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drand48 erand48 lrand48 nrand48 mrand48 jrand48 srand48 seed48 lcong48 - pseudo random number generators and initialization routines
Lb libc
For all the six generator routines described next, the first computational step is to perform a single iteration of the algorithm.
The
drand48 ();
and
erand48 ();
functions
return values of type double.
The full 48 bits of r(n+1) are
loaded into the mantissa of the returned value, with the exponent set
such that the values produced lie in the interval [0.0, 1.0).
The
lrand48 ();
and
nrand48 ();
functions
return values of type long in the range
[0, 2**31-1].
The high-order (31) bits of
r(n+1) are loaded into the lower bits of the returned value, with
the topmost (sign) bit set to zero.
The
mrand48 ();
and
jrand48 ();
functions
return values of type long in the range
[-2**31, 2**31-1].
The high-order (32) bits of
r(n+1) are loaded into the returned value.
The
drand48 (,);
lrand48 (,);
and
mrand48 ();
functions
use an internal buffer to store r(n).
For these functions
the initial value of r(0) = 0x1234abcd330e = 20017429951246.
On the other hand,
erand48 (,);
nrand48 (,);
and
jrand48 ();
use a user-supplied buffer to store the seed r(n),
which consists of an array of 3 shorts, where the zeroth member
holds the least significant bits.
All functions share the same multiplicand and addend.
The
srand48 ();
function
is used to initialize the internal buffer r(n) of
drand48 (,);
lrand48 (,);
and
mrand48 ();
such that the 32 bits of the seed value are copied into the upper 32 bits
of r(n), with the lower 16 bits of r(n) arbitrarily being set to 0x330e.
Additionally, the constant multiplicand and addend of the algorithm are
reset to the default values given above.
The
seed48 ();
function
also initializes the internal buffer r(n) of
drand48 (,);
lrand48 (,);
and
mrand48 (,);
but here all 48 bits of the seed can be specified in an array of 3 shorts,
where the zeroth member specifies the lowest bits.
Again,
the constant multiplicand and addend of the algorithm are
reset to the default values given above.
The
seed48 ();
function
returns a pointer to an array of 3 shorts which contains the old seed.
This array is statically allocated, thus its contents are lost after
each new call to
seed48 (.);
Finally,
lcong48 ();
allows full control over the multiplicand and addend used in
drand48 (,);
erand48 (,);
lrand48 (,);
nrand48 (,);
mrand48 (,);
and
jrand48 (,);
and the seed used in
drand48 (,);
lrand48 (,);
and
mrand48 (.);
An array of 7 shorts is passed as argument; the first three shorts are
used to initialize the seed; the second three are used to initialize the
multiplicand; and the last short is used to initialize the addend.
It is thus not possible to use values greater than 0xffff as the addend.
Note that all three methods of seeding the random number generator always also set the multiplicand and addend for any of the six generator calls.
For a more powerful random number generator, see random(3).
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