Unhandled exception in simple.exe. Hjælp !

Jeg har flg program i c:

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include "simple.h"

/* implement functions defined in simple.h */
list_element_p newElement(list_element_p prev, char *str) {
    /* allocate new element */
    list_element_p elem = (list_element_p)malloc(sizeof(struct list_element));
   
    /* copy str into the element */
    int str_len = strlen(str);
    elem->str = (char *)malloc(sizeof(char)*str_len);
    strncpy(elem->str, str, str_len);
   
    /* insert element */
    elem->prev = prev;
    if (prev != NULL) {
        list_element_p next = prev->next;
        elem->next = next;
        prev->next = elem;
        if (next != NULL)
            next->prev = elem;
    } else
        elem->next = NULL;
    return elem;
}

void deleteElement(list_element_p element) {
    /* delete the element from the list */
    list_element_p prev = element->prev;
    list_element_p next = element->next;
    if (prev != NULL)
        prev->next = next;
    if (next != NULL)
        next->prev = prev;
   
    /* free the list element */
    free(element->str);
    free(element);
}

void printElement(list_element_p element) {
    printf(element->str);
}

list_element_p findElement(list_element_p list_head, char *str) {
    list_element_p list = list_head;
    while (list != NULL) {
        if (strcmp(str, list->str) == 0)
            return list;
        list = list->next;
    }
    /* No element found */
    return NULL;
}

/* simple test program */
int main(int argc, char **argv) {
    list_element_p head;
    list_element_p elem;
    list_element_p traversal_element;
    list_element_p search_element;
   
    /* create list */
    head = newElement(NULL, "string 1");
    elem = newElement(head, "string 2");
    elem = newElement(elem, "string 3");
    elem = newElement(elem, "string 4");
   
    printElement(head);
    printf("\n");
    traversal_element = head;
    printf("All list elements:\n");
    while (traversal_element != NULL) {
        printElement(traversal_element);
        printf("\n");
        traversal_element = traversal_element->next;
    }
    search_element = findElement(head, "string 3");
    printf("Deleting element: ");
    printElement(search_element);
    printf("\n");
    deleteElement(search_element);
    printf("All list elements (again):\n");
    traversal_element = head;
    while (traversal_element != NULL) {
        printElement(traversal_element);
        printf("\n");
        traversal_element = traversal_element->next;
    }
    search_element = findElement(head, "string 4");
    printf("Deleting element: ");
    printElement(search_element);
    printf("\n");
    deleteElement(search_element);
    traversal_element = head;
    while (traversal_element != NULL) {
        printElement(traversal_element);
        printf("\n");
        traversal_element = traversal_element->next;
    }
}

og en header-fil:

#ifndef __SIMPLE_H_INCLUDED
#define __SIMPLE_H_INCLUDED

typedef struct list_element *list_element_p;

struct list_element {
    char *str;
    list_element_p next;
    list_element_p prev;
};

/* create a new list element with the specified str and add it after
prev */
list_element_p newElement(list_element_p prev, char *str);
/* delete the specified list element from its list */
void deleteElement(list_element_p element);
/* print the str value of a specified list element */
void printElement(list_element_p element);
/* search sequentially for a list element with the specified str value,
beginning at list_head */
list_element_p findElement(list_element_p list_head, char *str);

#endif /* __SIMPLE_H_INCLUDED */

- Når jeg kører programmet får jeg flg fejl:
  "Unhandled exception in simple.exe:0xC0000002: Access violation"

Jeg er temmelig ny i c-programmering, så der er sikkert noget basalt jeg har overset