ffix

[thirdparty/man-pages.git] / man3 / random.3

.\" Copyright 1993 David Metcalfe (david@prism.demon.co.uk)
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.\" References consulted:
.\"     Linux libc source code
.\"     Lewine's _POSIX Programmer's Guide_ (O'Reilly & Associates, 1991)
.\"     386BSD man pages
.\" Modified Sun Mar 28 00:25:51 1993, David Metcalfe
.\" Modified Sat Jul 24 18:13:39 1993 by Rik Faith (faith@cs.unc.edu)
.\" Modified Sun Aug 20 21:47:07 2000, aeb
.\"
.TH RANDOM 3  2000-08-20 "GNU" "Linux Programmer's Manual"
.SH NAME
random, srandom, initstate, setstate \- random number generator
.SH SYNOPSIS
.nf
.B #include <stdlib.h>
.sp
.B long int random(void);
.br
.BI "void srandom(unsigned int " seed );
.br
.BI "char *initstate(unsigned int " seed ", char *" state ", size_t " n );
.br
.BI "char *setstate(char *" state );
.fi
.SH DESCRIPTION
The
.BR random ()
function uses a non-linear additive feedback random
number generator employing a default table of size 31 long integers to
return successive pseudo-random numbers in
the range from 0 to \fBRAND_MAX\fR.
The period of this random number generator is very large, approximately
16*((2**31)\-1).
.PP
The
.BR srandom ()
function sets its argument as the seed for a new
sequence of pseudo-random integers to be returned by
.BR random ().
These sequences are repeatable by calling
.BR srandom ()
with the same
seed value.
If no seed value is provided, the
.BR random ()
function
is automatically seeded with a value of 1.
.PP
The
.BR initstate ()
function allows a state array \fIstate\fP to
be initialized for use by
.BR random ().
The size of the state array
\fIn\fP is used by
.BR initstate ()
to decide how sophisticated a
random number generator it should use \(em the larger the state array,
the better the random numbers will be.  \fIseed\fP is the seed for the
initialization, which specifies a starting point for the random number
sequence, and provides for restarting at the same point.
.PP
The
.BR setstate ()
function changes the state array used by the
.BR random ()
function.
The state array \fIstate\fP is used for
random number generation until the next call to
.BR initstate ()
or
.BR setstate ().
\fIstate\fP must first have been initialized
using
.BR initstate ()
or be the result of a previous call of
.BR setstate ().
.SH "RETURN VALUE"
The
.BR random ()
function returns a value between 0 and RAND_MAX.
The
.BR srandom ()
function returns no value.
The
.BR initstate ()
and
.BR setstate ()
functions return a pointer to the previous state
array, or NULL on error.
.SH ERRORS
.TP
.B EINVAL
A state array of less than 8 bytes was specified to
.BR initstate ().
.SH NOTES
Current "optimal" values for the size of the state array \fIn\fP are
8, 32, 64, 128, and 256 bytes; other amounts will be rounded down to
the nearest known amount.
Using less than 8 bytes will cause an
error.
.SH "CONFORMING TO"
4.3BSD, POSIX.1-2001.
.SH "SEE ALSO"
.BR rand (3),
.BR srand (3)