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



PROLOG
       This manual page is part of the POSIX Programmer's Manual.  The OS
       implementation of this interface may differ (consult the corresponding
       OS manual page for details of OS behavior), or the interface may not be
       implemented on OS.

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.

       These functions should be avoided whenever non-trivial requirements
       (including safety) have to be fulfilled.

RATIONALE
       None.

FUTURE DIRECTIONS
       None.

SEE ALSO
       drand48(), rand()

       The Base Definitions volume of POSIX.1-2017, <stdlib.h>

COPYRIGHT
       Portions of this text are reprinted and reproduced in electronic form
       from IEEE Std 1003.1-2017, Standard for Information Technology --
       Portable Operating System Interface (POSIX), The Open Group Base
       Specifications Issue 7, 2018 Edition, Copyright (C) 2018 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 .

       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 .



IEEE/The Open Group                  2017                         INITSTATE(3)
