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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 <&lt;stdlib.h>&gt;

       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]);

   Feature Test Macro Requirements for glibc (see feature_test_macros(7)):

       All functions shown above: _SVID_SOURCE || _XOPEN_SOURCE

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 >&gt;= 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
       SVr4, POSIX.1-2001.

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

SEE ALSO
       rand(3), random(3)

COLOPHON
       This  page  is  part of release 3.05 of the Linux man-pages project.  A
       description of the project, and information about reporting  bugs,  can
       be found at http://www.kernel.org/doc/man-pages/.



                                  2007-07-26                        DRAND48(3)