mirror of https://github.com/mkerrisk/man-pages
166 lines
4.5 KiB
Groff
166 lines
4.5 KiB
Groff
.\" Copyright 1993 David Metcalfe (david@prism.demon.co.uk)
|
|
.\"
|
|
.\" Permission is granted to make and distribute verbatim copies of this
|
|
.\" manual provided the copyright notice and this permission notice are
|
|
.\" preserved on all copies.
|
|
.\"
|
|
.\" Permission is granted to copy and distribute modified versions of this
|
|
.\" manual under the conditions for verbatim copying, provided that the
|
|
.\" entire resulting derived work is distributed under the terms of a
|
|
.\" permission notice identical to this one.
|
|
.\"
|
|
.\" Since the Linux kernel and libraries are constantly changing, this
|
|
.\" manual page may be incorrect or out-of-date. The author(s) assume no
|
|
.\" responsibility for errors or omissions, or for damages resulting from
|
|
.\" the use of the information contained herein. The author(s) may not
|
|
.\" have taken the same level of care in the production of this manual,
|
|
.\" which is licensed free of charge, as they might when working
|
|
.\" professionally.
|
|
.\"
|
|
.\" Formatted or processed versions of this manual, if unaccompanied by
|
|
.\" the source, must acknowledge the copyright and authors of this work.
|
|
.\"
|
|
.\" References consulted:
|
|
.\" Linux libc source code
|
|
.\" Lewine's _POSIX Programmer's Guide_ (O'Reilly & Associates, 1991)
|
|
.\" 386BSD man pages
|
|
.\"
|
|
.\" Modified 1993-03-29, David Metcalfe
|
|
.\" Modified 1993-04-28, Lars Wirzenius
|
|
.\" Modified 1993-07-24, Rik Faith (faith@cs.unc.edu)
|
|
.\" Modified 1995-05-18, Rik Faith (faith@cs.unc.edu) to add
|
|
.\" better discussion of problems with rand on other systems.
|
|
.\" (Thanks to Esa Hyyti{ (ehyytia@snakemail.hut.fi).)
|
|
.\" Modified 1998-04-10, Nicolás Lichtmaier <nick@debian.org>
|
|
.\" with contribution from Francesco Potorti <F.Potorti@cnuce.cnr.it>
|
|
.\" Modified 2003-11-15, aeb, added rand_r
|
|
.\"
|
|
.TH RAND 3 2008-04-28 "" "Linux Programmer's Manual"
|
|
.SH NAME
|
|
rand, rand_r, srand \- pseudo-random number generator
|
|
.SH SYNOPSIS
|
|
.nf
|
|
.B #include <stdlib.h>
|
|
.sp
|
|
.B int rand(void);
|
|
.sp
|
|
.BI "int rand_r(unsigned int *" seedp );
|
|
.sp
|
|
.BI "void srand(unsigned int " seed );
|
|
.fi
|
|
.sp
|
|
.in -4n
|
|
Feature Test Macro Requirements for glibc (see
|
|
.BR feature_test_macros (7)):
|
|
.in
|
|
.sp
|
|
.BR rand_r ():
|
|
_POSIX_C_SOURCE || _XOPEN_SOURCE
|
|
.SH DESCRIPTION
|
|
The
|
|
.BR rand ()
|
|
function returns a pseudo-random integer in
|
|
the range [0,\ \fBRAND_MAX\fR].
|
|
.PP
|
|
The
|
|
.BR srand ()
|
|
function sets its argument as the seed for a new
|
|
sequence of pseudo-random integers to be returned by
|
|
.BR rand ().
|
|
These sequences are repeatable by calling
|
|
.BR srand ()
|
|
with the same
|
|
seed value.
|
|
.PP
|
|
If no seed value is provided, the
|
|
.BR rand ()
|
|
function is automatically
|
|
seeded with a value of 1.
|
|
.PP
|
|
The function
|
|
.BR rand ()
|
|
is not reentrant or thread-safe, since it
|
|
uses hidden state that is modified on each call.
|
|
This might just be
|
|
the seed value to be used by the next call, or it might be something
|
|
more elaborate.
|
|
In order to get reproducible behavior in a threaded
|
|
application, this state must be made explicit.
|
|
The function
|
|
.BR rand_r ()
|
|
is supplied with a pointer to an
|
|
.IR "unsigned int" ,
|
|
to be used as state.
|
|
This is a very small amount of state, so this function will be a weak
|
|
pseudo-random generator.
|
|
Try
|
|
.BR drand48_r (3)
|
|
instead.
|
|
.SH "RETURN VALUE"
|
|
The
|
|
.BR rand ()
|
|
and
|
|
.BR rand_r ()
|
|
functions return a value
|
|
between 0 and
|
|
.BR RAND_MAX .
|
|
The
|
|
.BR srand ()
|
|
function returns no value.
|
|
.SH "CONFORMING TO"
|
|
The functions
|
|
.BR rand ()
|
|
and
|
|
.BR srand ()
|
|
conform to SVr4, 4.3BSD, C89, C99, POSIX.1-2001.
|
|
The function
|
|
.BR rand_r ()
|
|
is from POSIX.1-2001.
|
|
.\" FIXME . Mar 08: The next POSIX.1 revisions marks rand_r() obsolete.
|
|
.SH NOTES
|
|
The versions of
|
|
.BR rand ()
|
|
and
|
|
.BR srand ()
|
|
in the Linux C Library use
|
|
the same random number generator as
|
|
.BR random (3)
|
|
and
|
|
.BR srandom (3),
|
|
so
|
|
the lower-order bits should be as random as the higher-order bits.
|
|
However, on older
|
|
.BR rand ()
|
|
implementations, and on current implementations on different systems,
|
|
the lower-order bits are much less random than the higher-order bits.
|
|
Do not use this function in applications intended to be portable
|
|
when good randomness is needed.
|
|
(Use
|
|
.BR random (3)
|
|
instead.)
|
|
.SH EXAMPLE
|
|
POSIX.1-2001 gives the following example of an implementation of
|
|
.BR rand ()
|
|
and
|
|
.BR srand (),
|
|
possibly useful when one needs the same sequence on two different machines.
|
|
.sp
|
|
.in +4n
|
|
.nf
|
|
static unsigned long next = 1;
|
|
|
|
/* RAND_MAX assumed to be 32767 */
|
|
int myrand(void) {
|
|
next = next * 1103515245 + 12345;
|
|
return((unsigned)(next/65536) % 32768);
|
|
}
|
|
|
|
void mysrand(unsigned seed) {
|
|
next = seed;
|
|
}
|
|
.fi
|
|
.in
|
|
.SH "SEE ALSO"
|
|
.BR drand48 (3),
|
|
.BR random (3)
|