bionic (3) initstate.3posix.gz

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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(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 integers to return successive pseudo-random numbers in the range from
       0  to  231−1.  The  period of this random-number generator is approximately 16 x (231−1). 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 269.

       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-number 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
       setstate() 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, followed 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 POSIX.1‐2008.

       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 multiple threads is required.


RATIONALE
       None.


FUTURE DIRECTIONS
       None.


SEE ALSO
       drand48(), rand()

       The Base Definitions volume of POSIX.1‐2008, <stdlib.h>


       Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1, 2013 Edition,
       Standard  for  Information Technology -- Portable Operating System Interface (POSIX), The Open Group Base
       Specifications Issue 7, Copyright (C) 2013 by the Institute of Electrical and Electronics Engineers,  Inc
       and  The  Open Group.  (This is POSIX.1-2008 with the 2013 Technical Corrigendum 1 applied.) 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.unix.org/online.html .

       Any typographical or formatting errors that appear in this page are most likely to have  been  introduced
       during   the   conversion  of  the  source  files  to  man  page  format.  To  report  such  errors,  see
       https://www.kernel.org/doc/man-pages/reporting_bugs.html .