SearchListNode Error - c

Question

#define _CRT_SECURE_NO_WARNINGS
#include <stdio.h>
#include <stdlib.h>
typedef struct _listnode
{
int item;
struct _listnode *next;
} ListNode;
int search(ListNode *head, int value);
void printList(ListNode *head);
int main()
{
ListNode *head = NULL, *temp = NULL;
int i = 0;
int value = 0;
while (1)
{
printf("Enter a integer: ");
scanf("%d", &i);
if (i == -1)
break;
if (head == NULL)
{
head = malloc(sizeof(ListNode));
temp = head;
}
else
{
temp->next = malloc(sizeof(ListNode));
temp = temp->next;
}
temp->item = i;
}
printList(head);
printf("Enter the value to search for: ");
scanf("%d", &value);
printf("Value of %d found in index %d",value,search(&head, value));
return 0;
}
void printList(ListNode *head)
{
while (head != NULL)
{
printf("%d ", head->item);
head = head->next;
}
printf("\n");
}
int search(ListNode *head, int value)
{
if (head == NULL)
return NULL;
if (head->item != value)
{
head = head->next;
}
return value;
}
I am trying to search a value inside a LinkedList and print out the index of the node when the value is found. But after my printlist, my program just shut down, I did not even reach to the part that require me to enter the value I want to search. Is there something wrong with my printList?

Answer 1

First of all, you should check the return value of scanf() - it returns the number of items successfully matched and assigned. Only after checking the return value can you be sure that the values you use are valid.
The problem, however, is that you never terminate the list properly, that is, you never set the last node's next to NULL. Try to add this after the while:
temp->next = NULL;
Personally, though, I'd advice against doing so. In general, it is more elegant if the list itself is in a consistent state after each iteration of the loop. So, this is what I suggest instead:
while (1)
{
printf("Enter a integer: ");
if (scanf("%d", &i) != 1) {
/* Deal with invalid input... */
}
if (i == -1)
break;
ListNode *new_node;
if ((new_node = malloc(sizeof(*new_node))) == NULL) {
/* Deal with memory exhaustion */
}
new_node->item = i;
new_node->next = NULL;
if (head == NULL)
head = new_node;
else
temp->next = new_node;
temp = new_node;
}
That makes temp work as a pointer to the previous node, so you might want to rename it to prev. Note the check for the return value of scanf() and malloc(), as well as the operand to sizeof: it is a good idea not to hardcode the type when using sizeof for maintenance reasons. Finally, notice, if you will, that there is now a single call to malloc(). This is conceptually easier to understand, since a new node is allocated in the same place in the code, regardless of head being valid or not.
Also, your search() function is broken. It assumes that if value is not the first node's value, then it is the second - not what you want. Also, you are not returning the index of the node, instead, you are returning the value itself. In fact, there's a situation where you return NULL (?!). I doubt that this compiles without warnings.
You need something like this instead:
int search(ListNode *head, int value)
{
int res = 0;
for (; head != NULL && head->item != value; head = head->next, res++)
; /* Intentionally left blank */
if (head == NULL)
return -1;
else
return res;
}

Answer 2

You don't put temp->next to NULL after this line temp = temp->next; in the list creation so it can be NULL or at any other value. When you iterate the list in printList() you reach this random value pointer and you have a segfault.