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.

Node
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;
}NODE;

Head

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;
}HEAD;

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.

 <kernel/notifier.c>

 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;
 64 
 65     nb = rcu_dereference(*nl);
 66 
 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.

Notification Chains in Linux Kernel - Part 02

Continuation after PART-1.

Check the PART-3. 

Blocking Notifier chains
A blocking notifier chain runs in the process context. The calls in the notification list could be blocked as it runs in the process context. Notifications that are not highly time critical could use blocking notifier chains.

Linux modules use blocking notifier chains to inform the modules on a change in QOS value or the addition of a new device.

<kernel/notifier.c>

186 int blocking_notifier_chain_register(struct blocking_notifier_head *nh,
187         struct notifier_block *n)
188 {
.
199     down_write(&nh->rwsem);
200     ret = notifier_chain_register(&nh->head, n);
201     up_write(&nh->rwsem);
202     return ret;
203 }
204 EXPORT_SYMBOL_GPL(blocking_notifier_chain_register)
.
216 int blocking_notifier_chain_unregister(struct blocking_notifier_head *nh,
217         struct notifier_block *n)
218 {
.
229     down_write(&nh->rwsem);
230     ret = notifier_chain_unregister(&nh->head, n);
231     up_write(&nh->rwsem);
232     return ret;
233 }
234 EXPORT_SYMBOL_GPL(blocking_notifier_chain_unregister);

Notification Chains in Linux Kernel - Part 01

Linux is a monolithic kernel. Its subsystems or modules help to keep the kernel light by being flexible enough to load and unload at runtime. In most cases, the kernel modules are interconnected to one another. An event captured by a certain module might be of interest to another module.

Typically, communication systems implement request-reply messaging, or polling. In such models, a program that receives a request will have to send the data available since the last transaction. Such methods sometimes require high bandwidth or they waste polling cycles.

To fulfill the need for interaction, Linux uses so called notification chains. These notifier chains work in a Publish-Subscribe model. This model is more effective when compared to polling or the request-reply model.

For each notification chain there is a passive side (the notified) and an active side (the notifier), as in the so-called publish-and-subscribe model:

• The notified are the subsystems that ask to be notified about the event and that provide a callback function to invoke.

• The notifier is the subsystem that experiences an event and calls the callback function.

NOTE: All the code samples are taken from Linux 2.6.24 kernel.

struct notifier_block
The elements of the notification chain's list are of type notifier_block:

<include/linux/notifier.h>

 50 struct notifier_block {
 51     int (*notifier_call)
(struct notifier_block *, unsigned long, void *);
 52     struct notifier_block *next;
 53     int priority;
 54 };

• notifier_call - function to execute. 

• next - used to link together the elements of the list.

• priority - the priority of the function. Functions with higher priority are executed first. But in practice, almost all registrations leave the priority out of the notifier_block definition, which means it gets the default value of 0 and execution order ends up depending only on the registration order (i.e., it is a semirandom order).

The notifier_block data structure is a simple linked list of function pointers. The function pointers are registered with ‘functions’ that are to be called when an event occurs. Each module needs to maintain a notifier list. The functions are registered to this notification list. The notification module (publisher) maintains a list head that is used to manage and traverse the notifier block list. The function that subscribes to a module is added to the head of the module’s list by using the register_xxxxxx_notifier API and deletion from the list is done using unregister_xxxxxx_notifier.

Printing logs based on log levels in C

LOG LEVELS ??
As per my definition LOG LEVEL means a way to differentiate the importance of logs in our application. We can divide the logs into categories based on their importance and effect for e.g. ERROR logs are more important than DEBUG logs.

Why do we need to print logs based on LOG LEVEL ??
It is really helpful in projects with millions of lines of source code where the user can't use #defines or #ifdef's in order to maintain DEBUG prints. It is really tiresome to maintain #defines and #ifdef atleast for printing logs.

printk which is a part of LINUX KERNEL supports printing logs based on LOG LEVEL and it is really helpful in debugging kernel.

printf or any of its brothers & sisters don't support the option to print logs depending upon the log levels.