Hacking - The Art of Exploitation, 2nd Edition

Programming 51

7 - Hello, world!
8 - Hello, world!
9 - Hello, world!
10 - Hello, world!
11 - Hello, world!
reader@hacking:~/booksrc $

Format strings are used quite often, so familiarity with them is valuable.

In addition, the ability to output the values of variables allows for debugging in

the program, without the use of a debugger. Having some form of immediate

feedback is fairly vital to the hacker’s learning process, and something as

simple as printing the value of a variable can allow for lots of exploitation.

0x265 Typecasting

Typecasting is simply a way to temporarily change a variable’s data type, despite

how it was originally defined. When a variable is typecast into a different

type, the compiler is basically told to treat that variable as if it were the

new data type, but only for that operation. The syntax for typecasting is

as follows:

(typecast_data_type) variable

This can be used when dealing with integers and floating-point variables,

as typecasting.c demonstrates.

typecasting.c

#include <stdio.h>

int main() {
int a, b;
float c, d;

a = 13;
b = 5;

c = a / b; // Divide using integers.
d = (float) a / (float) b; // Divide integers typecast as floats.

printf("[integers]\t a = %d\t b = %d\n", a, b);
printf("[floats]\t c = %f\t d = %f\n", c, d);
}

The results of compiling and executing typecasting.c are as follows.

reader@hacking:~/booksrc $ gcc typecasting.c
reader@hacking:~/booksrc $ ./a.out
[integers] a = 13 b = 5
[floats] c = 2.000000 d = 2.600000
reader@hacking:~/booksrc $