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.\" Copyright 1993 David Metcalfe (david@prism.demon.co.uk)
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.\"
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.\" Permission is granted to make and distribute verbatim copies of this
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.\" manual provided the copyright notice and this permission notice are
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.\" preserved on all copies.
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.\"
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.\" Permission is granted to copy and distribute modified versions of this
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.\" manual under the conditions for verbatim copying, provided that the
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.\" entire resulting derived work is distributed under the terms of a
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.\" permission notice identical to this one.
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2007-04-12 22:42:49 +00:00
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.\"
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2004-11-03 13:51:07 +00:00
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.\" Since the Linux kernel and libraries are constantly changing, this
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.\" manual page may be incorrect or out-of-date. The author(s) assume no
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.\" responsibility for errors or omissions, or for damages resulting from
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.\" the use of the information contained herein. The author(s) may not
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.\" have taken the same level of care in the production of this manual,
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.\" which is licensed free of charge, as they might when working
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.\" professionally.
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2007-04-12 22:42:49 +00:00
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.\"
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2004-11-03 13:51:07 +00:00
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.\" Formatted or processed versions of this manual, if unaccompanied by
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.\" the source, must acknowledge the copyright and authors of this work.
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.\"
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.\" References consulted:
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.\" Linux libc source code
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.\" Lewine's _POSIX Programmer's Guide_ (O'Reilly & Associates, 1991)
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.\" 386BSD man pages
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.\" Modified Sun Mar 28 00:25:51 1993, David Metcalfe
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.\" Modified Sat Jul 24 18:13:39 1993 by Rik Faith (faith@cs.unc.edu)
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.\" Modified Sun Aug 20 21:47:07 2000, aeb
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.\"
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_exit.2, brk.2, capget.2, chdir.2, chmod.2, chown.2, chroot.2, getdtablesize.2, gethostname.2, getpagesize.2, getsid.2, killpg.2, mknod.2, mknodat.2, poll.2, posix_fadvise.2, pread.2, readlink.2, setpgid.2, setreuid.2, sigaltstack.2, stat.2, symlink.2, sync.2, truncate.2, vfork.2, wait.2, wait4.2, a64l.3, abs.3, acos.3, acosh.3, asin.3, asinh.3, atan.3, atan2.3, atoi.3, cbrt.3, ceil.3, copysign.3, cosh.3, dirfd.3, div.3, ecvt.3, erf.3, erfc.3, exp.3, exp2.3, fabs.3, fdim.3, ffs.3, floor.3, fma.3, fmax.3, fmin.3, fmod.3, fpclassify.3, frexp.3, ftw.3, fwide.3, gcvt.3, getcwd.3, getdate.3, getgrent.3, gethostid.3, getpass.3, getpwent.3, getsubopt.3, getw.3, hypot.3, ilogb.3, index.3, isalpha.3, isgreater.3, iswblank.3, j0.3, ldexp.3, lockf.3, log.3, log10.3, log1p.3, logb.3, lrint.3, lround.3, mkstemp.3, mktemp.3, modf.3, mq_receive.3, mq_send.3, nan.3, nextafter.3, posix_fallocate.3, posix_memalign.3, printf.3, qecvt.3, random.3, realpath.3, remainder.3, remquo.3, rint.3, round.3, scalb.3, scalbln.3, scanf.3, siginterrupt.3, signbit.3, sigset.3, sinh.3, sqrt.3, strcasecmp.3, strcat.3, strchr.3, strcmp.3, strcoll.3, strcpy.3, strfry.3, strpbrk.3, strsep.3, strspn.3, strstr.3, strtod.3, strtok.3, strtol.3, strtoul.3, strxfrm.3, tanh.3, tgamma.3, trunc.3, ttyslot.3, ualarm.3, usleep.3, wprintf.3, armscii-8.7, cp1251.7, iso_8859-10.7, iso_8859-11.7, iso_8859-13.7, iso_8859-14.7, iso_8859-3.7, iso_8859-5.7, iso_8859-6.7, iso_8859-8.7, koi8-u.7: tstamp
Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>
2010-09-19 17:19:13 +00:00
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.TH RANDOM 3 2010-09-20 "GNU" "Linux Programmer's Manual"
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2004-11-03 13:51:07 +00:00
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.SH NAME
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random, srandom, initstate, setstate \- random number generator
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.SH SYNOPSIS
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.nf
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.B #include <stdlib.h>
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.sp
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.B long int random(void);
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2007-12-23 13:45:24 +00:00
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2004-11-03 13:51:07 +00:00
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.BI "void srandom(unsigned int " seed );
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2007-12-23 13:45:24 +00:00
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2004-11-03 13:51:07 +00:00
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.BI "char *initstate(unsigned int " seed ", char *" state ", size_t " n );
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2006-07-20 16:16:51 +00:00
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.br
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2004-11-03 13:51:07 +00:00
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.BI "char *setstate(char *" state );
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.fi
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2007-07-08 12:11:40 +00:00
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.sp
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.in -4n
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Feature Test Macro Requirements for glibc (see
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.BR feature_test_macros (7)):
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.in
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.sp
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.ad l
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.BR random (),
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.BR srandom (),
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.BR initstate (),
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.BR setstate ():
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2010-09-18 07:52:49 +00:00
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.RS 4
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_SVID_SOURCE || _BSD_SOURCE || _XOPEN_SOURCE\ >=\ 500 ||
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_XOPEN_SOURCE\ &&\ _XOPEN_SOURCE_EXTENDED
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.RE
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.ad
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2004-11-03 13:51:07 +00:00
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.SH DESCRIPTION
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2007-05-12 09:06:04 +00:00
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The
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.BR random ()
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2010-01-16 16:58:03 +00:00
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function uses a nonlinear additive feedback random
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2004-11-03 13:51:07 +00:00
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number generator employing a default table of size 31 long integers to
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2007-04-12 22:42:49 +00:00
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return successive pseudo-random numbers in
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the range from 0 to \fBRAND_MAX\fR.
