DRAND48
NAME
drand48, erand48, lrand48, nrand48, mrand48, jrand48, srand48, seed48,
lcong48 - generate uniformly distributed pseudo-random numbers
SYNOPSIS
#include <stdlib.h>
double drand48(void);
I double erand48(unsigned short xsubi [3]);
long int lrand48(void);
I long int nrand48(unsigned short xsubi [3]);
long int mrand48(void);
I long int jrand48(unsigned short xsubi [3]);
I void srand48(long int seedval );
I unsigned short *seed48(unsigned short seed16v [3]);
I void lcong48(unsigned short param [7]);
DESCRIPTION
These functions generate pseudo-random numbers using the linear congruential
algorithm and 48-bit integer arithmetic.
The
R drand48 ()
and
R erand48 ()
functions return non-negative
double-precision floating-point values uniformly distributed between
[0.0, 1.0).
The
R lrand48 ()
and
R nrand48 ()
functions return non-negative
long integers uniformly distributed between 0 and 2^31.
The
R mrand48 ()
and
R jrand48 ()
functions return signed long
integers uniformly distributed between -2^31 and 2^31.
The
R srand48 (),
R seed48 ()
and
R lcong48 ()
functions are
initialization functions, one of which should be called before using
R drand48 (),
R lrand48 ()
or
R mrand48 ().
The functions
R erand48 (),
R nrand48 ()
and
R jrand48 ()
do not require
an initialization function to be called first.
All the functions work by generating a sequence of 48-bit integers,
Xi, according to the linear congruential formula:
Xn+1 = (aXn + c) mod m, where n >= 0
The parameter
m = 2^48, hence 48-bit integer arithmetic is performed.
Unless
R lcong48 ()
is called,
a and
c are given by:
a = 0x5DEECE66D
c = 0xB
The value returned by any of the functions
R drand48 (),
R erand48 (),
R lrand48 (),
R nrand48 (),
R mrand48 ()
or
R 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, is copied from the high-order bits of Xi and transformed
into the returned value.
The functions
R drand48 (),
R lrand48 ()
and
R mrand48 ()
store
the last 48-bit Xi generated in an internal buffer.
The functions
R erand48 (),
R nrand48 ()
and
R jrand48 ()
require the calling
program to provide storage for the successive Xi values in the array
argument xsubi.
The functions are initialized by placing the initial
value of Xi into the array before calling the function for the first
time.
The initializer function
R srand48 ()
sets the high order 32-bits of
Xi to the argument seedval.
The low order 16-bits are set
to the arbitrary value 0x330E.
The initializer function
R seed48 ()
sets the value of Xi to
the 48-bit value specified in the array argument seed16v.
The
previous value of Xi is copied into an internal buffer and a
pointer to this buffer is returned by
R seed48 ().
The initialization function
R lcong48 ()
allows the user to specify
initial values for Xi, a and c.
Array argument
elements param[0-2] specify Xi, param[3-5] specify
a, and param[6] specifies c.
After
R lcong48 ()
has been called, a subsequent call to either
R srand48 ()
or
R seed48 ()
will restore the standard values of a and c.
CONFORMING TO
SVr4, POSIX.1-2001.
NOTES
These functions are declared obsolete by SVID 3, which states that
rand(3)
should be used instead.
SEE ALSO
