using /dev/random

Lowell Gilbert freebsd-questions-local at
Tue Sep 23 15:51:33 UTC 2008

Robert Huff <roberthuff at> writes:

> 	What is the canonical way to get data from /dev/random?
> Specifically: having opened the file, how do I read the stream?
> I'm currently using
>   union {
>     float f;
>     char c[4];
>   } foo;
>   foo.f = 0.0;
>   fscanf(rand_fp,"%4c",foo.c);
> 	which doesn't seem to produce anywhere near "random bytes" as
> promised by the man page.

Have you turned off the "seeded" variable?  You'll fall back to a
software pseudorandom sequence if you don't.

Most computers don't have all that much real random data ("entropy") to
work with, and if you need a lot of random data, you're more or less
forced to use a good pseudorandom generator.  "Good" can vary a bit
depending on application, but random(3) is generally more than good
enough for monte carlo style simulation purposes.

Cryptography is another story.  I have a hardware random number
generator on my server, which helps with performance in some cases...

Lowell Gilbert, embedded/networking software engineer, Boston area

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