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  2020-06-09 "GNU" "Linux Programmer's Manual"
.SH NAME
random, srandom, initstate, setstate \- random number generator
.SH SYNOPSIS
.nf
.B #include <stdlib.h>
.PP
.B long int random(void);
.PP
.BI "void srandom(unsigned int " seed );
.PP
.BI "char *initstate(unsigned int " seed ", char *" state ", size_t " n );
.PP
.BI "char *setstate(char *" state );
.fi
.PP
.in -4n
Feature Test Macro Requirements for glibc (see
.BR feature_test_macros (7)):
.in
.PP
.ad l
.BR random (),
.BR srandom (),
.BR initstate (),
.BR setstate ():
.RS 4
_XOPEN_SOURCE\ >=\ 500
.\"    || _XOPEN_SOURCE\ &&\ _XOPEN_SOURCE_EXTENDED
    || /* Glibc since 2.19: */ _DEFAULT_SOURCE
    || /* Glibc versions <= 2.19: */ _SVID_SOURCE || _BSD_SOURCE
.RE
.ad
.SH DESCRIPTION
The
.BR random ()
function uses a nonlinear 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 2^31\ \-\ 1.
The period of this random number generator is very large, approximately
.IR "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\(emthe larger the state array,
the better the random numbers will be.
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 results in an error.
\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
.IR "(2^31)\ \-\ 1" .
The
.BR srandom ()
function returns no value.
.PP
The
.BR initstate ()
function returns a pointer to the previous state array.
On error,
.I errno
is set to indicate the cause.
.PP
On success,
.BR setstate ()
returns a pointer to the previous state array.
On error, it returns NULL, with
.I errno
set to indicate the cause of the error.
.SH ERRORS
.TP
.B EINVAL
The
.I state
argument given to
.BR setstate ()
was NULL.
.TP
.B EINVAL
A state array of less than 8 bytes was specified to
.BR initstate ().
.SH ATTRIBUTES
For an explanation of the terms used in this section, see
.BR attributes (7).
.TS
allbox;
lbw23 lb lb
l l l.
Interface	Attribute	Value
T{
.BR random (),
.BR srandom (),
.br
.BR initstate (),
.BR setstate ()
T}	Thread safety	MT-Safe
.TE
.SH CONFORMING TO
POSIX.1-2001, POSIX.1-2008, 4.3BSD.
.SH NOTES
The
.BR random ()
function should not be used in multithreaded programs
where reproducible behavior is required.
Use
.BR random_r (3)
for that purpose.
.PP
Random-number generation is a complex topic.
.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, 2007, 3rd ed.)
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, 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 BUGS
According to POSIX,
.BR initstate ()
should return NULL on error.
In the glibc implementation,
.I errno
is (as specified) set on error, but the function does not return NULL.
.\" http://sourceware.org/bugzilla/show_bug.cgi?id=15380
.SH SEE ALSO
.BR getrandom (2),
.BR drand48 (3),
.BR rand (3),
.BR random_r (3),
.BR srand (3)