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2004-11-03 13:51:07 +00:00
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The period of this random number generator is very large, approximately
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2008-07-12 21:11:54 +00:00
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.IR "16\ *\ ((2^31)\ \-\ 1)" .
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2004-11-03 13:51:07 +00:00
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.PP
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2007-05-12 09:06:04 +00:00
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The
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.BR srandom ()
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function sets its argument as the seed for a new
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sequence of pseudo-random integers to be returned by
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.BR random ().
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These sequences are repeatable by calling
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.BR srandom ()
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with the same
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2007-04-12 22:42:49 +00:00
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seed value.
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2007-05-12 09:06:04 +00:00
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If no seed value is provided, the
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.BR random ()
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function
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2004-11-03 13:51:07 +00:00
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is automatically seeded with a value of 1.
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.PP
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2007-05-12 09:06:04 +00:00
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The
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.BR initstate ()
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function allows a state array \fIstate\fP to
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be initialized for use by
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.BR random ().
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2007-04-12 22:42:49 +00:00
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The size of the state array
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2007-05-12 09:06:04 +00:00
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\fIn\fP is used by
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.BR initstate ()
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to decide how sophisticated a
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intro.1, fork.2, futex.2, open.2, rename.2, select_tut.2, semop.2, spu_create.2, stat.2, netlink.3, random.3, scanf.3, shm_open.3, strftime.3, console.4, console_codes.4, sk98lin.4, st.4, bootparam.7, cpuset.7, credentials.7, man-pages.7, path_resolution.7, uri.7: Global fix: remove spaces around em-dash
Normal English typographical convention is not to have
spaces around em dashes.
Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>
2011-10-03 06:47:35 +00:00
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random number generator it should use\(emthe larger the state array,
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2007-05-12 13:26:09 +00:00
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the better the random numbers will be.
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\fIseed\fP is the seed for the
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2004-11-03 13:51:07 +00:00
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initialization, which specifies a starting point for the random number
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sequence, and provides for restarting at the same point.
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.PP
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2007-05-12 09:06:04 +00:00
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The
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.BR setstate ()
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function changes the state array used by the
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.BR random ()
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function.
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2007-04-12 22:42:49 +00:00
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The state array \fIstate\fP is used for
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2007-05-12 09:06:04 +00:00
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random number generation until the next call to
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.BR initstate ()
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or
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.BR setstate ().
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\fIstate\fP must first have been initialized
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using
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.BR initstate ()
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or be the result of a previous call of
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.BR setstate ().
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2004-11-03 13:51:07 +00:00
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.SH "RETURN VALUE"
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2007-05-12 09:06:04 +00:00
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The
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.BR random ()
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2007-06-22 19:42:52 +00:00
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function returns a value between 0 and
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.BR RAND_MAX .
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2007-05-12 09:06:04 +00:00
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The
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.BR srandom ()
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function returns no value.
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The
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.BR initstate ()
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2010-09-19 18:08:14 +00:00
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function returns a pointer to the previous state array.
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The
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2007-05-12 09:06:04 +00:00
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.BR setstate ()
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2010-09-19 18:08:14 +00:00
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function returns a pointer to the previous state array, or NULL on error.
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2004-11-03 13:51:07 +00:00
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.SH ERRORS
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.TP
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.B EINVAL
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2007-05-12 09:06:04 +00:00
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A state array of less than 8 bytes was specified to
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.BR initstate ().
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2007-05-19 04:30:20 +00:00
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.SH "CONFORMING TO"
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4.3BSD, POSIX.1-2001.
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2004-11-03 13:51:07 +00:00
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.SH NOTES
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Current "optimal" values for the size of the state array \fIn\fP are
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8, 32, 64, 128, and 256 bytes; other amounts will be rounded down to
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2007-04-12 22:42:49 +00:00
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the nearest known amount.
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Using less than 8 bytes will cause an
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2004-11-03 13:51:07 +00:00
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error.
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2008-03-03 16:51:35 +00:00
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.PP
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This function should not be used in cases where multiple threads use
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.BR random ()
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and the behavior should be reproducible.
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Use
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.BR random_r (3)
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for that purpose.
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.PP
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Random-number generation is a complex topic.
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.I Numerical Recipes in C: The Art of Scientific Computing
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(William H. Press, Brian P. Flannery, Saul A. Teukolsky, William
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T. Vetterling; New York: Cambridge University Press, 2007, 3rd ed.)
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provides an excellent discussion of practical random-number generation
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issues in Chapter 7 (Random Numbers).
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.PP
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For a more theoretical discussion which also covers many practical issues
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in depth, see Chapter 3 (Random Numbers) in Donald E. Knuth's
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.IR "The Art of Computer Programming" ,
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volume 2 (Seminumerical Algorithms), 2nd ed.; Reading, Massachusetts:
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Addison-Wesley Publishing Company, 1981.
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2004-11-03 13:51:07 +00:00
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.SH "SEE ALSO"
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2008-03-03 16:51:35 +00:00
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.BR drand48 (3),
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2004-11-03 13:51:07 +00:00
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.BR rand (3),
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2009-02-02 05:34:41 +00:00
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.BR random_r (3),
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2004-11-03 13:51:07 +00:00
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.BR srand (3)
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