Re: Multi-channel rand()

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From	Keith Thompson <kst-u@mib.org>
Newsgroups	comp.lang.c
Subject	Re: Multi-channel rand()
Date	Thu, 27 Dec 2018 12:47:44 -0800
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James Kuyper <jameskuyper@alumni.caltech.edu> writes:
> On Thursday, December 27, 2018 at 2:29:01 PM UTC-5, Keith Thompson wrote:
>> James Kuyper <jameskuyper@alumni.caltech.edu> writes:
>> > On Thursday, December 27, 2018 at 9:46:42 AM UTC-5, Bart wrote:
>> [...]
>> >> OK (it sounds like random() doesn't support independent sequences).
>> >
>> > It does, through the initstate() and setstate() functions.
>> 
>> Yes, but you'd have to save and restore the entire state on each
>> call, or at least each time you want to switch to a different
>> sequence.
>
> Was the example code I gave to Rick coded incorrectly? This is a feature
> I've never personally used, so I may have made a mistake, but that code
> did not save or restore any state, because the interface doesn't seem to
> require doing so.

I was wrong.  The initstate() function returns a pointer to the state
(as a char*), and setstate() returns a pointer to the previous state.

I'm going to assume your example code was probably correct, but I didn't
save a copy.

So if you want multiple streams using random(), you could do something
like:

    setstate(pointer_to_state_for_this_thread);
    long int r = random();

But if you want a distinct stream for each thread, you'd have to use
some kind of synchronization to ensure that there's no context switch
between the setstate() and random() calls.

Since I misunderstood the state management the last time I read the man
page, it's entirely possible that I've misunderstood it again in some
new and embarrassing way.

-- 
Keith Thompson (The_Other_Keith) kst@mib.org  <http://www.ghoti.net/~kst>
Will write code for food.
"We must do something.  This is something.  Therefore, we must do this."
    -- Antony Jay and Jonathan Lynn, "Yes Minister"

