drand48(3C) Standard 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 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 automat‐
ically 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 val‐
ues, 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 aand the addend
value care 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 gen‐
erating 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 pro‐
gram 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 using 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] 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
above.
ATTRIBUTES
See attributes(5) for descriptions of the following attributes:
┌─────────────────────────────┬─────────────────────────────┐
│ ATTRIBUTE TYPE │ ATTRIBUTE VALUE │
├─────────────────────────────┼─────────────────────────────┤
│Interface Stability │Standard │
├─────────────────────────────┼─────────────────────────────┤
│MT-Level │Safe │
└─────────────────────────────┴─────────────────────────────┘
SEE ALSOrand(3C), attributes(5), standards(5)SunOS 5.10 24 Jul 2002 drand48(3C)
