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


Deleting a Node from a Singly Linked List

This article is part of article series - "Datastructures"

Previous Article: Implementation of Singly Linked List.
Next Article: Reversing a Singly Linked List

Deletion of a Node from a Singly Linked List
Similar to insertion we have three cases for deleting a Node from a Singly Linked List.

  • Deleting First Node in Singly Linked List

    To complete deletion of firstNode in the list we have to change Head pointing to Next of firstNode.

    firstNode = Head
    Head = firstNode->Next
    free firstNode
    Time Complexity: O(1)
    Space Complexity: O(1)

Detecting First Node in a Loop in the List

This article is part of article series - "Datastructures"

Previous Article: Detecting a Loop in Singly Linked List - Tortoise and Hare.
Next Article: Finding Nth node from end of a Singly Linked List.

Once we confirm that there is a Loop in a Singly Linked List we will see how to determine first node of the loop.


Finding Nth node from end of a Singly Linked List

This article is part of article series - "Datastructures"

Previous Article: Finding first node in a Loop in Singly Linked List.

Figure 1: Singly Linked List

Solution 01 - Brute Force Approach:
  1. Start at First Node of the List (call it curNodePtr).
  2. Assign curNodePtr to tmpPtr and count number of nodes after the curNodePtr.
  3. If number of nodes after curNodePtr are equal to N nodes or tmpPtr reaches END then break. If tmPtr reaches END but count not equal to N then return since we can't find the Nth node from the end of the Singly Linked List.
  4. Move the curNodePtr one step forward in the Linked List i.e curNodePtr now points to its next node in the list and start again from STEP-2.


Reversing a Singly Linked List

This article is part of article series - "Datastructures"

Previous Article: Deleting a Node from a Singly Linked List.
Next Article: Detecting a Loop in Singly Linked List - Tortoise and Hare.

Let us see how to reverse a Singly Linked List.

Figure-1: Singly Linked List
cur_ptr = HEAD->NEXT
prev_ptr = NULL


   if cur_ptr == NULL

   tmp_ptr  = prev_ptr
   prev_ptr = cur_ptr
   cur_ptr  = cur_ptr->NEXT
   prev_ptr->NEXT = tmp_ptr

HEAD->NEXT = prev_ptr
Time Complexity: O(n)
Space Complexity: O(3)

If we try on the example in Figure-1 we get the output as shown below

Figure-2: After reversing the Singly Linked List

Previous Article: Deleting a Node from a Singly Linked List.
Next Article: Detecting a Loop in Singly Linked List - Tortoise and Hare.


Detecting a Loop in Singly Linked List - Tortoise & Hare

This article is part of article series - "Datastructures"

Previous Article: Reversing a Singly Linked List.
Next Article: Finding first node in a Loop in Singly Linked List.

Eventhough there are multiple algorithms available we start with

Floyd's Cycle-Finding Algorithm
In simple terms it is also known as "Tortoise and Hare Algorithm" or "Floyd's Cycle Detection Algorithm" named after its inventor Robert Floyd. It is one of the simple cycle detection algorithm. It's a simple pointers based approach.

Robert Floyd


Write a C Program without main function

In a C program main function is the entry point so it is mandatory to have a main function.

But let us see how can we write a program without main (Kind of hiding main in some obfuscated code). This post is purely out of interest to know that we can do something weird like this, just for learning.


Implementing ls command in C

We know that ls command in unix displays the list of files in a directory. It has various options to display the data in various styles and formats.

By default its implementation is part of Coreutils package, which is default package in all Linux flavours.

I want to see how we can implement its basic functionality with a simple C program.


GIT - Adding diffmerge as visual merge in git

GIT is one of the popular distributed code repositories in opensource community, especially with developers working on opensource projects like linux kernel and others.

Operations like Merge and Diff are the most irritating & tricky tasks via command line when working on large code changes. So let us see how can we add some graphical stuff for these operations when using on GIT so that we can do merging and other options very easily. 


Decoding hardware information of a PC

Checking out machine hardware information is no more a geeky thing of olden days where you need to go through the hardware specification documents or to open up a physical machine to find out the hardware details if the specification documents are missing. Now we have some really cool handy software tools to help us out.

I thought to make a page with some important tools which help us in decoding the hardware related information on a Linux Box. Feel free to drop a comment with the tool names I missed out in this post.

