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>
```

`drand48`

(`erand48`

(`lrand48`

(`nrand48`

(`mrand48`

(`jrand48`

(`srand48`

(`srand48_deterministic`

(`seed48`

(`seed48_deterministic`

(`lcong48`

(`lcong48_deterministic`

(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.
`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.
November 10, 2015 | OpenBSD-current |