DRAND48(3) | Library Functions Manual | DRAND48(3) |

`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.

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

December 20, 2019 | OpenBSD-current |