RANDOM(3)

NAME
initstate, random, setstate, srandom, srandomdev - better random number generator; routines for changing generators

LIBRARY
libc

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) and srand(3) functions. The difference is that rand(3) 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), srandom() sets the initial seed value for future calls to random(). Like rand(3), 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 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), rand(3), srand(3), random(4), compat(5)

HISTORY
These functions appeared in 4.2BSD.

AUTHORS
Earl T. Cohen

BUGS
About 2/3 the speed of rand(3).

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).