Hackers Handbook - Millenium Edition/Hackers Handbook.iso/library/hack/cprogramming.txt

Text File | 1998-07-17 | 10.0 KB | 345 lines

C programming for the complete newbie

        Hello there im Krisis you may have seen me on irc.hackersclub.com.
Well I thought it was about time to write an article like everyone else. But
unlike many others
mine wont be on Hacking, Cracking, or Phreaking it's on C programming, you
see I'm not the best
hacker but I'm an ok programmer. So here it goes. This is based for absolute
beginners so those
of you  like my friend Chrak wouldn't be interested in it.

        1st lets talk about some of C 's history. C was invent by Dennis Ritchie
and Kenneth
 Thompson. They modeled it after the language they were using called B. C
was a subset of B hence
the name. C was made because B was going out of style and they needed a new
language to write
UNIX in. Yes UNIX was made in C. C was made popular very quickly because
every UNIX sold had a C
compiler. A compiler is a program thats looks at your source code and
transfers it into object
code, after it is transfered into object code it must be linked, once it's
linked it can be
executed.

/***************************************************************************
********************/
        2nd Lets talk about variables they are your integers and characters and so
on. You have
many data types they are.
                                int  integers

                                char characters

                                long int bigger integers

                                short int same as int

                                float decimal numbers

                                double even bigger decimal numbers

To define a variable 1st you must put something like   int MyNum;
MyNum = 2;
or for a character value it would be like              char name;       name
= 'Jim';

notice the ''  you must have those around character values, they are not
needed for integers or
decimals.

/***************************************************************************
********************/
        3rd I'll tell you about stuff like #include and #define. #include is used
to tell the
compiler that whatever is in the brackets just be included like its part of
your code.

#include <stdio.h>

#define is used to define something Like the color of a truck or car.

#define TRUCK "red"

#include and #define must come before any functions are even prototyped
(I'll talk about this
later).

/***************************************************************************
********************/
        4th I'll talk about functions. Every program must have at least one
function. That
functions name must be main(). The () tells the compiler that it is a
function. All functions
must return a value in the main() function a 0 is usually returned. In your
functions you will
want to use comment's to explain your code a comment is begun by using /*
and ended by using */ .
I will now show you your 1st program.


#include <stdio.h>      /* Used in most standard Input Output Programs */
main()
{                       /* Beginning Brackets used to show the beggining of
a block of code */
printf("Hello World");  /* A function already written in Stdio.h */
return 0;               /* Value returned from the program */
}                       /* Ending bracket used to show end of a block of code */

now compile your program in your compiler if your using UNIX do it like this
gcc hello.c -o Hello
and then run your program bye typing in ./Hello

/***************************************************************************
********************/
        5th I'll talk about output which is essential to almost all programs.
I'll start you out with printf(); It is defined in Stdio.h so every time you
call printf(); you
must include Stdio.h .
printf(); 's syntax is quite easy you just used it like this

printf("What ever you want outputted");

to output variables you do it like this

char dog='scruffy';
printf("My dogs name is %c", dog);  notice the %c it tells the compiler to
look for a character
                                    variable.

Now for Integers and Decimals

int age=16;
printf("I am %d year's old", age);  use %d to print out decimals and integers

/***************************************************************************
********************/
6th Lets talk about multiple functions. When you have more than one function
you must prototype
it. Here is an example.

#include <stdio.h>
void hello(); /* This is a prototype notice the void. Void tells the
compiler that this function
                 does not return a value like return 0; */
main() /* Main doesnt ever need to be prototyped */
{
hello();
return 0;
}

void hello(); /* Your prototype must look exactly like your real function */
{
printf("Im in the function hello!");
}

Void is your return type. Other return types are int for returning integers
use float to return
decimals and so on.

