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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  algo-
       rithm and 48-bit integer arithmetic.

       The  drand48()  and erand48() functions return non-negative double-precision float-
       ing-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 uniformly dis-
       tributed 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 func-
       tion 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 argu-
       ment 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  speci-
       fied  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 argument 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 information about reporting bugs, can be found at http://www.ker-
       nel.org/doc/man-pages/.



                                  2007-07-26                        DRAND48(3)

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