---
name: "random"
section: "3"
platform: "macos"
package: "Libc"
version: "1752.120.2"
release: "macOS 26.5"
platforms: ["linux", "freebsd", "macos"]
url: "https://new.carta.tech/random/3"
---

# random(3)

better random number generator; routines for changing generators

## NAME

**initstate**, **random**, **setstate**, **srandom**, **srandomdev** – better random number generator; routines for changing generators

## LIBRARY

Standard C Library (libc, -lc)

## SYNOPSIS

**#include \<stdlib.h\>**

*char* *\**\

**initstate**(*unsigned seed*, *char \*state*, *size\_t size*);

*long*\

**random**(*void*);

*char* *\**\

**setstate**(*const char \*state*);

*void*\

**srandom**(*unsigned seed*);

*void*\

**srandomdev**(*void*);

## DESCRIPTION

The **random**() function uses a non-linear, additive feedback, random number generator, employing a default table of size 31 long integers. It returns successive pseudo-random numbers in the range from 0 to (2\*\*31)−1. The period of this random number generator is very large, approximately 16\*((2\*\*31)−1).

The **random**() and **srandom**() functions have (almost) the same calling sequence and initialization properties as the [rand(3)](/rand/3.md) and [srand(3)](/srand/3.md) functions. The difference is that [rand(3)](/rand/3.md) produces a much less random sequence — in fact, the low dozen bits generated by rand go through a cyclic pattern. All of the bits generated by **random**() are usable. For example, ‘random()&01’ will produce a random binary value.

Like [srand(3)](/srand/3.md), **srandom**() sets the initial seed value for future calls to **random**(). Like [rand(3)](/rand/3.md), **random**() will by default produce a sequence of numbers that can be duplicated by calling **srandom**() with the same seed.

The **srandomdev**() routine initializes a state array, using the [random(4)](/random/4.md) random number device which returns good random numbers, suitable for cryptographic use. Note that this particular seeding procedure can generate states which are impossible to reproduce by calling **srandom**() with any value, since the succeeding terms in the state buffer are no longer derived from the LC algorithm applied to a fixed seed.

The **initstate**() routine allows a state array, passed in as an argument, to be initialized for future use. The size of the state array (in bytes) is used by **initstate**() to decide how sophisticated a random number generator it should use — the more state, the better the random numbers will be. (Current "optimal" values for the amount of state information 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.) The seed for the initialization (which specifies a starting point for the random number sequence and provides for restarting at the same point) is also an argument. The **initstate**() function returns a pointer to the previous state information array.

Once a state has been initialized, the **setstate**() routine provides for rapid switching between states. The **setstate**() function returns a pointer to the previous state array; its argument state array is used for further random number generation until the next call to **initstate**() or **setstate**().

Once a state array has been initialized, it may be restarted at a different point either by calling **initstate**() (with the desired seed, the state array, and its size) or by calling both **setstate**() (with the state array) and **srandom**() (with the desired seed). The advantage of calling both **setstate**() and **srandom**() is that the size of the state array does not have to be remembered after it is initialized.

With 256 bytes of state information, the period of the random number generator is greater than 2\*\*69 , which should be sufficient for most purposes.

## DIAGNOSTICS

If **initstate**() is called with less than 8 bytes of state information, or if **setstate**() detects that the state information has been garbled, error messages are printed on the standard error output.

## LEGACY SYNOPSIS

**#include \<stdlib.h\>**

*char* *\**\
 **initstate**(*unsigned long seed*, *char \*state*, *long size*);

*char* *\**\
 **setstate**(*char \*state*);

*void*\
 **srandom**(*unsigned long seed*);

The type of each parameter is different in the legacy version.

## SEE ALSO

[arc4random(3)](/arc4random/3.md), [rand(3)](/rand/3.md), [srand(3)](/srand/3.md), [random(4)](/random/4.md), [compat(5)](/compat/5.md)

## HISTORY

These functions appeared in 4.2BSD.

## AUTHORS

Earl T. Cohen

## BUGS

About 2/3 the speed of [rand(3)](/rand/3.md).

The historical implementation used to have a very weak seeding; the random sequence did not vary much with the seed. The current implementation employs a better pseudo-random number generator for the initial state calculation.

Applications requiring cryptographic quality randomness should use [arc4random(3)](/arc4random/3.md).