/***************************************************************************
********************/
7th I'll introduce you to input. Ill teach you how to use gets() and scanf()
and fgets() properly
gets() takes a variable and place data into as do scanf() and fgets() In the
next example I will
use all 3

#include <stdio.h>
#include <conio.h>

main()
{
int x, y, z, ans;

printf("What is X 's value \n ");
gets(x);
printf("What is Y 's value \n");
scanf("%d", &y); /* Scanf is odd I dont recommend using it try and use gets
and fgets more */
/* Whatever is used to print the variable type you are using is placed in
parantheses and & is
   used in front of whatever variable you are using */

printf("What is Z 's value");
fgets(z, 25, stdin); /* fgets is kinda tricky at first glance */

/* first off you put what variable you want then how many integers  or
characters
long it can be and then stdin,stdin is a macro defined in stdio.h it is used
to represent
standard input */

ans=x+y+z;
printf("Ans equals %d", ans);

/***************************************************************************
********************/
8th Lets talk about decision statements like if and else.
here is how if is used

if(VariableName==5)
  {
   printf("Your variable is 5");
  }

else is used after if, it is used like this

if(VariableName==5)
  {
   printf("Your variable is 5");
  }
else
  {
   printf("I dont know what your variable is");
  }

/***************************************************************************
********************/
9th Ill talk about While loops and do-while loops. Loops aren't as hard as
they may seem.
while loops are easy. Just watch and learn.

#include <stdio.h>
main()
{
int x=1;
        while(x<2600)
        {
        printf("X=%d",x);
        x++; /* adds 1 to x */
        }
return 0;
} /* While loops dont have to happen only if the right sequence happens do
they execute */

Loops can be placed inside of IF and else statements if you want. That can
be very helpful if you
want a process to happen a bunch if something happens Like the user pressing
X instead of Y.

Do-While loops are just as easy. They automatically execute at least once.

#include <stdio.h>
main()
{
int x=1;
        do
        {
        printf("X=%d",x);
        x++;
        }while(x<2600);
return 0;
}

The do tells the program to do this at least once and it doesn't see the
while until it has
already do the do.

/***************************************************************************
********************/
10th I'm going to tell you about another kind of loop the for loop. For
loops execute a given
number of times and then stop. For loops are executed like this.

for(x=1; x<100; x++)
{
printf("X=%d",x);
}
That prints 1 through 100. Thats about it about for loops there not very
hard. They can be pretty
useful. But I dont use them alot I'm into While loops.

/***************************************************************************
********************/
11th is all about Arrays. Arrays are consecutive places in memory. Arrays
can be integers and
characters. They can be just about any size. Here is an example.

#include <stdio.h>
main()
{
int i[2]

int i[1] =2600
int i[2] =1982 /* Year I was born */

printf(" I[1] = %d ",i[1]);
printf(" I[2] = %d ",i[2]);

return 0;
}

See how easy that was Arrays aren't very hard at all.

/***************************************************************************
********************/

12th I'll tell you about passing parameters to functions. Its nots to hard
but I've said that
about everything. First you must prototype it before main() I hope you
remember how to prototype.
Here is an example of passing parameters.

#include <stdio.h>
int next(int x);

main()
{
int age;
printf("please enter you age ");
fgets(age, 3 ,stdin);
age(age); /* Age is passed on to the next function */

return 0;
}

next(int x);
{
x++;
printf("Next year you will be %d", x);
return 0;
}

/***************************************************************************
********************/
13th I'll say a little sumthin about why C is good to Hacking. It's good
because it is so
portable C can be used on all processors and Operating Systems. So if your
exploit you just wrote works on
one UNIX like OS odds are it will work on another, therefore you dont have
to write a whole new
program just to get a root shell.

/***************************************************************************
********************/
Last but not least a sample program.

#include <stdio.h>
int blah(int x,int y);
main()
{
int a,b,r;
printf("Enter some numbers ");
scanf("%d", &a);
scanf("%d", &b);
r=blah(a,b);
printf("R = %d",r);
return 0;
}

int blah(int x, int y)
{ return x * y; }

What does blah do and how does it work?    You tell me.
/***************************************************************************
********************/
Thats it for my little tutorial on C. I hope it helped you some. But for
further info on C I
suggest reading
C programming in 12 easy lessons by Greg Perry from Sams Publishing.
It helped my a shitload on learning C. You also might want to get some books
on C++ a subset of C

You can mail me at [e-mail]