linked list problem when displaying the list in C - c

Question

having segmentation error while trying to access nodes
i can create new nodes with my add function after function executes i cant access my nodes. i think they deallocated in memory but i couldnt figure it out.
#include <stdio.h>
#include <stdlib.h>
struct node
{
int data;
struct node *nextNode;
};
struct node *head;
void add(int data)
{
struct node *new = (struct node *)malloc(sizeof(struct node));
new->data = data;
new->nextNode = NULL;
struct node *temp1;
temp1 = head;
while (temp1 != NULL)
{
temp1 = temp1->nextNode;
}
temp1 = new;
printf("\nValue of temp1:%d\nValue of new: %d\n",temp1,new);
printf("\nData of temp1:%d\nData of new:%d\n",temp1->data,new->data);
}
void printList()
{
int i = 1;
struct node *tempP;
tempP = head;
while (tempP != NULL)
{
printf("\nData of %dth element is : %d\n", i, tempP->data);
tempP = tempP->nextNode;
i++;
}
}
void main()
{
head = (struct node *)malloc(sizeof(struct node));
head->data = 10;
head->nextNode = NULL;
add(20);
add(30);
add(40);
printList();
}

Answer 1

This code snippet within the function add
struct node *temp1;
temp1 = head;
while (temp1 != NULL)
{
temp1 = temp1->nextNode;
}
temp1 = new;
is wrong. Within it there is changed the local variable temp1. It is not linked with the list.
Also using the conversion specifier %d to output a pointer invokes undefined behavior. You should use conversion specifier %p.
Using your approach to the function definition you could write instead.
void add(int data)
{
struct node *new = malloc( sizeof( *new ) );
new->data = data;
new->nextNode = NULL;
if ( head == NULL )
{
head = new;
}
else
{
struct node *temp1 = head;
while ( temp1->nextNode != NULL)
{
temp1 = temp1->nextNode;
}
temp1->nextNode = new;
}
printf("\nValue of temp1->nextNode:%p\nValue of new: %p\n",
( void * )temp1->nextNode, ( void * )new);
printf("\nData of temp1->nectNode:%d\nData of new:%d\n",
temp1->nextNode->data,new->data);
}
Pay attention to that it is a bad design when functions depend on a global variable as in your case where the functions depend on the global variable head.
And it is also a bad idea when the first node is added to the list bypassing the function add.
And you need check whether a node was successfully allocated.
Also according to the C Standard the function main without parameters shall be declared like
int main( void )
As for me I would declare the pointer to the head node in main like
int main( void )
{
struct node *head = NULL;
// ...
And the function add will look like
int add( struct node **head, int data )
{
struct node *new_node = malloc( sizeof( *new_node ) );
int success = new_node != NULL;
if ( success )
{
new_node->data = data;
new_node->nextNode = NULL;
while ( *head != NULL ) head = &( *head )->nextNode;
*head = new_node;
}
return success;
}
and called like
struct node *head = NULL;
add( &head, 10 );
add( &head, 20 );
add( &head, 30 );
add( &head, 40 );
In turn the function printList can look like
void printList( const struct node *head )
{
for ( size_t i = 1; head != NULL; head = head->nextNode )
{
printf( "Data of %zuth element is : %d\n", i++, head->data);
}
}
And you need at least to write one more function that will free all the allocated memory.

Answer 2

There were a handful of mistakes in your add() function, which I've highlighted and fixed below:
void add(int data)
{
struct node *new = malloc(sizeof(*new)); // suggested by ryyker
new->data = data;
new->nextNode = NULL;
struct node *temp1 = head; // just keep it short
while (temp1->nextNode != NULL) // temp1 != NULL will always result in it being NULL, last node is the node with NULL as next
{
temp1 = temp1->nextNode;
}
temp1->nextNode = new; // you want the next in the list to be the new node, not reassign the head to a new node. That's a memory leak.
// remember: temp1 == head, and head = new makes head lose the original node and point to the newly created one
printf("\nValue of temp1:%p\nValue of new: %p\n",temp1,new); // %p for pointers
printf("\nData of temp1:%d\nData of new:%d\n",temp1->data,new->data);
}
Output:
Value of temp1:0x55809a4b22a0
Value of new: 0x55809a4b22c0
Data of temp1:10
Data of new:20
Value of temp1:0x55809a4b22c0
Value of new: 0x55809a4b26f0
Data of temp1:20
Data of new:30
Value of temp1:0x55809a4b26f0
Value of new: 0x55809a4b2710
Data of temp1:30
Data of new:40
Data of 1th element is : 10
Data of 2th element is : 20
Data of 3th element is : 30
Data of 4th element is : 40