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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 <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]);

   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  congruential  algorithm  and  48-bit  integer
       arithmetic.

       The drand48() and erand48() functions return non-negative double-precision floating-point values uniformly dis-
       tributed 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 uniformly 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  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, 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 initial values for Xi, a and c.  Array argu-
       ment elements param[0-2] specify 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 3.22 of the Linux man-pages project.  A description of the project, and informa-
       tion about reporting bugs, can be found at http://www.kernel.org/doc/man-pages/.



                                  2007-07-26                        DRAND48(3)