srand48(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: _XOPEN_SOURCE
       || /* Glibc since 2.19: */ _DEFAULT_SOURCE
       || /* Glibc versions <= 2.19: */ _SVID_SOURCE

DESCRIPTION

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

   The  drand48()  and  erand48()  functions  return  nonnegative  double-
   precision floating-point values uniformly distributed over the interval
   [0.0, 1.0).

   The  lrand48() and nrand48() functions return nonnegative long integers
   uniformly distributed over the interval [0, 2^31).

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

ATTRIBUTES

   For   an   explanation   of   the  terms  used  in  this  section,  see
   attributes(7).

   ┌──────────────────────┬───────────────┬────────────────────────┐
   │Interface             │ Attribute     │ Value                  │
   ├──────────────────────┼───────────────┼────────────────────────┤
   │drand48(), erand48(), │ Thread safety │ MT-Unsafe race:drand48 │
   │lrand48(), nrand48(), │               │                        │
   │mrand48(), jrand48(), │               │                        │
   │srand48(), seed48(),  │               │                        │
   │lcong48()             │               │                        │
   └──────────────────────┴───────────────┴────────────────────────┘
   The above functions record global  state  information  for  the  random
   number generator, so they are not thread-safe.

CONFORMING TO

   POSIX.1-2001, POSIX.1-2008, SVr4.

SEE ALSO

   rand(3), random(3)

COLOPHON

   This  page  is  part of release 4.09 of the Linux man-pages project.  A
   description of the project, information about reporting bugs,  and  the
   latest     version     of     this    page,    can    be    found    at
   https://www.kernel.org/doc/man-pages/.

                              2016-03-15                        DRAND48(3)

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