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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 dif-
       fer (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 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 deter-
       mines 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  sran-
       dom() 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 init-
       state() 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; oth-
       erwise, 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 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, Stan-
       dard for Information Technology -- Portable Operating System Interface (POSIX), The Open Group Base  Specifica-
       tions  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 Stan-
       dard,  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)