Limiting random number generator

[Jenwhiz]

Hi, this should be a straightforward question, I am writing some code to randomly generate a number, and it works fine the only issue is I don't know how to limit the range of numbers it generates, and currently it prints between 0 and 250.

uint8_t rand[1];

sys_rand_get(rand, sizeof(rand));

printk("rand: %d\n", rand[0]);

This is the current code. I want to only generate numbers between 1 and 100.

Thanks!

[khelmutlord]

EDIT: My previous approach fell victim to "modulo bias" in generating a random number.

Per my colleague Einar Thorsrud:

https://research.kudelskisecurity.com/2020/07/28/the-definitive-guide-to-modulo-bias-and-how-to-avoid-it/

"It would be better to use a bit mask to limit the generated number to 0-127, and then throw away any generated numbers that are outside of the desired range (1 to 100 in this case)."

Instead, I would encourage you to use the approach here in this toy example: https://github.com/hlord2000/zephyr_rng_test

Thanks,

Helmut Lord

[Jenwhiz]

Thank you so much, this helps a lot!

[khelmutlord]

Hello Jennifer, I would encourage you to check the edit I made to my initial post.

Thanks,

Helmut Lord