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Diffstat (limited to 'lib/libc/stdlib/random.3')
-rw-r--r-- | lib/libc/stdlib/random.3 | 196 |
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diff --git a/lib/libc/stdlib/random.3 b/lib/libc/stdlib/random.3 new file mode 100644 index 0000000..c6502bf --- /dev/null +++ b/lib/libc/stdlib/random.3 @@ -0,0 +1,196 @@ +.\" 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. +.\" 4. 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 June 4, 1993 +.Dt RANDOM 3 +.Os +.Sh NAME +.Nm random , +.Nm srandom , +.Nm srandomdev , +.Nm initstate , +.Nm setstate +.Nd better random 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 long seed" +.Ft void +.Fn srandomdev void +.Ft char * +.Fn initstate "unsigned long seed" "char *state" "long n" +.Ft char * +.Fn setstate "char *state" +.Sh DESCRIPTION +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 random +and +.Fn srandom +functions have (almost) the same calling sequence and initialization properties as the +.Xr rand 3 +and +.Xr srand 3 +functions. +The difference is that +.Xr rand 3 +produces a much less random sequence \(em in fact, the low dozen bits +generated by rand go through a cyclic pattern. +All the bits generated by +.Fn random +are usable. +For example, +.Sq Li random()&01 +will produce a random binary +value. +.Pp +Like +.Xr rand 3 , +.Fn random +will by default produce a sequence of numbers that can be duplicated +by calling +.Fn srandom +with +.Ql 1 +as the seed. +.Pp +The +.Fn srandomdev +routine initializes a state array using the +.Xr random 4 +random number device which returns good random numbers, +suitable for cryptographic use. +Note that this particular seeding +procedure can generate states which are impossible to reproduce by +calling +.Fn srandom +with any value, since the succeeding terms in the +state buffer are no longer derived from the LC algorithm applied to +a fixed seed. +.Pp +The +.Fn initstate +routine allows a state array, passed in as an argument, to be initialized +for future use. +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 seed for the initialization (which specifies a starting point for +the random number sequence, and provides for restarting at the same +point) is also an argument. +The +.Fn initstate +function +returns a pointer to the previous state information array. +.Pp +Once a state has been initialized, the +.Fn setstate +routine provides for rapid switching between states. +The +.Fn setstate +function +returns a pointer to the previous state array; its +argument state array is used for further random number generation +until the next call to +.Fn initstate +or +.Fn setstate . +.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, error +messages are printed on the standard error output. +.Sh SEE ALSO +.Xr arc4random 3 , +.Xr lrand48 3 , +.Xr rand 3 , +.Xr srand 3 , +.Xr random 4 +.Sh HISTORY +These +functions appeared in +.Bx 4.2 . +.Sh AUTHORS +.An Earl T. Cohen +.Sh BUGS +About 2/3 the speed of +.Xr rand 3 . +.Pp +The historical implementation used to have a very weak seeding; the +random sequence did not vary much with the seed. +The current implementation employs a better pseudo-random number +generator for the initial state calculation. +.Pp +Applications requiring cryptographic quality randomness should use +.Xr arc4random 3 . |