erand48

DRAND48(3)                 Linux Programmer's Manual                DRAND48(3)



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 int lrand48(void);

       long int nrand48(unsigned short xsubi[3]);

       long int mrand48(void);

       long int jrand48(unsigned short xsubi[3]);

       void srand48(long int seedval);

       unsigned short *seed48(unsigned short seed16v[3]);

       void lcong48(unsigned short param[7]);

DESCRIPTION
       These functions generate pseudo-random numbers using the linear congru-
       ential algorithm and 48-bit integer arithmetic.

       The drand48() and erand48() functions return non-negative double-preci-
       sion floating-point values uniformly distributed between [0.0, 1.0).

       The lrand48() and nrand48() functions return non-negative long integers
       uniformly distributed between 0 and 2^31.

       The  mrand48() and jrand48() functions return signed long integers uni-
       formly distributed between -2^31 and 2^31.

       The srand48(), seed48()  and  lcong48()  functions  are  initialization
       functions,  one  of  which  should  be  called  before using drand48(),
       lrand48()  or  mrand48().   The  functions  erand48(),  nrand48()   and
       jrand48() do not require an initialization function to be called first.

       All the functions work by generating a sequence of 48-bit integers, Xi,
       according to the linear congruential formula:

              Xn+1 = (aXn + c) mod m, where n >= 0

       The  parameter  m = 2^48, hence 48-bit integer arithmetic is performed.
       Unless lcong48() is called, a and c are given by:

              a = 0x5DEECE66D
              c = 0xB

       The value returned  by  any  of  the  functions  drand48(),  erand48(),
       lrand48(),  nrand48(), mrand48() or jrand48() is computed by first gen-
       erating the next 48-bit Xi in the sequence.  Then the appropriate  num-
       ber  of  bits,  according  to  the type of data item to be returned, is
       copied from the high-order 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.  The functions erand48(), nrand48()
       and  jrand48()  require  the calling program to provide storage for the
       successive Xi values in the array argument xsubi.   The  functions  are
       initialized  by  placing  the initial value of Xi into the array before
       calling the function for the first time.

       The initializer function srand48() sets the high order 32-bits of Xi to
       the  argument  seedval.  The low order 16-bits are set to the arbitrary
       value 0x330E.

       The initializer function seed48() sets the value of Xi  to  the  48-bit
       value  specified  in the array argument seed16v.  The previous value of
       Xi is copied into an internal buffer and a pointer to  this  buffer  is
       returned by seed48().

       The  initialization  function lcong48() allows the user to specify ini-
       tial values for Xi, a and c.  Array argument elements param[0-2]  spec-
       ify  Xi,  param[3-5]  specify  a,  and  param[6]  specifies  c.   After
       lcong48() has been called, a subsequent call  to  either  srand48()  or
       seed48() will restore the standard values of a and c.

CONFORMING TO
       SVID 3

NOTES
       These  functions  are  declared  obsolete  by SVID 3, which states that
       rand(3) should be used instead.

SEE ALSO
       rand(3), random(3)



                                  1993-07-02                        DRAND48(3)