1INITSTATE(3P)              POSIX Programmer's Manual             INITSTATE(3P)
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PROLOG

6       This  manual  page is part of the POSIX Programmer's Manual.  The Linux
7       implementation of this interface may differ (consult the  corresponding
8       Linux  manual page for details of Linux behavior), or the interface may
9       not be implemented on Linux.
10

NAME

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

15       #include <stdlib.h>
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17       char *initstate(unsigned seed, char *state, size_t size);
18       long random(void);
19       char *setstate(char *state);
20       void srandom(unsigned seed);
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DESCRIPTION

23       The random() function shall use a non-linear additive feedback  random-
24       number  generator employing a default state array size of 31 long inte‐
25       gers to return successive pseudo-random numbers in the range from 0  to
26       231-1. The period of this random-number generator is approximately 16 x
27       (231-1). The size of the state array determines the period of the  ran‐
28       dom-number  generator.  Increasing  the state array size shall increase
29       the period.
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31       With 256 bytes of state information, the period  of  the  random-number
32       generator shall be greater than 269.
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34       Like  rand(),  random()  shall produce by default a sequence of numbers
35       that can be duplicated by calling srandom() with 1 as the seed.
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37       The srandom() function shall initialize the current state  array  using
38       the value of seed.
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40       The initstate() and setstate() functions handle restarting and changing
41       random-number generators.  The  initstate()  function  allows  a  state
42       array,  pointed  to by the state argument, to be initialized for future
43       use. The size argument, which specifies the size in bytes of the  state
44       array,  shall be used by initstate() to decide what type of random-num‐
45       ber generator to use; the larger the state array, the more  random  the
46       numbers. Values for the amount of state information are 8, 32, 64, 128,
47       and 256 bytes. Other values greater than 8 bytes are  rounded  down  to
48       the  nearest  one  of  these  values.  If  initstate()  is  called with
49       8≤size<32, then random() shall use a simple linear congruential  random
50       number  generator. The seed argument specifies a starting point for the
51       random-number sequence and provides for restarting at the  same  point.
52       The  initstate()  function shall return a pointer to the previous state
53       information array.
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55       If initstate() has not been  called,  then  random()  shall  behave  as
56       though initstate() had been called with seed=1 and size=128.
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58       Once  a state has been initialized, setstate() allows switching between
59       state arrays. The array defined by the state argument shall be used for
60       further  random-number  generation  until initstate() is called or set‐
61       state() is called again. The setstate() function shall return a pointer
62       to the previous state array.
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RETURN VALUE

65       If initstate() is called with size less than 8, it shall return NULL.
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67       The random() function shall return the generated pseudo-random number.
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69       The srandom() function shall not return a value.
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71       Upon  successful  completion, initstate() and setstate() shall return a
72       pointer to the previous state array; otherwise, a null pointer shall be
73       returned.
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ERRORS

76       No errors are defined.
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78       The following sections are informative.
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EXAMPLES

81       None.
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APPLICATION USAGE

84       After  initialization,  a  state  array can be restarted at a different
85       point in one of two ways:
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87        1. The initstate() function can be used, with the desired seed,  state
88           array, and size of the array.
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90        2. The  setstate() function, with the desired state, can be used, fol‐
91           lowed by srandom() with the desired seed. The  advantage  of  using
92           both  of  these  functions is that the size of the state array does
93           not have to be saved once it is initialized.
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95       Although some implementations of  random()  have  written  messages  to
96       standard error, such implementations do not conform to POSIX.1‐2008.
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98       Issue 5 restored the historical behavior of this function.
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100       Threaded  applications  should  use  erand48(), nrand48(), or jrand48()
101       instead of random() when an independent random number sequence in  mul‐
102       tiple threads is required.
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104       These  functions  should  be  avoided whenever non-trivial requirements
105       (including safety) have to be fulfilled.
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RATIONALE

108       None.
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FUTURE DIRECTIONS

111       None.
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SEE ALSO

114       drand48(), rand()
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116       The Base Definitions volume of POSIX.1‐2017, <stdlib.h>
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119       Portions of this text are reprinted and reproduced in  electronic  form
120       from  IEEE Std 1003.1-2017, Standard for Information Technology -- Por‐
121       table Operating System Interface (POSIX), The Open Group Base  Specifi‐
122       cations  Issue  7, 2018 Edition, Copyright (C) 2018 by the Institute of
123       Electrical and Electronics Engineers, Inc and The Open Group.   In  the
124       event of any discrepancy between this version and the original IEEE and
125       The Open Group Standard, the original IEEE and The Open Group  Standard
126       is  the  referee document. The original Standard can be obtained online
127       at http://www.opengroup.org/unix/online.html .
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129       Any typographical or formatting errors that appear  in  this  page  are
130       most likely to have been introduced during the conversion of the source
131       files to man page format. To report such errors,  see  https://www.ker
132       nel.org/doc/man-pages/reporting_bugs.html .
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136IEEE/The Open Group                  2017                        INITSTATE(3P)
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