random numbers

lggrossi · 05-14-2008, 05:04 AM

Hello everybody,

I need to generate random numbers inside the linux kernel, but i tried with the normal rand and srand and this is not possible because they are from stdlib.h of C.
Then i discover the function get_random_bytes (void *buf, int nbytes). I'm not understanding well this function. If i make this:

int i;
get_random_bytes ( &i, sizeof (i) );

this function will put in the address of "i" one random integer number?? And how can I bound the generation of numbers as in rand()%100 in C, that in this case generates only numbers at max 100?

Thanks a lot,
LUCAS

Hko · 05-14-2008, 06:41 AM

Quote:

Originally Posted by lggrossi

int i;
get_random_bytes ( &i, sizeof (i) );

this function will put in the address of "i" one random integer number?? And how can I bound the generation of numbers as in rand()%100 in C, that in this case generates only numbers at max 100?

Just the same way I suppose:

Code:

int i, lessthan100;
get_random_bytes(&i, sizeof(i));
lessthan100 = i % 100;

lggrossi · 05-16-2008, 11:00 AM

If i make this:

int i;
get_random_bytes(&i, 1);
i = i % 100;

I suppose that the function will generate a value between 0 and 255 because i specify the number of bytes to be 1 (second argument).
Then, for example if the value is 250, in the third line "i" will receive the remainder of 250/100.

It is ok?

Thanks,
LUCAS

Hko · 05-16-2008, 05:43 PM

I have no experience in the kernel.
But looks OK to me.

fantas · 05-16-2008, 09:02 PM

What you are aiming for is heavily dependant on the endianness of the system (read: least versus most significant byte). But since "i" initially will contain garbage as well (not initialised) and as you limit the range by the modulo operation this probably even won't make much of a difference to the result. Highly indeterministic though.

Hko · 05-17-2008, 04:47 AM

Ah, yes. That is true.
Better to fill the entire bitspace of the int instead of only one byte. So use:

Code:

get_random_bytes(&i, sizeof(int));

instead of:

Code:

get_random_bytes(&i, 1);

lggrossi · 05-18-2008, 07:48 AM

Hi,

Thanks a lot for the help.

LUCAS