NAME    [Toc]    [Back]

     drand48,  erand48,  lrand48,  nrand48,   mrand48,   jrand48,
srand48, seed48,
     lcong48 - pseudo-random number generators and initialization
routines

SYNOPSIS    [Toc]    [Back]

     #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]);

DESCRIPTION    [Toc]    [Back]

     The rand48() family  of  functions  generates  pseudo-random
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**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.

     mrand48() and jrand48() 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.

     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
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.

     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).

SEE ALSO    [Toc]    [Back]

     arc4random(3), rand(3), random(3)

AUTHORS    [Toc]    [Back]

     Martin Birgmeier

OpenBSD      3.6                          October     8,     1993