Description

   The drand48 function generates pseudorandom numbers using the
   linear congruential algorithm and 48-bit integer arithmetic.

   It returns nonnegative, double-precision, floating-point values
   uniformly distributed over the range of y values such that 0.0    y < 1.0.

   Before you call drand48, use either srand48, seed48, or lcong48
   to initialize the random-number generator. You must initialize
   prior to invoking the drand48 function because it stores the last
   48-bit Xi generated into an internal buffer. (Although it is not
   recommended, constant default initializer values are supplied
   automatically if the drand48, lrand48, or mrand48 functions are
   called without first calling an initialization function.)

   The drand48 function works 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 argument m equals 248, so 48-bit integer arithmetic is

   performed. Unless you invoke lcong48, the multiplier value a
   and the addend value c are:

        a = 5DEECE66D16 = 2736731631558

        c = B16 = 138

   The values returned by drand48 are computed by first generating
   the next 48-bit Xi in the sequence. Then the appropriate bits,
   according to the type of returned data item, are copied from the
   high-order (most significant) bits of Xi and transformed into the
   returned value.

   See also srand48, seed48, lcong48, lrand48, and mrand48.