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INITSTATE(3P)              POSIX Programmer's Manual             INITSTATE(3P)

PROLOG
       This  manual  page is part of the POSIX Programmer's Manual.  The Linux
       implementation of this interface may differ (consult the  corresponding
       Linux  manual page for details of Linux behavior), or the interface may
       not be implemented on Linux.
NAME
       initstate, random, setstate, srandom - pseudo-random number functions
SYNOPSIS
       #include <stdlib.h>
       char *initstate(unsigned seed, char *state, size_t size);
       long random(void);
       char *setstate(const char *state);
       void srandom(unsigned seed);

DESCRIPTION
       The random() function shall use a non-linear additive feedback  random-
       number  generator employing a default state array size of 31 long inte-
       gers to return successive pseudo-random numbers in the range from 0  to
       2**31-1.   The  period of this random-number generator is approximately
       16 x (2**31-1).  The size of the state array determines the  period  of
       the  random-number  generator.  Increasing  the  state array size shall
       increase the period.
       With 256 bytes of state information, the period  of  the  random-number
       generator shall be greater than 2**69.
       Like  rand(),  random()  shall produce by default a sequence of numbers
       that can be duplicated by calling srandom() with 1 as the seed.
       The srandom() function shall initialize the current state  array  using
       the value of seed.
       The initstate() and setstate() functions handle restarting and changing
       random-number generators.  The  initstate()  function  allows  a  state
       array,  pointed  to by the state argument, to be initialized for future
       use. The size argument, which specifies the size in bytes of the  state
       array,  shall be used by initstate() to decide what type of random-num-
       ber generator to use; the larger the state array, the more  random  the
       numbers. Values for the amount of state information are 8, 32, 64, 128,
       and 256 bytes. Other values greater than 8 bytes are  rounded  down  to
       the  nearest  one  of  these  values.  If  initstate()  is  called with
       8<=size<32, then random() shall use a simple linear congruential random
       number  generator. The seed argument specifies a starting point for the
       random-number sequence and provides for restarting at the  same  point.
       The  initstate()  function shall return a pointer to the previous state
       information array.
       If initstate() has not been  called,  then  random()  shall  behave  as
       though initstate() had been called with seed=1 and size=128.
       Once  a state has been initialized, setstate() allows switching between
       state arrays. The array defined by the state argument shall be used for
       further  random-number  generation  until initstate() is called or set-
       state() is called again. The setstate() function shall return a pointer
       to the previous state array.
RETURN VALUE
       If initstate() is called with size less than 8, it shall return NULL.
       The random() function shall return the generated pseudo-random number.
       The srandom() function shall not return a value.
       Upon  successful  completion, initstate() and setstate() shall return a
       pointer to the previous state array; otherwise, a null pointer shall be
       returned.
ERRORS
       No errors are defined.
       The following sections are informative.
EXAMPLES
       None.
APPLICATION USAGE
       After  initialization,  a  state  array can be restarted at a different
       point in one of two ways:
        1. The initstate() function can be used, with the desired seed,  state
           array, and size of the array.
        2. The  setstate() function, with the desired state, can be used, fol-
           lowed by srandom() with the desired seed. The  advantage  of  using
           both  of  these  functions is that the size of the state array does
           not have to be saved once it is initialized.
       Although some implementations of  random()  have  written  messages  to
       standard    error,    such    implementations   do   not   conform   to
       IEEE Std 1003.1-2001.
       Issue 5 restored the historical behavior of this function.
       Threaded applications should use  erand48(),  nrand48(),  or  jrand48()
       instead  of random() when an independent random number sequence in mul-
       tiple threads is required.
RATIONALE
       None.
FUTURE DIRECTIONS
       None.
SEE ALSO
       drand48(), rand(), the Base Definitions volume of IEEE Std 1003.1-2001,
       <stdlib.h>
COPYRIGHT
       Portions  of  this text are reprinted and reproduced in electronic form
       from IEEE Std 1003.1, 2003 Edition, Standard for Information Technology
       --  Portable  Operating  System  Interface (POSIX), The Open Group Base
       Specifications Issue 6, Copyright (C) 2001-2003  by  the  Institute  of
       Electrical  and  Electronics  Engineers, Inc and The Open Group. In the
       event of any discrepancy between this version and the original IEEE and
       The  Open Group Standard, the original IEEE and The Open Group Standard
       is the referee document. The original Standard can be  obtained  online
       at http://www.opengroup.org/unix/online.html .

IEEE/The Open Group                  2003                        INITSTATE(3P)