drand48(3C)                      DG/UX 5.4.2                     drand48(3C)


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,
       Xi, according to the linear congruential formula

                                 Xn+1=(aXn+c)modmn>0.

       The parameter m=2^48 (2 to the 48th power); hence 48-bit integer
       arithmetic is performed.  Unless lcong48 has been invoked, the



Licensed material--property of copyright holder(s)                         1

drand48(3C)                      DG/UX 5.4.2                     drand48(3C)


       multiplier value a and the addend value c are given by

             a = 5DEECE66D base 16 = 273673163155 base 8
             c = B base 16 = 13 base 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 Xi in the sequence.  Then the appropriate number of bits,
       according to the type of
       data item to be returned, are copied from the high-order (leftmost) 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.  Xi must be initialized prior to being
       invoked.  The functions erand48, nrand48, and jrand48 require the
       calling program to provide storage for the successive Xi values in the
       array specified as an argument when the functions are invoked.  These
       routines do not have to be initialized; the calling program must place
       the desired initial value of Xi into the array and pass it as an
       argument.  By using different 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 Xi to
       the 32 bits contained in its argument.  The low-order 16 bits of Xi are
       set to the arbitrary value 330E base 16.

       The initializer function seed48 sets the value of Xi to the 48-bit value
       specified in the argument array.  In addition, the previous value of Xi
       is copied into a 48-bit internal buffer, used only by seed48, 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 Xi value, and then use this value
       to reinitialize via seed48 when the program is restarted.

       The initialization function lcong48 allows the user to specify the
       initial Xi, the multiplier value a, and the addend value c.  Argument
       array elements param[0-2] specify Xi, 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).









Licensed material--property of copyright holder(s)                         2