672e12255d
rand(3)'s standard C API is extremely limiting, but we can do better than the historical 32-bit state Park-Miller LCG we've shipped since 2001: r73156. The justification provided at the time for not using random(3) was that rand_r(3) could not be made to use the same algorithm. That is still true. However, the irrelevance of rand_r(3) is increasingly obvious. Since that time, POSIX has marked the interface obsolescent. rand_r(3) never became part of the standard C library. If not for API compatibility reasons, I would just remove rand_r(3) entirely. So, I do not believe it is a problem for rand_r(3) and rand(3) to diverge. The 12 ABI is maintained with compatibility definitions, but this revision does subtly change the API of rand(3). The sequences of pseudorandom numbers produced in programs built against new versions of libc will differ from programs built against prior versions of libc. Reviewed by: kevans, markm MFC after: no Relnotes: yes Differential Revision: https://reviews.freebsd.org/D23290
179 lines
5.1 KiB
Groff
179 lines
5.1 KiB
Groff
.\" Copyright (c) 1983, 1991, 1993
|
|
.\" The Regents of the University of California. All rights reserved.
|
|
.\"
|
|
.\" Redistribution and use in source and binary forms, with or without
|
|
.\" modification, are permitted provided that the following conditions
|
|
.\" are met:
|
|
.\" 1. Redistributions of source code must retain the above copyright
|
|
.\" notice, this list of conditions and the following disclaimer.
|
|
.\" 2. Redistributions in binary form must reproduce the above copyright
|
|
.\" notice, this list of conditions and the following disclaimer in the
|
|
.\" documentation and/or other materials provided with the distribution.
|
|
.\" 3. Neither the name of the University nor the names of its contributors
|
|
.\" may be used to endorse or promote products derived from this software
|
|
.\" without specific prior written permission.
|
|
.\"
|
|
.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
|
|
.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
|
|
.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
|
|
.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
|
|
.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
|
|
.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
|
|
.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
|
|
.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
|
|
.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
|
|
.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
|
|
.\" SUCH DAMAGE.
|
|
.\"
|
|
.\" @(#)random.3 8.1 (Berkeley) 6/4/93
|
|
.\" $FreeBSD$
|
|
.\"
|
|
.Dd February 1, 2020
|
|
.Dt RANDOM 3
|
|
.Os
|
|
.Sh NAME
|
|
.Nm random ,
|
|
.Nm srandom ,
|
|
.Nm srandomdev ,
|
|
.Nm initstate ,
|
|
.Nm setstate
|
|
.Nd non-cryptographic pseudorandom number generator; routines for changing generators
|
|
.Sh LIBRARY
|
|
.Lb libc
|
|
.Sh SYNOPSIS
|
|
.In stdlib.h
|
|
.Ft long
|
|
.Fn random void
|
|
.Ft void
|
|
.Fn srandom "unsigned int seed"
|
|
.Ft void
|
|
.Fn srandomdev void
|
|
.Ft char *
|
|
.Fn initstate "unsigned int seed" "char *state" "size_t n"
|
|
.Ft char *
|
|
.Fn setstate "char *state"
|
|
.Sh DESCRIPTION
|
|
.Bf -symbolic
|
|
The functions described in this manual page are not secure.
|
|
Applications which require unpredictable random numbers should use
|
|
.Xr arc4random 3
|
|
instead.
|
|
.Ef
|
|
.Pp
|
|
Unless initialized with less than 32 bytes of state, 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
|
|
If initialized with less than 32 bytes of state,
|
|
.Fn random
|
|
uses the poor-quality 32-bit Park-Miller LCG.
|
|
.Pp
|
|
The
|
|
.Fn random
|
|
and
|
|
.Fn srandom
|
|
functions are analagous to
|
|
.Xr rand 3
|
|
and
|
|
.Xr srand 3 .
|
|
.Pp
|
|
Like
|
|
.Xr rand 3 ,
|
|
.Fn random
|
|
is implicitly initialized as if
|
|
.Fn srandom "1"
|
|
had been invoked explicitly.
|
|
.Pp
|
|
The
|
|
.Fn srandomdev
|
|
routine initializes the state array using random numbers obtained from the
|
|
kernel.
|
|
This can generate states which are impossible to reproduce by calling
|
|
.Fn srandom ,
|
|
because the succeeding terms in the state buffer are no longer derived from the
|
|
Park-Miller LCG algorithm applied to a fixed seed.
|
|
.Pp
|
|
The
|
|
.Fn initstate
|
|
routine initializes the provided state array of
|
|
.Vt uint32_t
|
|
values and uses it in future
|
|
.Fn random
|
|
invocations.
|
|
(Despite the
|
|
.Vt char *
|
|
type of
|
|
.Fa state ,
|
|
the underlying object must be a naturally aligned array of 32-bit values.)
|
|
The size of the state array (in bytes) is used by
|
|
.Fn initstate
|
|
to decide how sophisticated a random number generator it should use \(em the
|
|
more state, the better the random numbers will be.
|
|
(Current "optimal" values for the amount of state information are
|
|
8, 32, 64, 128, and 256 bytes; other amounts will be rounded down to
|
|
the nearest known amount.
|
|
Using less than 8 bytes will cause an error.)
|
|
The
|
|
.Fa seed
|
|
is used as in
|
|
.Fn srandom .
|
|
The
|
|
.Fn initstate
|
|
function
|
|
returns a pointer to the previous state information array.
|
|
.Pp
|
|
The
|
|
.Fn setstate
|
|
routine switches
|
|
.Fn random
|
|
to using the provided state.
|
|
It returns a pointer to the previous state.
|
|
.Pp
|
|
Once a state array has been initialized, it may be restarted at a
|
|
different point either by calling
|
|
.Fn initstate
|
|
(with the desired seed, the state array, and its size) or by calling
|
|
both
|
|
.Fn setstate
|
|
(with the state array) and
|
|
.Fn srandom
|
|
(with the desired seed).
|
|
The advantage of calling both
|
|
.Fn setstate
|
|
and
|
|
.Fn srandom
|
|
is that the size of the state array does not have to be remembered after
|
|
it is initialized.
|
|
.Pp
|
|
With 256 bytes of state information, the period of the random number
|
|
generator is greater than
|
|
.if t 2\u\s769\s10\d,
|
|
.if n 2**69
|
|
which should be sufficient for most purposes.
|
|
.Sh DIAGNOSTICS
|
|
If
|
|
.Fn initstate
|
|
is called with less than 8 bytes of state information, or if
|
|
.Fn setstate
|
|
detects that the state information has been garbled,
|
|
NULL is returned.
|
|
.Sh SEE ALSO
|
|
.Xr arc4random 3 ,
|
|
.Xr lrand48 3 ,
|
|
.Xr rand 3 ,
|
|
.Xr random 4
|
|
.Sh HISTORY
|
|
These
|
|
functions appeared in
|
|
.Bx 4.2 .
|
|
.Sh AUTHORS
|
|
.An Earl T. Cohen
|