Author: Abhilash

Hi, I’m Abhilash! A seasoned web developer with 15 years of experience specializing in Ruby and Ruby on Rails. Since 2010, I’ve built scalable, robust web applications and worked with frameworks like Angular, Sinatra, Laravel, Node.js, Vue and React. Passionate about clean, maintainable code and continuous learning, I share insights, tutorials, and experiences here. Let’s explore the ever-evolving world of web development together!

What is padding in C?

In C what is meant by padding of bits? Consider the following structure,

struct number {
int a;
char b;
int c;
};


What is the size of this structure? An integer takes 4 bytes, char takes 1 and the third integer 4 bytes. Total 9 bytes. This is wrong. We can see the size of the structure is 12 bytes. What happens here? the compiler pads 3 bytes to char element. This is for proper alignment. In a 32 bit processor it checks the next data only after 32 bits. Else the compiler will have related problems. Now the compiler knows each data is 4 bytes wider. So the benefits.

Is the study of pointers in C is easy?

Here explains a little about pointers.  Yes the pointers are easy, of course. But take care in your building path. If you go through pointers deeply, you will see the complexity.

In C address is only for l-values. It can be a variable or something. If we assign p = &a, the type of p is int * – pointer to an integer. type of ‘a’ is int and type of ‘&a’ is int *. C language is a weakly typed language. ie when we use assignment,  C allows to assign the right most thing to the left most without checking the types of these two.

int *p;
p = 1;
*p = 10;
Consider the above statements in C. We declared p as a pointer variable. Then 1 is assigned to p. In the third statement 10 is assigning to the memory location 1. What about the memory location 1, is there exists a memory address 1. Usually not. If yes, of course it isn’t accessible. OS allows the program to access only within a particular memory location. This is Memory protection. If OS takes no care about memory protection what happens? In that system so many programs are running and two programs want to store data on say 2000 th location then what happens? The programs access wrong data.

int *p;
p = 0;
*p = 10;

By executing above statements there is no error at compile time. Why? C takes zero as a pointer.

Its easy to go further.

HTML5

It is not a totally new version of HTML 4 . It support the features of HTML 4 and is a collection of individual features like canvas, video, geolocation etc. We can interacts with javascript through DOM (Document Object Model). It does not have any special notation to handle features like video. The API defines this. Upgrading to HTML5 need only change the doctype.

!DOCTYPE html

It defines new semantics like article, section, footer etc.

Local Storage :

HTML5 provides local storage. It stores information on the mechine, this allows the sites to retrieve it later.

Programming in UNIX Environment : Using the shell

A greater than sign ‘>’ followed by a name makes a file of that name. the -n option of echo command is for omiting the newline. the ‘-l’ option for the ‘who’ command shows the details of users. The shell is a command like all unix commands. It is represented by ‘sh’.

$ > file //creates a file named file.
$ echo -n you are lucky
$ echo /* //prints the all the folders in root (not recursively)
$ echo \* //prints a ‘*’ character
$ who -l

Programming in python : Classes

Create a class as follows,

>>> class abc():
… a = 1
… b = 2
…
create x as an instance of the class abc,

>>> x = abc()
>>> x

>>> x.b
2
Create another class with a function definition,

>>> class abc():
… a = 3
… b = 4
… def fun():
… print ‘hello’
…
>>> x = abc()
>>> x.a
3
>>> x.b
4
>>> x.fun()
Traceback (most recent call last):
File “”, line 1, in
TypeError: fun() takes no arguments (1 given)

We cannot call the function in class abc, as an attribute of x. x has no such an attribute. But the address of x is already given by the interpreter as a parameter to that function. Usually we give an extra parameter ‘self’, have no special meaning.

>>> class abc():
… a = 1
… b = 2
… def fun(self):
… p = 5
… q = 6
… print ‘hello’
…
>>> x = abc()
>>> x.a
>>> 1
>>> x.p
>>> x.fun()
The above statement causes an error. Because x has no attribute p. ‘x.fun()’ cannot print hello, so little changes are needed to our code and is shown below.