Thread

Multi-channel rand() "Rick C. Hodgin" <rick.c.hodgin@gmail.com> - 2018-12-24 19:54 -0500
  Re: Multi-channel rand() "Chris M. Thomasson" <invalid_chris_thomasson@invalid.invalid> - 2018-12-24 19:38 -0800
  Re: Multi-channel rand() nobody@example.org (Scott) - 2018-12-25 04:05 +0000
    Re: Multi-channel rand() "Rick C. Hodgin" <rick.c.hodgin@gmail.com> - 2018-12-24 23:57 -0500
      Re: Multi-channel rand() Mr Flibble <flibbleREMOVETHISBIT@i42.co.uk> - 2018-12-25 09:22 +0000
        Professional coders??? (Was: Multi-channel rand()) gazelle@shell.xmission.com (Kenny McCormack) - 2018-12-25 10:09 +0000
        Re: Multi-channel rand() "Rick C. Hodgin" <rick.c.hodgin@gmail.com> - 2018-12-25 07:59 -0500
          Re: Multi-channel rand() Mr Flibble <flibbleREMOVETHISBIT@i42.co.uk> - 2018-12-25 21:59 +0000
            Re: Multi-channel rand() nobody@example.org (Scott) - 2018-12-26 02:43 +0000
      Re: Multi-channel rand() rbowman <bowman@montana.com> - 2018-12-25 10:15 -0700
        Re: Multi-channel rand() Keith Thompson <kst-u@mib.org> - 2018-12-25 10:14 -0800
  Re: Multi-channel rand() Anton Shepelev <anton.txt@g{oogle}mail.com> - 2018-12-25 13:35 +0300
    Re: Multi-channel rand() fir <profesor.fir@gmail.com> - 2018-12-25 02:56 -0800
    Re: Multi-channel rand() "Rick C. Hodgin" <rick.c.hodgin@gmail.com> - 2018-12-25 08:02 -0500
  Re: Multi-channel rand() Bart <bc@freeuk.com> - 2018-12-25 11:52 +0000
    Re: Multi-channel rand() Anton Shepelev <anton.txt@g{oogle}mail.com> - 2018-12-25 14:58 +0300
    Re: Multi-channel rand() Richard Damon <Richard@Damon-Family.org> - 2018-12-25 06:59 -0500
      Re: Multi-channel rand() "Rick C. Hodgin" <rick.c.hodgin@gmail.com> - 2018-12-25 08:05 -0500
    Re: Multi-channel rand() "Rick C. Hodgin" <rick.c.hodgin@gmail.com> - 2018-12-25 08:04 -0500
  Re: Multi-channel rand() Bart <bc@freeuk.com> - 2018-12-26 15:48 +0000
    Re: Multi-channel rand() "Rick C. Hodgin" <rick.c.hodgin@gmail.com> - 2018-12-26 07:58 -0800
      Re: Multi-channel rand() Richard Damon <Richard@Damon-Family.org> - 2018-12-26 12:06 -0500
        Re: Multi-channel rand() "Rick C. Hodgin" <rick.c.hodgin@gmail.com> - 2018-12-26 12:45 -0500
          Re: Multi-channel rand() Richard Damon <Richard@Damon-Family.org> - 2018-12-26 13:20 -0500
            Re: Multi-channel rand() "Rick C. Hodgin" <rick.c.hodgin@gmail.com> - 2018-12-26 10:30 -0800
              Re: Multi-channel rand() "Chris M. Thomasson" <invalid_chris_thomasson@invalid.invalid> - 2018-12-26 11:35 -0800
                Re: Multi-channel rand() "Rick C. Hodgin" <rick.c.hodgin@gmail.com> - 2018-12-26 14:52 -0500
              Re: Multi-channel rand() Richard Damon <Richard@Damon-Family.org> - 2018-12-26 16:31 -0500
                Re: Multi-channel rand() "Rick C. Hodgin" <rick.c.hodgin@gmail.com> - 2018-12-26 22:28 -0500
                Re: Multi-channel rand() "Chris M. Thomasson" <invalid_chris_thomasson@invalid.invalid> - 2018-12-30 20:42 -0800
            Re: Multi-channel rand() Keith Thompson <kst-u@mib.org> - 2018-12-26 15:32 -0800
          Re: Multi-channel rand() Bart <bc@freeuk.com> - 2018-12-26 19:38 +0000
            Re: Multi-channel rand() Keith Thompson <kst-u@mib.org> - 2018-12-26 15:35 -0800
        Re: Multi-channel rand() "Rick C. Hodgin" <rick.c.hodgin@gmail.com> - 2018-12-26 12:59 -0500
          Re: Multi-channel rand() Richard Damon <Richard@Damon-Family.org> - 2018-12-26 16:05 -0500
        Re: Multi-channel rand() "Rick C. Hodgin" <rick.c.hodgin@gmail.com> - 2018-12-26 13:08 -0500
          Re: Multi-channel rand() Richard Damon <Richard@Damon-Family.org> - 2018-12-26 15:50 -0500
      Re: Multi-channel rand() Bart <bc@freeuk.com> - 2018-12-26 17:42 +0000
        Re: Multi-channel rand() "Rick C. Hodgin" <rick.c.hodgin@gmail.com> - 2018-12-26 12:48 -0500
        Re: Multi-channel rand() Richard Damon <Richard@Damon-Family.org> - 2018-12-26 12:58 -0500
          Re: Multi-channel rand() Bart <bc@freeuk.com> - 2018-12-26 19:03 +0000
            Re: Multi-channel rand() "Rick C. Hodgin" <rick.c.hodgin@gmail.com> - 2018-12-26 14:45 -0500
              Re: Multi-channel rand() Bart <bc@freeuk.com> - 2018-12-26 20:46 +0000
                Re: Multi-channel rand() "Rick C. Hodgin" <rick.c.hodgin@gmail.com> - 2018-12-26 16:25 -0500
                Re: Multi-channel rand() Richard Damon <Richard@Damon-Family.org> - 2018-12-26 17:13 -0500
