srand48 (3) - Linux Manuals
srand48: generate uniformly distributed pseudo-random numbers
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 xsubi[3]); long lrand48(void); long nrand48(unsigned short xsubi[3]); long mrand48(void); long jrand48(unsigned short xsubi[3]); void srand48(long 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
The
drand48()
and
erand48()
functions return nonnegative
double-precision floating-point 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 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.
The above
functions record global state information for the random number generator,
so they are not thread-safe.
DESCRIPTION
These functions generate pseudo-random numbers using the linear congruential
algorithm and 48-bit integer arithmetic.
ATTRIBUTES
For an explanation of the terms used in this section, see
attributes(7).
Interface Attribute Value
drand48(),
erand48(),
lrand48(),
nrand48(),
mrand48(),
jrand48(),
srand48(),
seed48(),
lcong48()
Thread safety
MT-Unsafe race:drand48
CONFORMING TO
POSIX.1-2001, POSIX.1-2008, SVr4.
COLOPHON
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