Tribblix: 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)