man pages section 3: Basic Library Functions
Updated: July 2014

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 231 −1. The period of this random-number generator is approximately 16 x (231 −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 269, 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.

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:

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

• 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.

Programmers should use /dev/urandom or /dev/random for most random-number generation, especially for cryptographic purposes. See random(7D) .

Examples

Example 1 Initialize an array.

The following example demonstrates the use of initstate() to initialize 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,
};
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

See attributes(5) for descriptions of the following attributes:

ATTRIBUTE TYPE
ATTRIBUTE VALUE
Interface Stability
Committed
MT-Level
See NOTES below.
Standard