### srandom (3)

#### Leading comments

Copyright 1993 David Metcalfe (david@prism.demon.co.uk) %%%LICENSE_START(VERBATIM) 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. Sin...

#### NAME

random, srandom, initstate, setstate - random number generator#### SYNOPSIS

#include <stdlib.h>long int random(void);void srandom(unsigned intseed);char *initstate(unsigned intseed, char *state, size_tn);

char *setstate(char *state);

Feature Test Macro Requirements for glibc (see
**feature_test_macros(7)**):

**random**(),
**srandom**(),
**initstate**(),
**setstate**():

- _SVID_SOURCE || _BSD_SOURCE || _XOPEN_SOURCE >= 500 || _XOPEN_SOURCE && _XOPEN_SOURCE_EXTENDED

#### DESCRIPTION

The**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

**RAND_MAX**. The period of this random number generator is very large, approximately

*16 * ((2^31) - 1)*.

The
**srandom**()
function sets its argument as the seed for a new
sequence of pseudo-random integers to be returned by
**random**().
These sequences are repeatable by calling
**srandom**()
with the same
seed value.
If no seed value is provided, the
**random**()
function
is automatically seeded with a value of 1.

The
**initstate**()
function allows a state array *state* to
be initialized for use by
**random**().
The size of the state array
*n* is used by
**initstate**()
to decide how sophisticated a
random number generator it should use---the larger the state array,
the better the random numbers will be.
*seed* is the seed for the
initialization, which specifies a starting point for the random number
sequence, and provides for restarting at the same point.

The
**setstate**()
function changes the state array used by the
**random**()
function.
The state array *state* is used for
random number generation until the next call to
**initstate**()
or
**setstate**().
*state* must first have been initialized
using
**initstate**()
or be the result of a previous call of
**setstate**().

#### RETURN VALUE

The**random**() function returns a value between 0 and

**RAND_MAX**. The

**srandom**() function returns no value.

The

**initstate**() function returns a pointer to the previous state array. On error,

*errno*is set to indicate the cause.

On success,

**setstate**() returns a pointer to the previous state array. On error, it returns NULL, with

*errno*set to indicate the cause of the error.

#### ERRORS

**EINVAL**-
The
*state*argument given to**setstate**() was NULL. **EINVAL**-
A state array of less than 8 bytes was specified to
**initstate**().

#### ATTRIBUTES

For an explanation of the terms used in this section, see**attributes(7)**.

Interface | Attribute | Value |

random(),
srandom(),
initstate(),
setstate()
| Thread safety | MT-Safe |

#### CONFORMING TO

POSIX.1-2001, POSIX.1-2008, 4.3BSD.#### NOTES

Current "optimal" values for the size of the state array*n*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.

This function should not be used in cases where multiple threads use
**random**()
and the behavior should be reproducible.
Use
**random_r(3)**
for that purpose.

Random-number generation is a complex topic.
*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).

For a more theoretical discussion which also covers many practical issues
in depth, see Chapter 3 (Random Numbers) in Donald E. Knuth's
*The Art of Computer Programming*,
volume 2 (Seminumerical Algorithms), 2nd ed.; Reading, Massachusetts:
Addison-Wesley Publishing Company, 1981.

#### BUGS

According to POSIX,**initstate**() should return NULL on error. In the glibc implementation,

*errno*is (as specified) set on error, but the function does not return NULL.

#### SEE ALSO

**drand48(3)**,

**getrandom(2)**,

**rand(3)**,

**random_r(3)**,

**srand(3)**

#### COLOPHON

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