c - Inserting a node before a given node in doubly linked list -

i trying insert node before given node. not able required output.

#include<stdio.h> #include<stdlib.h>  struct node{      int data;     struct node* prev;     struct node* next; };  void insert_beg(struct node** head, int new_data){     struct node* temp = (struct node*)malloc(sizeof(struct node));     temp->data = new_data;      if(*head == null){          temp->next = *head;         temp->prev = null;                   *head = temp;     }     else{         temp->next = *head;              (*head)->prev = temp;         *head = temp;      } }  void insert_before(struct node* next_node,int new_data){     struct node* temp = (struct node*)malloc(sizeof(struct node));     temp->data = new_data;      if(next_node == null)         printf("invalid!!!!");       temp->prev = next_node->prev;     temp->next = next_node;     next_node->prev = temp;      if(temp->prev!=null)         temp->prev->next = temp; }  void printlist(struct node* head){      if(head == null)         printf("the list empty\n");      else         {             while(head!=null){                  printf("%d\n",head->data);                               head = head->next;                             }          } }  int main(){      struct node* head = null;        printlist(head);         insert_beg(&head,10);     insert_beg(&head,20);     insert_before(head,70);      insert_beg(&head,30);      printlist(head); } 

here trying insert node(with data = 70) before 20.

output: 30,20,10

expected output: 30,70,20,10

when call insert_before, if given node head, new node new head. need pass the address of head in order modify it.

what have right looks this:

head   |   v ------          ------          ------ - 30 -   --->   - 20 -   --->   - 10 -  ------   <---   ------   <---   ------                   ^ ------            | - 70 -   ---------| ------ 

to fix this, include address of head in parameters insert_before.

void insert_before(struct node **head, struct node *next_node, int new_data){     struct node* temp = malloc(sizeof(struct node));   // don't cast return value of malloc     temp->data = new_data;      if(next_node == null)         printf("invalid!!!!");       temp->prev = next_node->prev;     temp->next = next_node;     next_node->prev = temp;      if(temp->prev!=null) {         temp->prev->next = temp;     } else {         *head = temp;     } } 

then call this:

insert_before(&head,head,70);