.\" 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.\" manual page may be incorrect or out-of-date.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, Nicol?s 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 2008-08-29 "" "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\ >=\ 1 || _XOPEN_SOURCE || _POSIX_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.
POSIX.1-2008 marks
.BR rand_r ()
as 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)
.SH COLOPHON
This page is part of release 3.22 of the Linux
.I man-pages
project.
A description of the project,
and information about reporting bugs,
can be found at
http://www.kernel.org/doc/man-pages/.
