drand48, erand48, lrand48, nrand48, mrand48, jrand48, srand48, seed48, lcong48 — generate uniformly distributed pseudo-random numbers
#include <stdlib.h>
double drand48( |
void) ; |
double erand48( |
unsigned short | xsubi) ; |
long
int lrand48( |
void) ; |
long
int nrand48( |
unsigned short | xsubi) ; |
long
int mrand48( |
void) ; |
long
int jrand48( |
unsigned short | xsubi) ; |
void
srand48( |
long int | seedval) ; |
unsigned short *seed48( |
unsigned short | seed16v) ; |
void
lcong48( |
unsigned short | param) ; |
These functions generate pseudo-random numbers using the linear congruential algorithm and 48-bit integer arithmetic.
The drand48
() and
erand48
() functions return
non-negative double-precision floating-point values uniformly
distributed between [0.0, 1.0).
The lrand48
() and
nrand48
() functions return
non-negative long integers uniformly distributed between 0
and 2^31.
The mrand48
() and
jrand48
() functions return
signed long integers uniformly distributed between
−2^31 and 2^31.
The srand48
(), seed48
() and lcong48
() functions are initialization
functions, one of which should be called before using
drand48
(), lrand48
() or mrand48
(). The functions erand48
(), nrand48
() and 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
lcong48
() is called, a
and c
are given by:
a = 0x5DEECE66D c = 0xB
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, is copied
from the high-order 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. The functions erand48
(), nrand48
() and 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 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 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 seed48
().
The initialization function 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 lcong48
() has been called, a subsequent
call to either srand48
() or
seed48
() will restore the
standard values of a
and c
.
These functions are declared obsolete by SVID 3, which states that rand(3) should be used instead.
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