drand48(3C) LIBRARY FUNCTIONS 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
1
drand48(3C) LIBRARY FUNCTIONS drand48(3C)
formula
Xn+1 = (aXn+c)mod m n>0.
The parameter m=248; hence 48-bit integer arithmetic is per-
formed. 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 ini-
tialized 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 rou-
tines 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, i.e., 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.
The initialization function lcong48 allows the user to
specify the initial Xi, the multiplier value a, and the
2
drand48(3C) LIBRARY FUNCTIONS drand48(3C)
addend value c. Argument array elements param[0-2] specify
Xi, param[3-5] specify the multiplier a, and param[6] speci-
fies 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, speci-
fied on the previous page.
SEE ALSO
rand(3C).
3