drand48, erand48, lrand48, nrand48, mrand48, jrand48,
srand48, seed48,
lcong48  pseudorandom number generators and initialization
routines
#include <stdlib.h>
double
drand48(void);
double
erand48(unsigned short xseed[3]);
long
lrand48(void);
long
nrand48(unsigned short xseed[3]);
long
mrand48(void);
long
jrand48(unsigned short xseed[3]);
void
srand48(long seed);
unsigned short *
seed48(unsigned short xseed[3]);
void
lcong48(unsigned short p[7]);
The rand48() family of functions generates pseudorandom
numbers using a
linear congruential algorithm working on integers 48 bits in
size. The
particular formula employed is r(n+1) = (a * r(n) + c) mod m
where the
default values are for the multiplicand a = 0xfdeece66d =
25214903917 and
the addend c = 0xb = 11. The modulus is always fixed at m =
2 ** 48.
r(n) is called the seed of the random number generator.
For all the six generator routines described next, the first
computational
step is to perform a single iteration of the algorithm.
drand48() and erand48() 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].
lrand48() and nrand48() return values of type long in the
range [0,
2**311]. The highorder (31) bits of r(n+1) are loaded into the lower
bits of the returned value, with the topmost (sign) bit set
to zero.
mrand48() and jrand48() return values of type long in the
range [2**31,
2**311]. The highorder (32) bits of r(n+1) are loaded into the returned
value.
drand48(), lrand48(), and mrand48() 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
usersupplied
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.
srand48() 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.
seed48() 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. seed48() returns a pointer
to an array of 3 shorts which contains the old seed. This
array is statically
allocated, so 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 parameter; 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).
arc4random(3), rand(3), random(3)
Martin Birgmeier
OpenBSD 3.6 October 8, 1993
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