PROGRAMMING - Hacking: The Art of Exploitation - page 101

function_ptr is 0x0804838d
This is function two
value returned was 2
reader@hacking:~/booksrc $
0x286 Pseudo-random Numbers
Since computers are deterministic machines, it is impossible for them to
produce truly random numbers. But many applications require some form of
randomness. The pseudo-random number generator functions fill this need
by generating a stream of numbers that is pseudo-random . These functions
can produce a seemingly random sequence of numbers started from a seed
number; however, the same exact sequence can be generated again with the
same seed. Deterministic machines cannot produce true randomness, but if
the seed value of the pseudo-random generation function isn't known, the
sequence will seem random. The generator must be seeded with a value
using the function srand() , and from that point on, the function rand() will
return a pseudo-random number from 0 to RAND_MAX . These functions and
RAND_MAX are defined in stdlib.h. While the numbers rand() returns will appear
to be random, they are dependent on the seed value provided to srand() .
To maintain pseudo-randomness between subsequent program executions,
the randomizer must be seeded with a different value each time. One common
practice is to use the number of seconds since epoch (returned from the time()
function) as the seed. The rand_example.c program demonstrates this
technique.
rand_example.c
#include <stdio.h>
#include <stdlib.h>
int main() {
int i;
printf("RAND_MAX is %u\n", RAND_MAX);
srand(time(0));
printf("random values from 0 to RAND_MAX\n");
for(i=0; i < 8; i++)
printf("%d\n", rand());
printf("random values from 1 to 20\n");
for(i=0; i < 8; i++)
printf("%d\n", (rand()%20)+1);
}
Notice how the modulus operator is used to obtain random values from
1 to 20.
reader@hacking:~/booksrc $ gcc rand_example.c
reader@hacking:~/booksrc $ ./a.out
RAND_MAX is 2147483647
random values from 0 to RAND_MAX