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>

COPYRIGHT

       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 .