RAND48(3) | Library Functions Manual | RAND48(3) |

`drand48`

, `erand48`

,
`lrand48`

, `nrand48`

,
`mrand48`

, `jrand48`

,
`srand48`

, `seed48`

,
`lcong48`

— 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`);

`unsigned short *`

`seed48`

(`unsigned
short xseed[3]`);

`void`

`lcong48`

(`unsigned
short p[7]`);

The `rand48`

() family of functions generates
pseudo-random 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`

() 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.

`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`

() 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`

() 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`

() 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`

().

Finally, `lcong48`

() 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.

For a more powerful random number generator, see arc4random(3).

The `drand48`

(),
`erand48`

(), `jrand48`

(),
`lcong48`

(), `lrand48`

(),
`mrand48`

(), `nrand48`

(),
`seed48`

(), and `srand48`

()
functions conform to IEEE Std 1003.1-2008
(“POSIX.1”).

Martin Birgmeier

January 21, 2014 | OpenBSD-5.5 |