drand48(3C)            UNIX System V(C Development Set)             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)mod m        n>0.




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drand48(3C)            UNIX System V(C Development Set)             drand48(3C)


      The parameter m=248; hence 48-bit integer arithmetic is performed.
      Unless lcong48 has been invoked, the multiplier value a and the addend
      value c are given by

            a = 5DEECE66D16 = 2736731631558
            c = B16 = 138.

      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 330E16.

      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).




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