RAND48(3) | Library Functions Manual | RAND48(3) |
drand48
, erand48
,
lrand48
, nrand48
,
mrand48
, jrand48
,
srand48
, seed48
,
lcong48
—
#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]);
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 = 0x5deece66d = 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,
thus 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 random(3).
October 8, 1993 | NetBSD 9.0 |