NAME
drand48,
erand48,
lrand48,
nrand48,
mrand48,
jrand48,
srand48,
seed48,
lcong48 —
pseudo-random number generators and initialization
routines
LIBRARY
Standard C Library (libc, -lc)
SYNOPSIS
#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]);
DESCRIPTION
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 = 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).
SEE ALSO
rand(3),
random(3)
AUTHORS
Martin Birgmeier