DRAND48(3) Library Functions Manual DRAND48(3)
delim $$
NAME
drand48, erand48, lrand48, nrand48, mrand48, jrand48, srand48, seed48,
lcong48  generate uniformly distributed pseudorandom numbers
SYNOPSIS
double drand48()
double erand48(xsubi)
unsigned short xsubi[3];
long lrand48()
long nrand48(xsubi)
unsigned short xsubi[3];
long mrand48()
long jrand48(xsubi)
unsigned short xsubi[3];
void srand48(seedval)
long seedval;
unsigned short *seed48(seed16v)
unsigned short seed16v[3];
void lcong48(param)
unsigned short param[7];
DESCRIPTION
This family of functions generates pseudorandom numbers using the
wellknown linear congruential algorithm and 48bit integer arithmetic.
drand48() and erand48() return nonnegative doubleprecision floating
point values uniformly distributed over the interval $[0.0,~1.0).$
lrand48() and nrand48() return nonnegative long integers uniformly
distributed over the interval (0, ~2**31).
mrand48() and jrand48() return signed long integers uniformly distrib
uted over the interval [2**31 ~2**31).
srand48(), seed48(), and lcong48() are initialization entry points, one
of which should be invoked before either drand48(), lrand48(), or
mrand48() is called. Although it is not recommended practice, constant
default initializer values will be supplied automatically if drand48(),
lrand48(), or mrand48() is called without a prior call to an initial
ization entry point. erand48(), nrand48(), and jrand48() do not
require an initialization entry point to be called first.
All the routines work by generating a sequence of 48bit integer val
ues, $X sub i ,$ according to the linear congruential formula
X sub{n+1}~=~(aX sub n^+^c) sub{roman mod~m}~~~~~~~~n>=0.
The parameter m=2**48; hence 48bit integer arithmetic is performed.
Unless lcong48() has been invoked, the multiplier value $a$ and the
addend value $c$ are given by
a~mark =~roman 5DEECE66D^sub 16~=~roman 273673163155^sub 8
c~lineup =~roman B^sub 16~=~roman 13^sub 8 .
The value returned by any of the functions drand48(), erand48(),
lrand48(), nrand48(), mrand48(), or jrand48() is computed by first gen
erating the next 48bit $X sub i$ in the sequence. Then the appropri
ate number of bits, according to the type of data item to be returned,
are copied from the highorder (leftmost) bits of $X sub i$ and trans
formed into the returned value.
drand48(), lrand48(), and mrand48() store the last 48bit $X sub i$
generated in an internal buffer; that is why they must be initialized
prior to being invoked. The functions erand48(), nrand48(), and
jrand48() require the calling program to provide storage for the suc
cessive $X sub i$ values in the array specified as an argument when the
functions are invoked. That is why these routines do not have to be
initialized; the calling program merely has to place the desired ini
tial value of $X sub i$ into the array and pass it as an argument. By
using different arguments, functions erand48(), nrand48(), and
jrand48() allow separate modules of a large program to generate several
independent streams of pseudorandom numbers, that is, the sequence of
numbers in each stream will not depend upon how many times the routines
have been called to generate numbers for the other streams.
The initializer function srand48() sets the highorder 32 bits of $X
sub i$ to the 32 bits contained in its argument. The loworder 16 bits
of $X sub i$ are set to the arbitrary value $roman 330E sub 16 .$
The initializer function seed48() sets the value of $X sub i$ to the
48bit value specified in the argument array. In addition, the previ
ous value of $X sub i$ is copied into a 48bit internal buffer, used
only by seed48(), and a pointer to this buffer is the value returned by
seed48(). This returned pointer, which can just be ignored if not
needed, is useful if a program is to be restarted from a given point at
some future time  use the pointer to get at and store the last $X sub
i$ value, and then use this value to reinitialize via seed48() when the
program is restarted.
The initialization function lcong48() allows the user to specify the
initial $X sub i ,$ the multiplier value $a,$ and the addend value $c.$
Argument array elements param[02] specify $X sub i ,$ param[35] spec
ify the multiplier $a,$ and param[6] specifies the 16bit addend $c.$
After lcong48() has been called, a subsequent call to either srand48()
or seed48() will restore the "standard" multiplier and addend values,
$a$ and $c,$ specified on the previous page.
SEE ALSO
rand(3V)
7 February 1989 DRAND48(3)
