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.\"	$OpenBSD: rand.3,v 1.20 2021/02/12 17:03:51 deraadt Exp $
.\"
.Dd $Mdocdate: February 12 2021 $
.Dt RAND 3
.Os
.Sh NAME
.Nm rand ,
.Nm rand_r ,
.Nm srand ,
.Nm srand_deterministic
.Nd bad pseudo-random number generator
.Sh SYNOPSIS
.In stdlib.h
.Ft void
.Fn srand "unsigned int seed"
.Ft void
.Fn srand_deterministic "unsigned int seed"
.Ft int
.Fn rand void
.Ft int
.Fn rand_r "unsigned int *seed"
.Sh DESCRIPTION
.Bf -symbolic
Standards insist that this interface return deterministic results.
Unsafe usage is very common, so
.Ox
changed the subsystem to return non-deterministic results by default.
.Ef
.Pp
To satisfy portable code,
.Fn srand
may be called to initialize the subsystem.
In
.Ox
the
.Ar seed
variable is ignored, and strong random number results will be provided from
.Xr arc4random 3 .
In other systems, the
.Ar seed
variable primes a simplistic deterministic algorithm.
.Pp
If the standardized behavior is required
.Fn srand_deterministic
can be substituted for
.Fn srand ,
then subsequent
.Fn rand
calls will return results using the deterministic algorithm.
The deterministic sequence algorithm changed a number of times since
original development, is underspecified, and should not be relied upon to
remain consistent between platforms and over time.
.Pp
The
.Fn rand
function returns a result in the range of 0 to
.Dv RAND_MAX .
By default, this result comes from
.Xr arc4random 3 .
If
.Fn srand_deterministic
was called, the result will be computed using the deterministic algorithm.
.Pp
The
.Fn rand_r
is a thread-safe version of
.Fn rand .
Storage for the seed must be provided through the
.Fa seed
argument, and needs to have been initialized by the caller.
It always operates using the deterministic algorithm.
.Sh SEE ALSO
.Xr arc4random 3 ,
.Xr rand48 3 ,
.Xr random 3
.Sh STANDARDS
The
.Fn rand
function conforms to
.St -ansiC .
.Pp
The
.Fn rand_r
function conforms to
.St -p1003.1-2008 .
.Pp
The
.Fn srand
function does not conform to
.St -ansiC ,
intentionally.
.Pp
The
.Fn srand_deterministic
function is an
.Ox
extension.
.Sh HISTORY
The functions
.Fn rand
and
.Fn srand
first appeared in
.At v3 .