>>> class abc():
… x = 1
… y = 2
… def fun(self):
… self.p = 3
… self.q = 4
… print ‘Hello’
…
>>> m = abc()
>>> m.p
Traceback (most recent call last):
File “”, line 1, in
AttributeError: abc instance has no attribute ‘p’

Here the interpreter says that the abc instance has no attribute ‘p’.’p’ is an attribute of the function ‘fun’. So create an instance of abc and pass the address of m to ‘self’.

>>> m = abc()
>>> m .fun()
Hello

Programming in UNIX Environment : Commands – sort, tail, cmp, diff

The command ‘sort’ is for sorting the line of the text. ‘tail’ prints the last ten lines of the file. ‘cmp’ for showing the difference between the files. It shows only the first difference. But ‘diff’ shows all the difference between two files.

sort filename1 file2
sort -r // reverse normal order.
sort -n // numeric order.
sort -f // fold upper and lower case together.
sort -b // Ignore the leading blanks.

tail filename // prints last 10 lines.
tail -3 filename // print last 3 lines.

cmp filename1 filename2 // prints only the 1st difference.

diff filen1 filen2 // prints all differences.

pwd – For printing current working directory.
cp – For copying a file.

cp /home/abhi/Pclass/num.c num.c

ed /home/abhi/Pclass/num.c

Programming in UNIX Environment : Commands – mv, rm, wc, grep

The ‘mv’ command is using for renaming. And ‘rm’ command for removing files. ‘wc’ for counting lines,words,characters. The ‘grep’ command is for searching a word. ‘-v’ for searching all words except the indicating word in a line.

mv filename newname

rm filename1 filename2

wc filename1 filename2 // count lines,words,characters.
wc -l //only counts the new lines.
wc -w //print the word count.
wc -m //print the character counts.
wc -c //printf the byte counts.

grep int filename // for searching the word int.
grep -v int filename // all except int.

Programming in UNIX Environment : Commands – kill, ps, nohup, nice

The ampersand at the end of the command says that run this command and also take another from the terminal without stopping the first. It prints the process id. For stopping a process we use ‘kill’ command followed by process id. ‘ps’ command prints what that user running. Using ‘nohup’, the command will run even if the user logged out. ‘nice’ is for using another system to run our job.

$wc a* >wc.out &

$kill 6578

$ps

$ps -ag // all process that are currently running.

$nohup command & // No hangup for this command even if user logged out.

$nice expensive-command & // if your command take a lot of processor resources.

The ‘at’ command followed by time is to run our job at whatever time we want.

$ at 2340 <file

There is a file ‘.profile’ in the user’s login directory, this file contains commands which the user want to run when the user logged in. A user can set things in it.

a=/long/directory/name

$cd $a

We can put ‘a’ as given above in our profile. So we can refer that directory by the value of shell variable.

Unix file system : Devices

/dev/rp00 and /dev/rp01 are named after the DEC RP06 disc drive attached to the system. We cannot make a link to a file in another subsystem. One reason is that the inode numbers are not unique in different file systems.
$ df
$ tty
$ mesg n
$ mesg y

The ‘df’ command tells the available space in the mounted file subsystem. The ‘tty’ command tells which terminal we are using. So we can send files and informations to that terminal and it appears in the corresponding terminal. ‘mesg n’ turn off the messages. /dev/tty is a synonym for our terminal.

If we want to run a program and its output files are not importent we can redirect it to ‘/dev/null’ causes its output to be thrown away.

Unix file system : The directory hierarchy

When the system starts the file /boot is read, it then reads /unix (it is the program for the UNIX kernal itself).

The following shows some directories which reside in root and its contents.

/bin : basic programs like who,ed reside.

/etc/rc : This is a file of shell commands which is executed after the system is bootstrapped.

/etc/group : lists the members of each group.

/lib : Contains the parts of the C compiler.

/tmp : Contains temporary (short-lived) files that are created in the execution of programs.