PROLOG | NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | EXAMPLES | APPLICATION USAGE | RATIONALE | FUTURE DIRECTIONS | SEE ALSO | COPYRIGHT

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

PROLOG         top

       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         top

       initstate, random, setstate, srandom — pseudo-random number functions

SYNOPSIS         top

       #include <stdlib.h>

       char *initstate(unsigned seed, char *state, size_t size);
       long random(void);
       char *setstate(char *state);
       void srandom(unsigned seed);

DESCRIPTION         top

       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         top

       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         top

       No errors are defined.

       The following sections are informative.

EXAMPLES         top

       None.

APPLICATION USAGE         top

       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         top

       None.

FUTURE DIRECTIONS         top

       None.

SEE ALSO         top

       drand48(3p), rand(3p)

       The Base Definitions volume of POSIX.1‐2008, stdlib.h(0p)

COPYRIGHT         top

       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 .

IEEE/The Open Group                 2013                       INITSTATE(3P)

Pages that refer to this page: stdlib.h(0p)gethostid(3p)random(3p)setstate(3p)srandom(3p)