RANDOM(3) | Library Functions Manual | RANDOM(3) |

`random`

, `srandom`

,
`srandomdev`

, `initstate`

,
`setstate`

— better random
number generator; routines for changing generators

```
#include
<stdlib.h>
```

`long`

`random`

(`void`);

`void`

`srandom`

(`unsigned
int seed`);

`void`

`srandomdev`

(`void`);

`char *`

`initstate`

(`unsigned
int seed`, `char
*state`, `size_t
n`);

`char *`

`setstate`

(`char
*state`);

The `random`

() function uses a non-linear
additive feedback random number generator employing a default table of size
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 very
large, approximately 16*((2**31)-1).

The `random`

() and
`srandom`

() functions have (almost) the same calling
sequence and initialization properties as
rand(3)/srand(3).
The difference is that rand
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. For example,
‘`random()&01`

’ will produce a
random binary value.

Like rand(3),
`random`

() will by default produce a sequence of
numbers that can be duplicated by calling `srandom`

()
with ‘`1`

’ as the seed.

The `srandomdev`

() routine initializes a
state array using random numbers obtained from the kernel, 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.

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.

The `random`

(),
`srandom`

(), `initstate`

(), and
`setstate`

() functions conform to
X/Open Portability Guide Issue 4, Version 2
(“XPG4.2”).

The `srandomdev`

() function is an
extension.

These functions appeared in 4.2BSD.

Earl T. Cohen

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

June 5, 2013 | OpenBSD-5.5 |