/* Ajith - Syntax Higlighter - End ----------------------------------------------- */


Inserting a Node in Doubly Linked List

This article is part of article series - "Datastructures"

Previous Article: Doubly Linked List                                           Next Article: Deletion of a Node from Doubly Linked List.

Insertion of a Node
Before we discuss about how to insert a NODE let us discuss few rules to follow at the time of insertion.

  • Check the location into which the user want to insert a new NODE. The possible locations where an user can insert a new node is in the range of 1 <= loc <= (length of list)+1. Let us say the length of the list is 10 & the user want to insert at location 12 (sounds stupid).

  • As we know we can traverse Bi-Directional in case of Doubly Linked Lists so we have to take care of PREV and NEXT variables in the NODE structure. We should also update the neighboring Nodes which are affected by this operation. If not we might break up the List somewhere or the other by creating a BROKEN LIST.

We have following scenarios in the case of insertion of a NODE.
  • Adding a Node at the start of the Empty List
  • Figure 1: Empty List and the newNode we want to add
    As shown in Figure-1 we have a Empty List with LENGTH set to 0 and FIRST pointing to NULL. Let us add newNode at Location 1. 
    In HEAD - FIRST variable points to newNode (head->FIRST = newNode).
    In newNode - NEXT and  PREV points to NULL as we don't have any other Nodes in the List.
    Increment the LENGTH variable in HEAD once insertion is successful to maintain the count of number of Nodes in the List.
    HEAD->FIRST = newNode
    newNode->PREV = NULL
    newNode->NEXT = NULL
Figure 2: After adding newNode in Empty List. (Changes in BLUE)


Implementation of Doubly Linked List in C

In computer science, a doubly linked list is a linked data structure that consists of a set of sequentially linked records called Nodes. Each Node contains two fields, called Links, that are references to the Previous and to the Next Node in the sequence of Nodes as well as field named Data.
For every Linked List we have something called Head which marks the starting of a list. So we have two main structures namely
  • Node
  • Head

Why we need a new structure for HEAD variable ? 
Just for convenience I decided to have HEAD its own structure. You can even use the Node structure.

Every Node in a Doubly Linked List has three main members namely
  • PREV
  • DATA
  • NEXT
As their names say
  • PREV - holds the memory location of the Previous Node in the List. If there are none we point towards NULL. For the First Node in the List PREV points to NULL.
  • NEXT - holds the memory location of the Next Node in the List. If there are none we point towards NULL. For the Last Node in the List NEXT points to NULL.
  • DATA - In simple words it holds Data. In our case it holds the memory location to the actual data to be held by the Node.
typedef struct node
    struct node *prev;
    void        *data;
    struct node *next;

Head acts as the  "head" of the List. Head structure has two members namely
  • LENGTH - holds the count of number of Nodes in the List.
  • FIRST - hold the memory location of the first Node in the List. If the List is EMPTY it points to NULL.
typedef struct head 
    unsigned int length;
    struct node  *first;

NOTE: Our Head structure doesn't contain any pointer to the Tail of the List. Eventhough its a best way to include a pointer to Tail Node we decided to cover that implementation in Circular Doubly Linked List.


Notification Chains in Linux Kernel - Part 03

Continuation after PART-2.

Notifying Events on a Chain 
Notifications are generated with notifier_call_chain. This function simply invokes, in order of priority, all the callback routines registered against the chain. Note that callback routines are executed in the context of the process that calls notifier_call_chain. A callback routine could, however, be implemented so that it queues the notification somewhere and wakes up a process that will look at it.

NOTE: Similar to register and unregister functions we don't directly call notifier_call_chain function as we have wrapper functions for respective chains.

 58 static int __kprobes notifier_call_chain(struct notifier_block **nl,
 59                     unsigned long val, void *v,
 60                     int nr_to_call, int *nr_calls)
 61 {
 62     int ret = NOTIFY_DONE;
 63     struct notifier_block *nb, *next_nb;
 65     nb = rcu_dereference(*nl);
 67     while (nb && nr_to_call) {
 68         next_nb = rcu_dereference(nb->next);
 69         ret = nb->notifier_call(nb, val, v);
 76         nb = next_nb;
 77         nr_to_call--;
 78     }
 79     return ret;
 80 }
  • nl
    Notification chain. 

  • val
    Event type. The chain itself identifies a class of events; val unequivocally identifies an event type (i.e., NETDEV_REGISTER). 

  • v
    Input parameter that can be used by the handlers registered by the various clients. This can be used in different ways under different circumstances. For instance, when a new network device is registered with the kernel, the associated notification uses v to identify the net_device data structure.

  • nr_to_call
    Number of notifier functions to be called. Don't care value of this parameter is -1. 

  • nr_calls
    Records the number of notifications sent. Don't care value of this field is NULL.