Hacking Concepts

Hacking Concepts

Now that the syntax of C feels more familiar and some fundamental programming
concepts have been explained, actually programming in C isn’t that big
of a step. C compilers exist for just about every operating system and processor
architecture out there, but for this book, Linux and an x 86-based processor
will be used exclusively. Linux is a free operating system that everyone has
access to, and x86-based processors are the most popular consumer-grade
processor on the planet. Since hacking is really about experimenting, it’s
probably best if you have a C compiler to follow along with.
Included with this book is a LiveCD you can use to follow along if your
computer has an x86 processor. Just put the CD in the drive and reboot
your computer. It will boot into a Linux environment without modifying your
existing operating system. From this Linux environment you can follow
along with the book and experiment on your own.
Let’s get right to it. The firstprog.c program is a simple piece of C code
that will print “Hello, world!” 10 times.
firstprog.c
#include <stdio.h>
int main()
{
int i;
for(i=0; i < 10; i++) // Loop 10 times.
{
puts("Hello, world!\n"); // put the string to the output.
}
return 0; // Tell OS the program exited without errors.
}
The main execution of a C program begins in the aptly named main()
function. Any text following two forward slashes (//) is a comment, which is
ignored by the compiler.
The first line may be confusing, but it’s just C syntax that tells the compiler
to include headers for a standard input/output (I/O) library named
stdio. This header file is added to the program when it is compiled. It is
located at /usr/include/stdio.h, and it defines several constants and function
prototypes for corresponding functions in the standard I/O library.
Since the main() function uses the printf() function from the standard I/O
20 0x200
library, a function prototype is needed for printf() before it can be used.
This function prototype (along with many others) is included in the stdio.h
header file. A lot of the power of C comes from its extensibility and libraries.
The rest of the code should make sense and look a lot like the pseudo-code
from before. You may have even noticed that there’s a set of curly braces that
can be eliminated. It should be fairly obvious what this program will do, but
let’s compile it using GCC and run it just to make sure.
The GNU Compiler Collection (GCC) is a free C compiler that translates C
into machine language that a processor can understand. The outputted translation
is an executable binary file, which is called a.out by default. Does the
compiled program do what you thought it would?
reader@hacking:~/booksrc $ gcc firstprog.c
reader@hacking:~/booksrc $ ls -l a.out
-rwxr-xr-x 1 reader reader 6621 2007-09-06 22:16 a.out
reader@hacking:~/booksrc $ ./a.out
Hello, world!
Hello, world!
Hello, world!
Hello, world!
Hello, world!
Hello, world!
Hello, world!
Hello, world!
Hello, world!
Hello, world!
reader@hacking:~/booksrc $