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 (con-
       sult 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 integers 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 ran-
       dom-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() func-
       tion 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 genera-
       tor 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 argu-
       ment 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 advan-
           tage  of  using both of these functions is that the size of the state array does not have to be saved once it is ini-
           tialized.

       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 multiple 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, Copy-
       right (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.open-
       group.org/unix/online.html .



IEEE/The Open Group                                           2003                                                 INITSTATE(3P)