DRAND48
Section: Linux Programmer's Manual (3)
Updated: 20170915
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NAME
drand48, erand48, lrand48, nrand48, mrand48, jrand48, srand48, seed48,
lcong48  generate uniformly distributed pseudorandom numbers
SYNOPSIS
#include <stdlib.h>
double drand48(void);
double erand48(unsigned short xsubi[3]);
long int lrand48(void);
long int nrand48(unsigned short xsubi[3]);
long int mrand48(void);
long int jrand48(unsigned short xsubi[3]);
void srand48(long int seedval);
unsigned short *seed48(unsigned short seed16v[3]);
void lcong48(unsigned short param[7]);
Feature Test Macro Requirements for glibc (see
feature_test_macros(7)):
All functions shown above:
_XOPEN_SOURCE
 /* Glibc since 2.19: */ _DEFAULT_SOURCE
 /* Glibc versions <= 2.19: */ _SVID_SOURCE
DESCRIPTION
These functions generate pseudorandom numbers using the linear congruential
algorithm and 48bit integer arithmetic.
The
drand48()
and
erand48()
functions return nonnegative
doubleprecision floatingpoint values uniformly distributed over the interval
[0.0, 1.0).
The
lrand48()
and
nrand48()
functions return nonnegative
long integers uniformly distributed over the interval [0, 2^31).
The
mrand48()
and
jrand48()
functions return signed long
integers uniformly distributed over the interval [2^31, 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 48bit integers,
Xi,
according to the linear congruential formula:
Xn+1 = (aXn + c) mod m, where n >= 0
The parameter
m
= 2^48, hence 48bit 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 48bit
Xi
in the sequence.
Then the appropriate number of bits, according to the type of data item to
be returned, is copied from the highorder bits of
Xi
and transformed
into the returned value.
The functions
drand48(),
lrand48()
and
mrand48()
store
the last 48bit
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 32bits of
Xi
to the argument
seedval.
The low order 16bits are set
to the arbitrary value 0x330E.
The initializer function
seed48()
sets the value of
Xi
to
the 48bit 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[02]
specify
Xi,
param[35]
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.
ATTRIBUTES
For an explanation of the terms used in this section, see
attributes(7).
T}  Thread safety 
MTUnsafe race:drand48

The above
functions record global state information for the random number generator,
so they are not threadsafe.
CONFORMING TO
POSIX.12001, POSIX.12008, SVr4.
SEE ALSO
rand(3),
random(3)
COLOPHON
This page is part of release 4.13 of the Linux
manpages
project.
A description of the project,
information about reporting bugs,
and the latest version of this page,
can be found at
https://www.kernel.org/doc/manpages/.
Index
 NAME

 SYNOPSIS

 DESCRIPTION

 ATTRIBUTES

 CONFORMING TO

 SEE ALSO

 COLOPHON
