Standard C Library Functions                          drand48(3C)



NAME
     drand48,  erand48,  lrand48,  nrand48,   mrand48,   jrand48,
     srand48,  seed48,  lcong48  - generate uniformly distributed
     pseudo-random numbers

SYNOPSIS
     #include 

     double drand48(void)


     double erand48(unsigned short x(i)[3]);


     long lrand48(void)


     long nrand48(unsigned short x(i)[3]);


     long mrand48(void)


     long jrand48(unsigned short x(i)[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, 2  ^31
     ].






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Standard C Library Functions                          drand48(3C)



     Functions  mrand48()  and  jrand48()  return   signed   long
     integers  uniformly distributed over the interval [-2 ^31  ,
     2 ^31  ].


     Functions srand48(), seed48(), and lcong48() are initializa-
     tion  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(i ), according to the linear congruential
     formula


     X(n]1)= (aX (n)]c)(mod m) n>=0.


     The parameter m = 2^48; hence 48-bit integer  arithmetic  is
     performed. Unless lcong48() has been invoked, the multiplier
     value aand the addend value care given by
       a = 5DECE66D(16) = 273673163155(8)
       c = B(16) = 13(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(i)  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 X(i) and transformed into the
     returned value.


     The functions drand48(), lrand48(), and mrand48() store  the
     last  48-bit X(i) generated in an internal buffer. X(i) must
     be  initialized  prior  to  being  invoked.  The   functions
     erand48(), nrand48(), and jrand48() require the calling pro-
     gram to provide storage for the successive  X(i)  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 X(i)
     into the array and pass it as an  argument.  By  using  dif-
     ferent   arguments,   functions  erand48(),  nrand48(),  and
     jrand48() allow separate modules of a large program to  gen-
     erate  several independent streams of pseudo-random numbers,



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Standard C Library Functions                          drand48(3C)



     that is, 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(i) to the 32 bits contained in its argument. The
     low-order 16 bits of X(i) are set  to  the  arbitrary  value
     330E(16) .


     The initializer function seed48() sets the value of X(i)  to
     the  48-bit  value specified in the argument array. In addi-
     tion, the previous value of X(i) 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
     X(i) value, and then use this value  to  reinitialize  using
     seed48() when the program is restarted.


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

ATRIBUTES
     See attributes(5) for descriptions of the  following  attri-
     butes:



     
           ATRIBUTE TYPE               ATRIBUTE VALUE       
    
     Interface Stability          Standard                    
    
     MT-Level                     Safe                        
    


SEE ALSO
     rand(3C), attributes(5), standards(5)






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