# random man page

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**():

_XOPEN_SOURCE >= 500

|| /* Glibc since 2.19: */ _DEFAULT_SOURCE

|| /* Glibc versions <= 2.19: */ _SVID_SOURCE || _BSD_SOURCE

## 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. 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 results in an error. *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

The **random**() function should not be used in multithreaded programs where reproducible behavior is required. 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

## Colophon

This page is part of release 4.16 of the Linux *man-pages* project. A description of the project, information about reporting bugs, and the latest version of this page, can be found at https://www.kernel.org/doc/man-pages/.

## Referenced By

arc4random.3bsd(3), drand48(3), drand48_r(3), guestfs-release-notes(1), rand(3), random_r(3), wipe(1).

The man pages initstate(3), setstate(3) and srandom(3) are aliases of random(3).