mirror of https://github.com/mkerrisk/man-pages
184 lines
5.2 KiB
Groff
184 lines
5.2 KiB
Groff
.\" 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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.\"
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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.\" manual page may be incorrect or out-of-date.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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.\"
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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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.\"
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.TH RAND 3 2003-11-15 "" "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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.sp
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.B int rand(void);
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.sp
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.BI "int rand_r(unsigned int *" seedp );
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.sp
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.BI "void srand(unsigned int " seed );
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.fi
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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 between 0
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and \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
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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
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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 or thread-safe, since it
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uses hidden state that is modified on each call.
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This might just be
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the seed value to be used by the next call, or it might be something
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more elaborate.
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In order to get reproducible behaviour in a threaded
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application, this state must be made explicit.
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The function
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.BR rand_r ()
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is supplied with a pointer to an unsigned int, to be used as state.
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This is a very small amount of state, so this function will be a weak
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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
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between 0 and RAND_MAX.
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The
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.BR srand ()
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function returns no value.
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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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.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
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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
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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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.PP
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In
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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, 1992 (2nd ed.,
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p. 277)), the following comments are made:
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.RS
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"If you want to generate a random integer between 1 and 10, you should
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always do it by using high-order bits, as in
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.RS
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.sp
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j = 1 + (int) (10.0 * (rand() / (RAND_MAX + 1.0)));
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.sp
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.RE
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and never by anything resembling
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.RS
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.sp
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j = 1 + (rand() % 10);
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.sp
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.RE
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(which uses lower-order bits)."
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.RE
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.PP
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Random-number generation is a complex topic.
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The
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.I Numerical Recipes in C
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book (see reference above)
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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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.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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.sp
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.nf
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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 seed) {
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next = seed;
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}
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.fi
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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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