Insertion in a Linked List - c

Question

I am trying to insert nodes in a linked list. I have tried to insert nodes at front, at the end and after a particular node.
I think that the code is fine and should work. However, this code is giving run time error. Please explain why is it giving a run time error?
Insertion in a linked list
#include <stdio.h>
#include <stdlib.h>
// A linked list node
struct node
{
int data;
struct node *next;
};
void push(struct node* head, int new_data)
{
/* 1. allocate node */
struct node* new_node = (struct node*) malloc(sizeof(struct node));
/* 2. put in the data */
new_node->data = new_data;
/* 3. Make next of new node as head */
new_node->next = head;
/* 4. move the head to point to the new node */
head = new_node;
}
/* Given a node prev_node, insert a new node after the given prev_node */
void insertAfter(struct node* prev_node, int new_data)
{
/*1. check if the given prev_node is NULL */
if (prev_node == NULL)
{
printf("the given previous node cannot be NULL");
return;
}
/* 2. allocate new node */
struct node* new_node =(struct node*) malloc(sizeof(struct node));
/* 3. put in the data */
new_node->data = new_data;
/* 4. Make next of new node as next of prev_node */
new_node->next = prev_node->next;
/* 5. move the next of prev_node as new_node */
prev_node->next = new_node;
}
void append(struct node* head, int new_data)
{
/* 1. allocate node */
struct node* new_node = (struct node*) malloc(sizeof(struct node));
struct node *last = head;
/* 2. put in the data */
new_node->data = new_data;
/* 3. This new node is going to be the last node, so make next of it as NULL*/
new_node->next = NULL;
/* 4. If the Linked List is empty, then make the new node as head */
if (head == NULL)
{
head = new_node;
return;
}
/* 5. Else traverse till the last node */
while (last->next != NULL)
last = last->next;
/* 6. Change the next of last node */
last->next = new_node;
return;
}
// This function prints contents of linked list starting from the given node
void printList(struct node *node)
{
while (node != NULL)
{
printf(" %d ", node->data);
node = node->next;
}
}
/* Driver program to test above functions*/
int main()
{
/* Start with the empty list */
struct node* head = NULL;
append(head, 6);
push(head, 7);
push(head, 1);
append(head, 4);
insertAfter(head->next, 8);
printf("\n Created Linked list is: ");
printList(head);
getchar();
return 0;
}

Answer 1

Every place you're loosing your head pointer
You should preserve its address in all functions, use struct node**, and update the pointer
using *head and (*prev_node)
void push(struct node** head, int new_data)
{ //...
// *head
}
void insertAfter(struct node** prev_node, int new_data)
{ //...
// (*prev_node)
}
void append(struct node** head, int new_data)
{ //...
// *head
}
See here
PS:There may be logical mistake
Also, make sure you're releasing memory using free() calls once you're done with everything.