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.\" $FreeBSD: release/9.1.0/share/man/man9/random.9 206622 2010-04-14 19:08:06Z uqs $
.\" "
.Dd September 25, 2000
.Dt RANDOM 9
.Os
.Sh NAME
.Nm arc4rand ,
.Nm arc4random ,
.Nm random ,
.Nm read_random ,
.Nm srandom
.Nd supply pseudo-random numbers
.Sh SYNOPSIS
.In sys/libkern.h
.Ft void
.Fn srandom "u_long seed"
.Ft u_long
.Fn random "void"
.Ft void
.Fn arc4rand "void *ptr" "u_int length" "int reseed"
.Ft u_int32_t
.Fn arc4random "void"
.Pp
.In sys/random.h
.Ft int
.Fn read_random "void *buffer" "int count"
.Sh DESCRIPTION
The
.Fn random
function will by default produce a sequence of numbers that can be duplicated
by calling
.Fn srandom
with
.Ql 1
as the
.Fa seed .
The
.Fn srandom
function may be called with any arbitrary
.Fa seed
value to get slightly more unpredictable numbers.
It is important to remember that the
.Fn random
function is entirely predictable, and is therefore not of use where
knowledge of the sequence of numbers may be of benefit to an attacker.
.Pp
The
.Fn arc4rand
function will return very good quality random numbers, slightly better
suited for security-related purposes.
The random numbers from
.Fn arc4rand
are seeded from the entropy device if it is available.
Automatic reseeds happen after a certain timeinterval and after a
certain number of bytes have been delivered.
A forced reseed can be forced by passing a non-zero value in the
.Fa reseed
argument.
.Pp
The
.Fn read_random
function is used to return entropy directly from the entropy device
if it has been loaded.
If the entropy device is not loaded, then
the
.Fa buffer
is filled with output generated by
.Fn random .
The
.Fa buffer
is filled with no more than
.Fa count
bytes.
It is advised that
.Fn read_random
is not used; instead use
.Fn arc4rand
.Pp
All the bits generated by
.Fn random ,
.Fn arc4rand
and
.Fn read_random
are usable.
For example,
.Sq Li random()&01
will produce a random binary value.
.Pp
The
.Fn arc4random
is a convenience function which calls
.Fn arc4rand
to return a 32 bit pseudo-random integer.
.Sh RETURN VALUES
The
.Fn random
function
uses a non-linear additive feedback random number generator employing a
default table of size 31 long integers to return successive pseudo-random
numbers in the range from 0 to
.if t 2\u\s731\s10\d\(mi1.
.if n (2**31)\(mi1.
The period of this random number generator is very large, approximately
.if t 16\(mu(2\u\s731\s10\d\(mi1).
.if n 16*((2**31)\(mi1).
.Pp
The
.Fn arc4rand
function uses the RC4 algorithm to generate successive pseudo-random
bytes.
The
.Fn arc4random
function
uses
.Fn arc4rand
to generate pseudo-random numbers in the range from 0 to
.if t 2\u\s732\s10\d\(mi1.
.if n (2**32)\(mi1.
.Pp
The
.Fn read_random
function returns the number of bytes placed in
.Fa buffer .
.Sh AUTHORS
.An Dan Moschuk
wrote
.Fn arc4random .
.An Mark R V Murray
wrote
.Fn read_random .