C segmentation fault when trying to implement LinkedList - c

Question

I've got the following code:
#include <stdio.h>
#include <stdlib.h>
#define MAXN 100
typedef int key;
typedef int data;
struct list * createElement(key k, data info);
struct list{
key k;
data info;
struct list *next;
};
struct list *L;
void init(key k, data info)
{
L = createElement(k, info);
}
struct list * createElement(key k, data info)
{
struct list *temp;
temp = (struct list *) malloc(sizeof(*temp));
temp->k = k;
temp->info = info;
return temp;
}
void insert(struct list * element)
{
element->next = L;
L = element;
}
void insertBefore(struct list * element, key k)
{
struct list * currentElement = L;
while(currentElement != NULL)
{
if(currentElement->k == k)
{
struct list *temp = currentElement;
currentElement = element;
currentElement->next = temp;
return;
}
currentElement = currentElement->next;
}
}
void insertAfter(struct list * element, key k)
{
struct list * currentElement = L;
while(currentElement != NULL)
{
if(currentElement->k == k)
{
struct list *temp = currentElement->next;
currentElement->next = element;
element->next = temp;
return;
}
currentElement = currentElement->next;
}
}
void deleteElement(struct list * element)
{
struct list * currentElement = L;
while(currentElement != NULL)
{
if(currentElement == element)
{
struct list * temp = currentElement;
currentElement = currentElement->next;
free(temp);
return;
}
currentElement = currentElement->next;
}
}
struct list * getElementByKey(key k) {
printf("\n1");
struct list *currentElement = L;
printf("2");
while(currentElement != NULL)
{
printf("3");
if(currentElement->k == k)
{
printf("4");
return currentElement;
}
printf("5");
currentElement = currentElement->next;
printf("6");
}
printf("There is no such element in the list");
}
struct list * pop()
{
struct list *element = L;
L = L->next;
return element;
}
int main()
{
init(0, 13);
struct list * element = createElement(5, 155);
insert(element);
struct list * k = createElement(7, 243);
insert(k);
//insertBefore(createElement(3, 100), 5);
printf("The first element value is: %d", pop()->info);
printf("The second element value is: %d", pop()->info);
printf("The element value is: %d", getElementByKey(5)->info);
return 0;
}
So when i execute it the debugger gives me segmantation fault on line 84 which is if(currentElement->k == k) on method getElementByKey. I know that i'm trying to access an unexisting element(because i use the pop method and it removes the first element in the list) but it should print me a warning message. it seems that there is a problem in the previous element's key or something that i failed to notice.

Answer 1

In your createElement function, you forgot to initialize the next pointer to NULL.
struct list * createElement(key k, data info)
{
struct list *temp;
temp = (struct list *) malloc(sizeof(*temp));
temp->k = k;
temp->info = info;
temp->next = NULL;
return temp;
}
So when you're browsing your list, it never finds the last element of the list because next pointer never equals NULL, and goes at some random position in the memory, trying to access to parts it is not allowed to do so.

Answer 2

When deleting an element (using deleteElement()) you miss to re-initialise L.
The segmenataion violation in if(currentElement->k == k)most likey dues to currentElement referring to an invalid memory address. As you test if against NULL some lines before line 84, it can only be the case that currentElement had either been assigned an invalid address explicitly or the address it points had been freed already.
From the code shown in deleteElement() I assume the latter.

Answer 3

two problem
1)at createElement()
need
temp->next = NULL;
2)at insertBefore()
not change pointer of list
currentElement = element;
e.g)
void insertBefore(struct list * element, key k)
{
struct list * currentElement = L; //add case of L->k
while(currentElement != NULL)
{
if(currentElement->next && currentElement->next->k == k)
{
element->next = currentElement->next;
currentElement->next = element;
return;
}
currentElement = currentElement->next;
}
}