mirror of https://github.com/mkerrisk/man-pages
240 lines
5.9 KiB
Groff
240 lines
5.9 KiB
Groff
.\" Copyright 1993 David Metcalfe (david@prism.demon.co.uk)
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.\"
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.\" %%%LICENSE_START(VERBATIM)
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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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.\"
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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.\" manual page may be incorrect or out-of-date.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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.\"
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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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.\" %%%LICENSE_END
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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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.\"
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.\" Modified 1993-03-29, David Metcalfe
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.\" Modified 1993-04-28, Lars Wirzenius
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.\" Modified 1993-07-24, Rik Faith (faith@cs.unc.edu)
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.\" Modified 1995-05-18, Rik Faith (faith@cs.unc.edu) to add
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.\" better discussion of problems with rand on other systems.
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.\" (Thanks to Esa Hyyti{ (ehyytia@snakemail.hut.fi).)
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.\" Modified 1998-04-10, Nicolás Lichtmaier <nick@debian.org>
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.\" with contribution from Francesco Potorti <F.Potorti@cnuce.cnr.it>
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.\" Modified 2003-11-15, aeb, added rand_r
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.\" 2010-09-13, mtk, added example program
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.\"
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.TH RAND 3 2017-07-13 "" "Linux Programmer's Manual"
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.SH NAME
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rand, rand_r, srand \- pseudo-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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.PP
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.B int rand(void);
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.PP
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.BI "int rand_r(unsigned int *" seedp );
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.PP
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.BI "void srand(unsigned int " seed );
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.fi
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.PP
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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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.PP
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.BR rand_r ():
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.RS 4
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Since glibc 2.24:
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_POSIX_C_SOURCE >= 199506L
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.br
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Glibc 2.23 and earlier
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_POSIX_C_SOURCE
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.RE
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.SH DESCRIPTION
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The
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.BR rand ()
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function returns a pseudo-random integer in the range 0 to
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.BR RAND_MAX
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inclusive (i.e., the mathematical range [0,\ \fBRAND_MAX\fR]).
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.PP
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The
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.BR srand ()
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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 rand ().
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These sequences are repeatable by calling
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.BR srand ()
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with the same seed value.
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.PP
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If no seed value is provided, the
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.BR rand ()
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function is automatically seeded with a value of 1.
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.PP
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The function
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.BR rand ()
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is not reentrant, since it
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uses hidden state that is modified on each call.
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This might just be the seed value to be used by the next call,
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or it might be something more elaborate.
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In order to get reproducible behavior in a threaded
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application, this state must be made explicit;
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this can be done using the reentrant function
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.BR rand_r ().
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.PP
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Like
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.BR rand (),
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.BR rand_r ()
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returns a pseudo-random integer in the range [0,\ \fBRAND_MAX\fR].
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The
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.I seedp
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argument is a pointer to an
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.IR "unsigned int"
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that is used to store state between calls.
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If
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.BR rand_r ()
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is called with the same initial value for the integer pointed to by
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.IR seedp ,
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and that value is not modified between calls,
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then the same pseudo-random sequence will result.
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.PP
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The value pointed to by the
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.I seedp
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argument of
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.BR rand_r ()
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provides only a very small amount of state,
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so this function will be a weak pseudo-random generator.
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Try
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.BR drand48_r (3)
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instead.
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.SH RETURN VALUE
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The
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.BR rand ()
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and
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.BR rand_r ()
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functions return a value between 0 and
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.BR RAND_MAX
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(inclusive).
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The
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.BR srand ()
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function returns no value.
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.SH ATTRIBUTES
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For an explanation of the terms used in this section, see
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.BR attributes (7).
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.TS
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allbox;
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lbw25 lb lb
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l l l.
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Interface Attribute Value
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T{
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.BR rand (),
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.BR rand_r (),
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.BR srand ()
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T} Thread safety MT-Safe
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.TE
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.SH CONFORMING TO
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The functions
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.BR rand ()
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and
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.BR srand ()
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conform to SVr4, 4.3BSD, C89, C99, POSIX.1-2001.
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The function
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.BR rand_r ()
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is from POSIX.1-2001.
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POSIX.1-2008 marks
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.BR rand_r ()
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as obsolete.
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.SH NOTES
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The versions of
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.BR rand ()
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and
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.BR srand ()
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in the Linux C Library use the same random number generator as
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.BR random (3)
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and
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.BR srandom (3),
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so the lower-order bits should be as random as the higher-order bits.
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However, on older
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.BR rand ()
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implementations, and on current implementations on different systems,
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the lower-order bits are much less random than the higher-order bits.
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Do not use this function in applications intended to be portable
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when good randomness is needed.
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(Use
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.BR random (3)
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instead.)
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.SH EXAMPLE
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POSIX.1-2001 gives the following example of an implementation of
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.BR rand ()
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and
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.BR srand (),
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possibly useful when one needs the same sequence on two different machines.
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.PP
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.in +4n
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.EX
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static unsigned long next = 1;
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/* RAND_MAX assumed to be 32767 */
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int myrand(void) {
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next = next * 1103515245 + 12345;
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return((unsigned)(next/65536) % 32768);
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}
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void mysrand(unsigned int seed) {
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next = seed;
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}
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.EE
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.in
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.PP
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The following program can be used to display the
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pseudo-random sequence produced by
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.BR rand ()
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when given a particular seed.
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.PP
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.in +4n
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.EX
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#include <stdlib.h>
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#include <stdio.h>
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int
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main(int argc, char *argv[])
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{
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int j, r, nloops;
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unsigned int seed;
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if (argc != 3) {
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fprintf(stderr, "Usage: %s <seed> <nloops>\\n", argv[0]);
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exit(EXIT_FAILURE);
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}
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seed = atoi(argv[1]);
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nloops = atoi(argv[2]);
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srand(seed);
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for (j = 0; j < nloops; j++) {
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r = rand();
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printf("%d\\n", r);
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}
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exit(EXIT_SUCCESS);
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}
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.EE
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.in
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.SH SEE ALSO
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.BR drand48 (3),
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.BR random (3)
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