illumos: manual page: random.3c

RANDOM(3C) Standard C Library Functions RANDOM(3C)

NAME

random, srandom, initstate, setstate - pseudorandom number functions

SYNOPSIS

#include <stdlib.h>

long random(void);

void srandom(unsigned int seed);

char *initstate(unsigned int seed, char *state, size_t size);

char *setstate(const char *state);

DESCRIPTION

The random() function uses a nonlinear additive feedback random-number
generator employing a default state array size of 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 approximately 16 x (2^31
-1). The size of the state array determines the period of the random-
number generator. Increasing the state array size increases the period.

The srandom() function initializes the current state array using the
value of seed.

The random() and srandom() functions have (almost) the same calling
sequence and initialization properties as rand() and srand() (see
rand(3C)). The difference is that rand(3C) produces a much less random
sequence--in fact, the low dozen bits generated by rand go through a
cyclic pattern. All the bits generated by random() are usable.

The algorithm from rand() is used by srandom() to generate the 31 state
integers. Because of this, different srandom() seeds often produce,
within an offset, the same sequence of low order bits from random(). If
low order bits are used directly, random() should be initialized with
setstate() using high quality random values.

Unlike srand(), srandom() does not return the old seed because the amount
of state information used is much more than a single word. Two other
routines are provided to deal with restarting/changing random number
generators. 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.

Like rand(3C), random() produces by default a sequence of numbers that
can be duplicated by calling srandom() with 1 as the seed.

The initstate() and setstate() functions handle restarting and changing
random-number generators. The initstate() function allows a state array,
pointed to by the state argument, to be initialized for future use. The
size argument, which specifies the size in bytes of the state array, is
used by initstate() to decide what type of random-number generator to
use; the larger the state array, the more random the numbers. Values for
the amount of state information are 8, 32, 64, 128, and 256 bytes. Other
values greater than 8 bytes are rounded down to the nearest one of these
values. For values smaller than 8, random() uses a simple linear
congruential random number generator. The seed argument specifies a
starting point for the random-number sequence and provides for restarting
at the same point. The initstate() function returns a pointer to the
previous state information array.

If initstate() has not been called, then random() behaves as though
initstate() had been called with seed=1 and size=128.

If initstate() is called with size<8, then random() uses a simple linear
congruential random number generator.

Once a state has been initialized, setstate() allows switching between
state arrays. The array defined by the state argument is used for further
random-number generation until initstate() is called or setstate() is
called again. The setstate() function returns a pointer to the previous
state array.

For a more powerful random number generator, see arc4random(3C).

RETURN VALUES

The random() function returns the generated pseudo-random number.

The srandom() function returns no value.

Upon successful completion, initstate() and setstate() return a pointer
to the previous state array. Otherwise, a null pointer is returned.

ERRORS

No errors are defined.

USAGE

After initialization, a state array can be restarted at a different point
in one of two ways:

o The initstate() function can be used, with the desired seed,
state array, and size of the array.

o The setstate() function, with the desired state, can be used,
followed by srandom() with the desired seed. The advantage of
using both of these functions is that the size of the state
array does not have to be saved once it is initialized.

EXAMPLES

Example 1: Initialize an array.

The following example demonstrates the use of initstate() to intialize an
array. It also demonstrates how to initialize an array and pass it to
setstate().

# include <stdlib.h>
static unsigned int state0[32];
static unsigned int state1[32] = {
3,
0x9a319039, 0x32d9c024, 0x9b663182, 0x5da1f342,
0x7449e56b, 0xbeb1dbb0, 0xab5c5918, 0x946554fd,
0x8c2e680f, 0xeb3d799f, 0xb11ee0b7, 0x2d436b86,
0xda672e2a, 0x1588ca88, 0xe369735d, 0x904f35f7,
0xd7158fd6, 0x6fa6f051, 0x616e6b96, 0xac94efdc,
0xde3b81e0, 0xdf0a6fb5, 0xf103bc02, 0x48f340fb,
0x36413f93, 0xc622c298, 0xf5a42ab8, 0x8a88d77b,
0xf5ad9d0e, 0x8999220b, 0x27fb47b9
};
main() {
unsigned seed;
int n;
seed = 1;
n = 128;
(void)initstate(seed, (char *)state0, n);
printf("random() = %d0\n", random());
(void)setstate((char *)state1);
printf("random() = %d0\n", random());
}

ATTRIBUTES

NOTES

The random() and srandom() functions are unsafe in multithreaded
applications.

Use of these functions in multithreaded applications is unsupported.

For initstate() and setstate(), the state argument must be aligned on an
int boundary.

arc4random(3C) is a newer and better performing random number generator.
Use it instead.

August 14, 2002 RANDOM(3C)