**drand48**,

**erand48**,

**lrand48**,

**nrand48**,

**mrand48**,

**jrand48**,

**srand48**,

**srand48_deterministic**,

**seed48**,

**seed48_deterministic**,

**lcong48**,

**lcong48_deterministic** —

pseudo-random number generators and initialization
routines
**#include
<stdlib.h>**
`double`
**drand48**(

`void`);

`double`
**erand48**(

`unsigned
short xseed[3]`);

`long`
**lrand48**(

`void`);

`long`
**nrand48**(

`unsigned
short xseed[3]`);

`long`
**mrand48**(

`void`);

`long`
**jrand48**(

`unsigned
short xseed[3]`);

`void`
**srand48**(

`long
seed`);

`void`
**srand48_deterministic**(

`long
seed`);

`unsigned short *`
**seed48**(

`unsigned
short xseed[3]`);

`unsigned short *`
**seed48_deterministic**(

`unsigned
short xseed[3]`);

`void`
**lcong48**(

`unsigned
short p[7]`);

`void`
**lcong48_deterministic**(

`unsigned
short p[7]`);

Standards insist
that this interface return deterministic results. Unsafe usage is very common,
so OpenBSD changed the subsystem to return
non-deterministic results by default.

To satisfy portable code,

**srand48**(),

**seed48**(), or

**lcong48**() should be called to initialize the
subsystem. In

OpenBSD the seeding parameters are
ignored, and strong random number results will be provided from

arc4random(3). In other
systems, the parameters prime a simplistic deterministic algorithm.

If the standardized behavior is required then

**srand48_deterministic**(),

**seed48_deterministic**(), and

**lcong48_deterministic**() can be substituted for

**srand48**(),

**seed48**(),
and

**lcong48**(). That will cause subsequent calls
to

**drand48**(),

**lrand48**(), and

**jrand48**() to return results using the
deterministic algorithm.

**drand48**() and

**erand48**() return values of type double. The full
48 bits of r(n+1) are loaded into the mantissa of the returned value, with the
exponent set such that the values produced lie in the interval [0.0, 1.0].

**lrand48**() and

**nrand48**() return values of type long in the range
[0, 2**31-1]. The high-order (31) bits of r(n+1) are loaded into the lower
bits of the returned value, with the topmost (sign) bit set to zero.

**mrand48**() and

**jrand48**() return values of type long in the range
[-2**31, 2**31-1]. The high-order (32) bits of r(n+1) are loaded into the
returned value.

In the deterministic mode, the

**rand48**() family of
functions generates numbers using a linear congruential algorithm working on
integers 48 bits in size. The particular formula employed is r(n+1) = (a *
r(n) + c) mod m where the default values are for the multiplicand a =
0xfdeece66d = 25214903917 and the addend c = 0xb = 11. The modulus is always
fixed at m = 2 ** 48. r(n) is called the seed of the random number generator.

For all the six generator routines described next, the first computational step
is to perform a single iteration of the algorithm.

**drand48**(),

**lrand48**(),
and

**mrand48**() use an internal buffer to store
r(n). For these functions the initial value of r(0) = 0x1234abcd330e =
20017429951246.

On the other hand,

**erand48**(),

**nrand48**(), and

**jrand48**() use a user-supplied buffer to store the
seed r(n), which consists of an array of 3 shorts, where the zeroth member
holds the least significant bits.

All functions share the same multiplicand and addend.

**srand48_deterministic**() is used to initialize the
internal buffer r(n) of

**drand48**(),

**lrand48**(), and

**mrand48**() such that the 32 bits of the seed value
are copied into the upper 32 bits of r(n), with the lower 16 bits of r(n)
arbitrarily being set to 0x330e. Additionally, the constant multiplicand and
addend of the algorithm are reset to the default values given above.

**seed48_deterministic**() also initializes the
internal buffer r(n) of

**drand48**(),

**lrand48**(), and

**mrand48**(), but here all 48 bits of the seed can
be specified in an array of 3 shorts, where the zeroth member specifies the
lowest bits. Again, the constant multiplicand and addend of the algorithm are
reset to the default values given above.

**seed48_deterministic**() returns a pointer to an
array of 3 shorts which contains the old seed. This array is statically
allocated, so its contents are lost after each new call to

**seed48_deterministic**().

Finally,

**lcong48_deterministic**() allows full
control over the multiplicand and addend used in

**drand48**(),

**erand48**(),

**lrand48**(),

**nrand48**(),

**mrand48**(), and

**jrand48**(), and the seed used in

**drand48**(),

**lrand48**(), and

**mrand48**(). An array of 7 shorts is passed as
parameter; the first three shorts are used to initialize the seed; the second
three are used to initialize the multiplicand; and the last short is used to
initialize the addend. It is thus not possible to use values greater than
0xffff as the addend.

Note that all three methods of seeding the random number generator always also
set the multiplicand and addend for any of the six generator calls.

arc4random(3),

rand(3),

random(3)
The

**drand48**(),

**erand48**(),

**jrand48**(),

**lrand48**(),

**mrand48**(), and

**nrand48**(), functions conform to

IEEE Std 1003.1-2008
(“POSIX.1”).

The

**seed48**(),

**srand48**(), and

**lcong48**() function do not conform to

ANSI X3.159-1989
(“ANSI C89”), intentionally.

The

**seed48_deterministic**(),

**srand48_deterministic**(), and

**lcong48_deterministic**() functions are

OpenBSD extensions.

Martin Birgmeier