    {
        listHeadIS = createNode(inputData);
        return listHeadIS;
    }
    while (currIS->next != NULL)
    {
        nextIS = currIS->next;

        if (currIS->data < inputData)
        {
            if (nextIS->data >= inputData)
            {

                nextIS->next = createNode(inputData);
                newNodeIS = nextIS->next;
                if (newNodeIS->data > listHeadIS->data)
                {
                    listHeadIS = newNodeIS;
                }
            }
        }
        currIS = currIS->next;
    }

    return listHeadIS;
}

/* Remove data from list pointed to by headRef, changing head if necessary.
* Make no assumptions as to whether the list is sorted.
* Memory for removed node should be freed.
* Return 1 if data was present, 0 if not found. */
int removeItem(struct ListNode** headRef, int data)
{
    struct ListNode * tempRem = *headRef;
    int filterVal = data;

    while (tempRem->next != NULL)
    {
        if (tempRem->data == filterVal)
        {
            free(tempRem);
            tempRem = tempRem->next;
            return 1;
        }
    }


    return 0;
}
/* Insert data at head of list, return new list head. */
struct ListNode* push(struct ListNode* head, int data)
{
    printf("push started \n");
    int dataPush = data;

    struct ListNode * tempPush = (struct  ListNode*)malloc(sizeof(struct ListNode));
    tempPush->data = dataPush;
    tempPush->next = head;
    *head = *tempPush;
    return tempPush;
}

/* Remove and return data from head of non-empty list, changing head.
* Memory for removed node should be freed. */
int pop(struct ListNode** headRef)
{
    struct ListNode * tempPop = *headRef;
    int tempData;

    tempData = tempPop->data;
    free(tempPop);
    tempPop = tempPop->next;

    return tempData;
}
/* Return length of the list. */
int listLength(struct ListNode* head)
{
    int i;
    while (head->next != NULL)
    {
        i++;
        head = head->next;
    }
    return i;
}
/* Print list data on single line, separated with spaces. */
void printList(struct ListNode* head)
{
    printf("PrintList Started \n");
    if (head != NULL)
    {

        while (head->next != NULL)
        {

            printf("%d\n", head->data);
            head = head->next;
        }
    }
}
/* Free memory used by the list. */
void freeList(struct ListNode* head)
{
    while (head != NULL)
    {
        free(head);
        head = head->next;
    }
}
/* Reverse order of elements in the list */
void reverseList(struct ListNode** headRef)
{
    struct ListNode * origRL = *headRef;
    struct ListNode * nextRL = NULL;
    struct ListNode * prevRL = NULL;
    while (origRL->next != NULL);
    {
        nextRL = origRL->next;
        prevRL = origRL;
        origRL = nextRL;
        origRL->next = prevRL;
   }

}

The code from listtest.c (I did not write this):

#include <stdio.h>
#include <stdlib.h>
#include "linkedlist.h"

int main(int argc, char** argv)
{
  int i, n;
  struct ListNode* head = NULL;
  struct ListNode* stack = NULL;

  printf("empty list: ");
  printList(head);

  for(i = 0; i < 23; ++i)
  {
    n = (i*17+11) % 23;
    head = insertSorted(head, n);
    printf("sort succesful /n");
    stack = push(stack, n);
  }

  printf("filled list: ");
  printList(head);
  printf("list length = %d\n", listLength(head));

  printf("filled stack: ");
  printList(stack);
  printf("stack size = %d\n", listLength(stack));

  for(i = -4; i < 25; i+=4)
  {
    n = removeItem(&head, i);
    if(!n) printf("remove did not find %d\n", i);  
  }

  printf("list after removes: ");
  printList(head);
  printf("list length = %d\n", listLength(head));

  for(i = 0; i < 5; ++i)
  {
    printf("pop: %d\n", pop(&stack));
  }

  printf("stack after pops: ");
  printList(stack);
  printf("stack size = %d\n", listLength(stack));

  reverseList(&head);
  printf("list after reverse: ");
  printList(head);

  freeList(head);
  head = NULL;

  freeList(stack);
  stack = NULL;

  return 0;
}

According to both Valgrind and GDB the segmentation fault is being caused by something in main. Valgrind gives me the error:

 Access not within mapped region at address 0x6FFFFFFE3
==6300==    at 0x400953: main 

My question is what exactly is causing the segmentation fault, how could I fix it, and could anything else in my code cause a segmentation fault? I am not allowed to edit listtest.c, so any changes will have to be in linkedlist.c. Thank You.

Answer 1

1. struct ListNode *newNodeCN; newNodeCN->data = inputData; No new node is actually allocated

2. int listLength(struct ListNode* head) - i is never initialized.

3. oid freeList(struct ListNode* head) - free, then dereference

Answer 2

In function insertSorted , the condition if (newNodeIS->data...) derefferences the pointer without verifing it's not NULL , thatay be the source of the segmentation fault. try fixing that the report back :)

Answer 3

first in the function createNode,u use the element "data" but didn't malloc a real memory for the newNodeCN whitch's type is struct ListNode. besides, the "segmentation fault" usually comes up when u attend to reference a wrong memory address(such as a pointer u not malloc, or a address out of a array and so on).