.\" Copyright 1993 David Metcalfe (david@prism.demon.co.uk)
.\"
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.\" 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.\" manual page may be incorrect or out-of-date.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
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.\" References consulted:
.\"     Linux libc source code
.\"     Lewine's _POSIX Programmer's Guide_ (O'Reilly & Associates, 1991)
.\"     386BSD man pages
.\" Modified Mon Mar 29 22:48:44 1993, David Metcalfe
.\" Modified Wed Apr 28 01:35:00 1993, Lars Wirzenius
.\" Modified Sat Jul 24 18:39:41 1993, Rik Faith (faith@cs.unc.edu)
.\" Modified Thu May 18 10:10:13 1995, 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 Fri Apr 10 01:47:03 1998, Nicolás Lichtmaier <nick@debian.org>
.\"          with contribution from Francesco Potorti <F.Potorti@cnuce.cnr.it>
.\"
.TH RAND 3  "18 May 1995" "GNU" "Linux Programmer's Manual"
.SH NAME
rand, srand \- random number generator.
.SH SYNOPSIS
.nf
.B #include <stdlib.h>
.sp
.B int rand(void);
.sp
.BI "void srand(unsigned int " seed );
.fi
.SH DESCRIPTION
The \fBrand()\fP function returns a pseudo-random integer between 0
and \fBRAND_MAX\fR.
.PP
The \fBsrand()\fP function sets its argument as the seed for a new
sequence of pseudo-random integers to be returned by \fBrand()\fP.
These sequences are repeatable by calling \fBsrand()\fP with the same
seed value.
.PP
If no seed value is provided, the \fBrand()\fP function is automatically 
seeded with a value of 1.
.SH "RETURN VALUE"
The \fBrand()\fP function returns a value between 0 and RAND_MAX.
The \fBsrand()\fP returns no value.
.SH NOTES
The versions of \fBrand()\fP and \fBsrand()\fP in the Linux C Library use
the same random number generator as \fBrandom()\fP and \fBsrandom()\fP, so
the lower-order bits should be as random as the higher-order bits.
However, on older
.B rand()
implementations, the lower-order bits are much less random than the
higher-order bits.
.PP
In
.I Numerical Recipes in C: The Art of Scientific Computing
(William H. Press, Brian P. Flannery, Saul A. Teukolsky, William
T. Vetterling; New York: Cambridge University Press, 1992 (2nd ed.,
p. 277)), the following comments are made:
.RS
"If you want to generate a random integer between 1 and 10, you should
always do it by using high-order bits, as in
.RS
.sp
j=1+(int) (10.0*rand()/(RAND_MAX+1.0));
.sp
.RE
and never by anything resembling
.RS
.sp
j=1+(rand() % 10);
.sp
.RE
(which uses lower-order bits)."
.RE
.PP
Random-number generation is a complex topic.  The
.I Numerical Recipes in C
book (see reference above)
provides an excellent discussion of practical random-number generation
issues in Chapter 7 (Random Numbers).
.PP
For a more theoretical discussion which also covers many practical issues
in depth, please see Chapter 3 (Random Numbers) in Donald E. Knuth's
.IR "The Art of Computer Programming" ,
volume 2 (Seminumerical Algorithms), 2nd ed.; Reading, Massachusetts:
Addison-Wesley Publishing Company, 1981.
.SH "CONFORMING TO"
SVID 3, BSD 4.3, ISO 9899
.SH "SEE ALSO"
.BR random "(3), " srandom "(3), " initstate "(3), " setstate (3)