                Re: Multi-channel rand() "Rick C. Hodgin" <rick.c.hodgin@gmail.com> - 2018-12-26 22:23 -0500
                Re: Multi-channel rand() Bart <bc@freeuk.com> - 2018-12-26 23:39 +0000
                Re: Multi-channel rand() Richard Damon <Richard@Damon-Family.org> - 2018-12-26 17:36 -0500
            Re: Multi-channel rand() Richard Damon <Richard@Damon-Family.org> - 2018-12-26 16:27 -0500
    Re: Multi-channel rand() rbowman <bowman@montana.com> - 2018-12-26 19:19 -0700
      Re: Multi-channel rand() Richard Damon <Richard@Damon-Family.org> - 2018-12-26 22:17 -0500
      Re: Multi-channel rand() James Kuyper <jameskuyper@alumni.caltech.edu> - 2018-12-26 19:32 -0800
      Re: Multi-channel rand() Keith Thompson <kst-u@mib.org> - 2018-12-26 19:34 -0800
        Re: Multi-channel rand() Bart <bc@freeuk.com> - 2018-12-27 11:29 +0000
          Re: Multi-channel rand() James Kuyper <jameskuyper@alumni.caltech.edu> - 2018-12-27 05:27 -0800
            Re: Multi-channel rand() "Rick C. Hodgin" <rick.c.hodgin@gmail.com> - 2018-12-27 09:41 -0500
              Re: Multi-channel rand() "Rick C. Hodgin" <rick.c.hodgin@gmail.com> - 2018-12-27 09:42 -0500
                Re: Multi-channel rand() James Kuyper <jameskuyper@alumni.caltech.edu> - 2018-12-27 07:01 -0800
                Re: Multi-channel rand() "Rick C. Hodgin" <rick.c.hodgin@gmail.com> - 2018-12-27 10:28 -0500
                Re: Multi-channel rand() Richard Damon <Richard@Damon-Family.org> - 2018-12-27 10:48 -0500
                Re: Multi-channel rand() "Rick C. Hodgin" <rick.c.hodgin@gmail.com> - 2018-12-27 11:08 -0500
                Re: Multi-channel rand() "Chris M. Thomasson" <invalid_chris_thomasson@invalid.invalid> - 2018-12-27 13:49 -0800
                Re: Multi-channel rand() "Rick C. Hodgin" <rick.c.hodgin@gmail.com> - 2018-12-27 17:18 -0500
                Re: Multi-channel rand() "Chris M. Thomasson" <invalid_chris_thomasson@invalid.invalid> - 2018-12-29 13:59 -0800
                Re: Multi-channel rand() "Chris M. Thomasson" <invalid_chris_thomasson@invalid.invalid> - 2018-12-30 14:01 -0800
                Re: Multi-channel rand() Bart <bc@freeuk.com> - 2018-12-30 22:07 +0000
                Re: Multi-channel rand() "Rick C. Hodgin" <rick.c.hodgin@gmail.com> - 2018-12-30 17:20 -0500
                Re: Multi-channel rand() Richard Damon <Richard@Damon-Family.org> - 2018-12-27 17:47 -0500
                Re: Multi-channel rand() "Chris M. Thomasson" <invalid_chris_thomasson@invalid.invalid> - 2018-12-30 20:45 -0800
                Re: Multi-channel rand() James Kuyper <jameskuyper@alumni.caltech.edu> - 2018-12-27 08:22 -0800
                Re: Multi-channel rand() "Rick C. Hodgin" <rick.c.hodgin@gmail.com> - 2018-12-27 11:40 -0500
                Re: Multi-channel rand() James Kuyper <jameskuyper@alumni.caltech.edu> - 2018-12-27 09:21 -0800
                Re: Multi-channel rand() "Rick C. Hodgin" <rick.c.hodgin@gmail.com> - 2018-12-27 12:45 -0500
                Re: Multi-channel rand() James Kuyper <jameskuyper@alumni.caltech.edu> - 2018-12-27 10:57 -0800
                Re: Multi-channel rand() "Rick C. Hodgin" <rick.c.hodgin@gmail.com> - 2018-12-27 14:35 -0500
                Re: Multi-channel rand() Tim Rentsch <txr@alumni.caltech.edu> - 2018-12-28 09:25 -0800
                Re: Multi-channel rand() Tim Rentsch <txr@alumni.caltech.edu> - 2018-12-28 09:55 -0800
                Re: Multi-channel rand() Keith Thompson <kst-u@mib.org> - 2018-12-28 12:36 -0800
                Re: Multi-channel rand() Robert Wessel <robertwessel2@yahoo.com> - 2018-12-27 20:34 -0600
              Re: Multi-channel rand() Richard Damon <Richard@Damon-Family.org> - 2018-12-27 10:37 -0500
            Re: Multi-channel rand() Bart <bc@freeuk.com> - 2018-12-27 14:46 +0000
              Re: Multi-channel rand() James Kuyper <jameskuyper@alumni.caltech.edu> - 2018-12-27 07:05 -0800
                Re: Multi-channel rand() Keith Thompson <kst-u@mib.org> - 2018-12-27 11:28 -0800
                Re: Multi-channel rand() James Kuyper <jameskuyper@alumni.caltech.edu> - 2018-12-27 12:07 -0800
                Re: Multi-channel rand() Keith Thompson <kst-u@mib.org> - 2018-12-27 12:47 -0800
                Re: Multi-channel rand() James Kuyper <jameskuyper@alumni.caltech.edu> - 2018-12-27 14:47 -0800
  Re: Multi-channel rand() "Chris M. Thomasson" <invalid_chris_thomasson@invalid.invalid> - 2018-12-26 11:58 -0800

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