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dc36d6f9bb
Remove ancient SCCS tags from the tree, automated scripting, with two minor fixup to keep things compiling. All the common forms in the tree were removed with a perl script. Sponsored by: Netflix
147 lines
4.2 KiB
Groff
147 lines
4.2 KiB
Groff
.\" Copyright (c) 1990, 1991, 1993
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.\" The Regents of the University of California. All rights reserved.
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.\"
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.\" This code is derived from software contributed to Berkeley by
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.\" the American National Standards Committee X3, on Information
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.\" Processing Systems.
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.\"
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.\" Redistribution and use in source and binary forms, with or without
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.\" modification, are permitted provided that the following conditions
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.\" are met:
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.\" 1. Redistributions of source code must retain the above copyright
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.\" notice, this list of conditions and the following disclaimer.
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.\" 2. Redistributions in binary form must reproduce the above copyright
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.\" notice, this list of conditions and the following disclaimer in the
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.\" documentation and/or other materials provided with the distribution.
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.\" 3. Neither the name of the University nor the names of its contributors
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.\" may be used to endorse or promote products derived from this software
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.\" without specific prior written permission.
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.\"
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.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
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.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
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.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
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.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
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.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
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.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
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.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
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.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
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.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
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.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
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.\" SUCH DAMAGE.
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.\"
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.Dd February 1, 2020
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.Dt RAND 3
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.Os
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.Sh NAME
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.Nm rand ,
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.Nm srand ,
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.Nm rand_r
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.Nd bad random number generator
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.Sh LIBRARY
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.Lb libc
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.Sh SYNOPSIS
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.In stdlib.h
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.Ft void
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.Fn srand "unsigned seed"
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.Ft int
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.Fn rand void
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.Ft int
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.Fn rand_r "unsigned *ctx"
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.Sh DESCRIPTION
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.Bf -symbolic
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The functions described in this manual page are not cryptographically
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secure.
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Applications which require unpredictable random numbers should use
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.Xr arc4random 3
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instead.
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.Ef
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.Pp
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The
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.Fn rand
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function computes a sequence of pseudo-random integers in the range
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of 0 to
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.Dv RAND_MAX ,
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inclusive.
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.Pp
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The
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.Fn srand
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function seeds the algorithm with the
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.Fa seed
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parameter.
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Repeatable sequences of
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.Fn rand
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output may be obtained by calling
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.Fn srand
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with the same
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.Fa seed .
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.Fn rand
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is implicitly initialized as if
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.Fn srand "1"
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had been invoked explicitly.
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.Pp
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In
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.Fx 13 ,
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.Fn rand
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is implemented using the same 128-byte state LFSR generator algorithm as
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.Xr random 3 .
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However, the legacy
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.Fn rand_r
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function is not (and can not be, because of its limited
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.Fa *ctx
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size).
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.Fn rand_r
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implements the historical, poor-quality Park-Miller 32-bit LCG and should not
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be used in new designs.
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.Sh IMPLEMENTATION NOTES
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Since
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.Fx 13 ,
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.Fn rand
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is implemented with the same generator as
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.Xr random 3 ,
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so the low-order bits should no longer be significantly worse than the
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high-order bits.
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.Sh SEE ALSO
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.Xr arc4random 3 ,
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.Xr random 3 ,
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.Xr random 4
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.Sh STANDARDS
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The
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.Fn rand
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and
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.Fn srand
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functions
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conform to
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.St -isoC .
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.Pp
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The
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.Fn rand_r
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function is not part of
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.St -isoC
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and is marked obsolescent in
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.St -p1003.1-2008 .
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It may be removed in a future revision of POSIX.
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.Sh CAVEATS
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Prior to
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.Fx 13 ,
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.Fn rand
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used the historical Park-Miller generator with 32 bits of state and produced
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poor quality output, especially in the lower bits.
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.Fn rand
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in earlier versions of
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.Fx ,
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as well as other standards-conforming implementations, may continue to produce
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poor quality output.
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.Pp
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.Em These functions should not be used in portable applications that want a
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.Em high quality or high performance pseudorandom number generator .
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One possible replacement,
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.Xr random 3 ,
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is portable to Linux — but it is not especially fast, nor standardized.
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.Pp
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If broader portability or better performance is desired, any of the widely
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available and permissively licensed SFC64/32, JSF64/32, PCG64/32, or SplitMix64
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algorithm implementations may be embedded in your application.
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These algorithms have the benefit of requiring less space than
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.Xr random 3
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and being quite fast (in header inline implementations).
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