drand48(3C) drand48(3C)
NAME
drand48, erand48, lrand48, nrand48, mrand48, jrand48, srand48,
seed48, lcong48 - generate uniformly distributed pseudo-random
numbers
SYNOPSIS
#include <stdlib.h>
double drand48(void);
double erand48(unsigned short xsubi[3]);
long lrand48(void);
long nrand48(unsigned short xsubi[3]);
long mrand48(void);
long jrand48(unsigned short xsubi[3]);
void srand48(long seedval);
unsigned short *seed48(unsigned short seed16v[3]);
void lcong48(unsigned short param[7]);
DESCRIPTION
This family of functions generates pseudo-random numbers using
the well-known linear congruential algorithm and 48-bit
integer arithmetic.
Functions drand48 and erand48 return non-negative double-
precision floating-point values uniformly distributed over the
interval [0.0, 1.0).
Functions lrand48 and nrand48 return non-negative long
integers uniformly distributed over the interval [0, 231).
Functions mrand48 and jrand48 return signed long integers
uniformly distributed over the interval [-231, 231).
Functions 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 initialization entry point.)
Functions erand48, nrand48, and jrand48 do not require an
initialization entry point to be called first.
All the routines work by generating a sequence of 48-bit
integer values, Xi, according to the linear congruential
formula
Copyright 1994 Novell, Inc. Page 1
drand48(3C) drand48(3C)
Xn+1 = (aXn + c)mod m n>0.
The parameter m=248; hence 48-bit integer arithmetic is
performed. Unless lcong48 has been invoked, the multiplier
value a and the addend value c are given by
a = 5DEECE66D16 = 2736731631558
c = B16 = 138 .
The value returned by any of the functions drand48, erand48,
lrand48, nrand48, mrand48, or jrand48 is computed by first
generating the next 48-bit Xi in the sequence. Then the
appropriate number of bits, according to the type of data item
to be returned, are copied from the high-order (leftmost) bits
of Xi and transformed into the returned value.
The functions drand48, lrand48, and mrand48 store the last
48-bit Xi generated in an internal buffer. Xi must be
initialized prior to being invoked. The functions erand48,
nrand48, and jrand48 require the calling program to provide
storage for the successive Xi values in the array specified as
an argument when the functions are invoked. These routines do
not have to be initialized; the calling program must place the
desired initial value of Xi 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 pseudo-random
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 high-order 32 bits
of Xi to the 32 bits contained in its argument. The low-order
16 bits of Xi are set to the arbitrary value 330E16.
The initializer function seed48 sets the value of Xi to the
48-bit value specified in the argument array. In addition,
the previous value of Xi is copied into a 48-bit 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 Xi value, and then
use this value to reinitialize via seed48 when the program is
restarted.
Copyright 1994 Novell, Inc. Page 2
drand48(3C) drand48(3C)
The initialization function lcong48 allows the user to specify
the initial Xi, the multiplier value a, and the addend value
c. Argument array elements param[0-2] specify Xi, param[3-5]
specify the multiplier a, and param[6] specifies the 16-bit
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.
REFERENCES
rand(3C)
Copyright 1994 Novell, Inc. Page 3