drand48(3C)         (C Programming Language Utilities)          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.





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   drand48(3C)         (C Programming Language Utilities)          drand48(3C)


         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.

         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


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   drand48(3C)         (C Programming Language Utilities)          drand48(3C)


         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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