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]);

DESCRIPTION

This family of functions generates pseudo-random numbers using the well-known linear congruential algorithm and 48-bit integer arithmetic.

Functions drand48 and erand48 return non-negative double-precision floating-point values uniformly distributed over the interval [0.0, 1.0).

Functions lrand48 and nrand48 return non-negative long integers uniformly distributed over the interval [0, 231).

Functions mrand48 and jrand48 return signed long integers uniformly distributed over the interval [-231, 231).

Functions srand48, seed48, and lcong48 are initialization entry points, one of which should be invoked before either drand48, lrand48, or mrand48 is called. (Although it is not recommended practice, constant default initializer values will be supplied automatically if drand48, lrand48, or mrand48 is called without a prior call to an initialization entry point.) Functions erand48, nrand48, and jrand48 do not require an initialization entry point to be called first.

All the routines work by generating a sequence of 48-bit integer values, X , according to the linear congruential formula i

              X    = (aX +c)             n>0.
               n+1      n   mod m
The parameter m=248; hence 48-bit integer arithmetic is per-
formed.   Unless  lcong48  has  been invoked, the multiplier
value a and the addend value c are given by

      a = 5DEECE66D   = 273673163155
      c = B   = 13 .6               8
           16     8
The value returned by any of the functions drand48, erand48,
lrand48,  nrand48,  mrand48, or jrand48 is computed by first
generating the next 48-bit X  in  the  sequence.   Then  the
appropriate  number  of  biti,  according  to  the  type  of
data item to be returned, are  copied  from  the  high-order
(leftmost)  bits  of  X   and  transformed into the returned
value.                 i

The functions drand48, lrand48, and mrand48 store the last 48-bit X generated in an internal buffer. X must be initialized irior to being invoked. The functions erand48, nrand48, and jrand48 require the calling program to provide storage for the successive X values in the array specified as an argument when the fuictions are invoked. These routines do not have to be initialized; the calling program must place the desired initial value of X into the array and pass it as an argument. By using diffeient arguments, functions erand48, nrand48, and jrand48 allow separate modules of a large program to generate several independent streams of pseudo-random numbers, i.e., the sequence of numbers in each stream will not depend upon how many times the routines have been called to generate numbers for the other streams.

The initializer function srand48 sets the high-order 32 bits
of  X   to  the 32 bits contained in its argument.  The low-
orderi16 bits of X  are set to the arbitrary value 330E  .
                  i                                    16
The initializer function seed48 sets the value of X  to  the
48-bit  value specified in the argument array.  In iddition,
the previous value of X  is copied into  a  48-bit  internal
buffer, used only by seid48, and a pointer to this buffer is
the value returned by seed48.  This returned pointer,  which
can just be ignored if not needed, is useful if a program is
to be restarted from a given point at some future time - use
the  pointer to get at and store the last X  value, and then
use this value to reinitialize via seed48 wien  the  program
is restarted.

The initialization function lcong48 allows the user to specify the initial X , the multiplier value a, and the addend value c. Argumeni array elements param[0-2] specify X , param[3-5] specify the multiplier a, and param[6] specifies the 16-bit addend c. After lcong48 has been called, a subsequent call to either srand48 or seed48 will restore the ``standard'' multiplier and addend values, a and c, specified on the previous page.

SEE ALSO

rand(3C).