NOTE: All of them are ordered in alphabetical order and I am trying these tools on a Ubuntu machine running in virtual box. So some of the tool outputs might be displaying names likes VirtualBox and Oracle Corporation.


Connecting via SSH without password prompt

SSH is the common way to connect remote machines these days. Almost all kinds of applications use SSH in background for communicating with end machines.

But sometimes typing password repeatedly for establishing a SSH connection with the trusted end machine is quite daunting task.

Let us see how to automate the things in 3 simple steps where we can ssh to "user@host" without asking a password every time. Replace user with the username and host with the remote machine ip-address or hostname. For E.g. john@

NOTE: Only do this with trusted machines.


Daemon-izing a Process in Linux

A Linux process works either in foreground or background.

A process running in foreground can interact with the user in front of the terminal. To run a.out in foreground we execute as shown below.
When a process runs as background process then it runs by itself without any user interaction. The user can check its status but he doesn't (need to) know what it is doing. To run a.out in background we execute as shown below.
$ ./a.out &
[1] 3665
As shown above when we run a process with & at the end then the process runs in background and returns the process id (3665 in above example).

what is a DAEMON Process?
A 'daemon' process is a process that runs in background, begins execution at startup
(not neccessarily), runs forever, usually do not die or get restarted, waits for requests to arrive and respond to them and frequently spawn other processes to handle these requests.

So running a process in BACKGROUND with a while loop logic in code to loop forever makes a Daemon ? Yes and also No. But there are certain things to be considered when we create a daemon process. We follow a step-by-step procedure as shown below to create a daemon process.

1. Create a separate child process - fork() it.
Using fork() system call create a copy of our process(child), then let the parent process exit. Once the parent process exits the Orphaned child process will become the child of init process (this is the initial system process, in other words the parent of all processes). As a result our process will be completely detached from its parent and start operating in background.

if (pid<0) exit(1); /* fork error */

if (pid>0) exit(0); /* parent exits */

/* child (daemon) continues */


Memory Layout of a C program - Part 2

Continuation of PART-1

As we have seen so much theory in the PART-1 now let us see a real-time example to understand about these segments. we will use size(1) command to list various section sizes in a C code.

A simple C program is given below
#include <stdio.h>

int main()
    return 0;

$ gcc test.c 
$ size a.out 
   text    data     bss     dec     hex filename
    836     260       8    1104     450 a.out
Now add a global variable as shown below
#include <stdio.h>

int global; /* Uninitialized variable stored in bss*/

int main()
    return 0;

$ gcc test.c 
$ size a.out 
   text    data     bss     dec     hex filename
    836     260      12    1108     454 a.out
As you can see BSS is incremented by 4 bytes.

Memory Layout of a C program - Part 1

A running program is called a process.

When we run a program, its executable image is loaded into memory area that normally called a process address space in an organized manner. It is organized into following areas of memory, called segments:
  • text segment 
  • data segment 
  • stack segment 
  • heap segment 
Memory layout of a C program
Figure 1: Memory layout

text segment

It is also called the code segment.

This is the area where the compiled code of the program itself resides. This is the machine language representation of the program steps to be carried out, including all functions making up the program, both user defined and system.

For example, Linux/Unix arranges things so that multiple running instances of the same program share their code if possible. Only one copy of the instructions for the same program resides in memory at any time and also it is often read-only, to prevent a program from accidentally modifying its instructions.

The portion of the executable file containing the text segment is the text section.


Deletion of a Node from Doubly Linked List

This article is part of article series - "Datastructures".

Previous Article: Inserting a Node in Doubly Linked List.

Deletion of a Node 
Let us say our current Doubly Linked List is as shown in Figure-1.
Figure 1: Current Doubly Linked List

  • Deleting First Node of the List
  • Now we have to delete First Node from the List shown in Figure-1. Because of this operation HEAD and Node-2 are affected.
    In HEAD - FIRST variable should now point to NODE-2 (i.e HEAD->FIRST = HEAD->FIRST->NEXT). If you see HEAD->FIRST->NEXT actually HEAD->FIRST currently points to NODE-1 so HEAD->FIRST->NEXT is equivalent to NODE-1->NEXT which is NODE-2.
    In NODE-2 - NEXT variable remains unchanged and PREV variable should now point to NULL since it is the first Node in the List.
    Decrement LENGTH variable in HEAD so that it maintains proper count of Nodes in the List.
Figure 2: After deleting the First Node in the List.


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.


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.

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:

 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.