Citation :
C 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 xi[3] );
long lrand48(void)
long nrand48(unsigned short xi[3] );
long mrand48(void)
long jrand48(unsigned short xi[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, 2**31].
Functions mrand48() and jrand48() return signed long
integers uniformly distributed over the interval [-2**31 , 2
**31 ].
Functions srand48(), seed48(), and lcong48() are initializa-
tion 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.
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C Library Functions drand48(3C)
All the routines work by generating a sequence of 48-bit
integer values, Xi , according to the linear congruential
formula
X n+1= (aX n+c) mod m n>=0.
The parameter m = 2**48; 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 pro-
vide 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 argu-
ments, 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 addi-
tion, 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 using
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C Library Functions drand48(3C)
seed48() when the program is restarted.
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] 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,
specified above.
ATTRIBUTES
See attributes(5) for descriptions of the following attri-
butes:
___________________________________________________________
| ATTRIBUTE TYPE | ATTRIBUTE VALUE |
|_____________________________|_____________________________|
| MT-Level | Safe |
|_____________________________|_____________________________|
SEE ALSO
rand(3C), attributes(5)
|
