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Re: Memory protection between compilation units?

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From Tim Rentsch <tr.17687@z991.linuxsc.com>
Newsgroups comp.lang.c
Subject Re: Memory protection between compilation units?
Date Tue, 01 Jul 2025 09:54:36 -0700
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Mateusz Viste <mateusz@not.gonna.tell> writes:

> On 14.06.2025 01:31, Tim Rentsch wrote:
>
>> It isn't wrong to think of bitwise-and as masking-in (or possibly
>> masking-out) of certain bits, but it still isn't a modulo.  A
>> modulo operation is what is desired;
>
> By "different viewpoints," I meant that while you approach the
> problem by applying a modulo operation to the index so it fits the
> array size, I tend to think in terms of ensuring the index
> correctly maps to a location within an n-bit address space.
> Naturally, the array should accommodate the maximum possible index
> for the given address space, and that?s where the original code
> fell short.  And you're absolutely right that hardcoded values are
> problematic, the size of the array should have been linked with
> the n-bits address space expectation.

I understand what you're doing.  However one thinks of it, what is
needed is a way to ensure the produced index value is in the range
of array index values, and that the mapping covers the full range of
array index values.  Using bitwise-and is a way of solving a less
general problem.  Unfortunately:  one, although it is known that
using bitwise-and works only for certain array sizes, there was no
check or assertion in the code to verify that requirement;  two,
it's a holdover from earlier times when the performance difference
might matter, but now it's a premature optimization (and in most
cases does not result in any improvement);  and three, in this case
using bitwise-and contributed to the bug, which wouldn't have
happened if modulo had been used instead.

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Memory protection between compilation units? Mateusz Viste <mateusz@x.invalid> - 2025-06-11 15:32 +0200
  Re: Memory protection between compilation units? Josef Möllers <josef@invalid.invalid> - 2025-06-11 16:06 +0200
    Re: Memory protection between compilation units? scott@slp53.sl.home (Scott Lurndal) - 2025-06-11 14:32 +0000
      Re: Memory protection between compilation units? Michael S <already5chosen@yahoo.com> - 2025-06-12 20:01 +0300
        Re: Memory protection between compilation units? Mateusz Viste <mateusz@x.invalid> - 2025-06-13 09:13 +0200
      Re: Memory protection between compilation units? Richard Heathfield <rjh@cpax.org.uk> - 2025-06-12 19:15 +0100
        Re: Memory protection between compilation units? Rosario19 <Ros@invalid.invalid> - 2025-06-16 18:14 +0200
          Re: Memory protection between compilation units? Richard Heathfield <rjh@cpax.org.uk> - 2025-06-16 17:53 +0100
  Re: Memory protection between compilation units? Lew Pitcher <lew.pitcher@digitalfreehold.ca> - 2025-06-11 14:30 +0000
  Re: Memory protection between compilation units? David Brown <david.brown@hesbynett.no> - 2025-06-11 17:14 +0200
    Re: Memory protection between compilation units? Mateusz Viste <mateusz@x.invalid> - 2025-06-12 14:31 +0200
      Re: Memory protection between compilation units? David Brown <david.brown@hesbynett.no> - 2025-06-12 15:29 +0200
        Re: Memory protection between compilation units? scott@slp53.sl.home (Scott Lurndal) - 2025-06-12 14:27 +0000
  Re: Memory protection between compilation units? Opus <ifonly@youknew.org> - 2025-06-11 17:19 +0200
    Re: Memory protection between compilation units? wij <wyniijj5@gmail.com> - 2025-06-11 23:38 +0800
    Re: Memory protection between compilation units? Mateusz Viste <mateusz@x.invalid> - 2025-06-12 14:41 +0200
      Re: Memory protection between compilation units? scott@slp53.sl.home (Scott Lurndal) - 2025-06-12 13:21 +0000
  Re: Memory protection between compilation units? Kaz Kylheku <643-408-1753@kylheku.com> - 2025-06-11 15:36 +0000
  Re: Memory protection between compilation units? Mateusz Viste <mateusz@x.invalid> - 2025-06-12 10:28 +0200
    Re: Memory protection between compilation units? scott@slp53.sl.home (Scott Lurndal) - 2025-06-12 13:18 +0000
    Re: Memory protection between compilation units? Kaz Kylheku <643-408-1753@kylheku.com> - 2025-06-12 18:59 +0000
      Re: Memory protection between compilation units? Mateusz Viste <mateusz@x.invalid> - 2025-06-13 08:42 +0200
        Re: Memory protection between compilation units? Kaz Kylheku <643-408-1753@kylheku.com> - 2025-06-13 16:17 +0000
      Re: Memory protection between compilation units? pozz <pozzugno@gmail.com> - 2025-06-13 09:21 +0200
        Re: Memory protection between compilation units? Mateusz Viste <mateusz@x.invalid> - 2025-06-13 14:14 +0200
          Re: Memory protection between compilation units? Michael S <already5chosen@yahoo.com> - 2025-06-13 16:56 +0300
            Re: Memory protection between compilation units? Richard Heathfield <rjh@cpax.org.uk> - 2025-06-13 15:43 +0100
              Re: Memory protection between compilation units? Michael S <already5chosen@yahoo.com> - 2025-06-14 22:07 +0300
            Re: Memory protection between compilation units? Kaz Kylheku <643-408-1753@kylheku.com> - 2025-06-13 17:14 +0000
            Re: Memory protection between compilation units? Mateusz Viste <mateusz@not.gonna.tell> - 2025-06-14 21:37 +0000
              Re: Memory protection between compilation units? antispam@fricas.org (Waldek Hebisch) - 2025-06-15 13:57 +0000
                Re: Memory protection between compilation units? Mateusz Viste <mateusz@not.gonna.tell> - 2025-06-15 20:27 +0000
                Re: Memory protection between compilation units? antispam@fricas.org (Waldek Hebisch) - 2025-06-15 23:50 +0000
                Re: Memory protection between compilation units? Kaz Kylheku <643-408-1753@kylheku.com> - 2025-06-16 01:01 +0000
                Re: Memory protection between compilation units? antispam@fricas.org (Waldek Hebisch) - 2025-06-16 10:00 +0000
                Re: Memory protection between compilation units? James Kuyper <jameskuyper@alumni.caltech.edu> - 2025-06-16 06:12 -0400
                Re: Memory protection between compilation units? Tim Rentsch <tr.17687@z991.linuxsc.com> - 2025-06-16 06:10 -0700
                Re: Memory protection between compilation units? antispam@fricas.org (Waldek Hebisch) - 2025-06-16 16:47 +0000
          Re: Memory protection between compilation units? Kaz Kylheku <643-408-1753@kylheku.com> - 2025-06-13 16:23 +0000
          Re: Memory protection between compilation units? wij <wyniijj5@gmail.com> - 2025-06-14 02:10 +0800
  Re: Memory protection between compilation units? Mikko <mikko.levanto@iki.fi> - 2025-06-12 11:40 +0300
    Re: Memory protection between compilation units? Mateusz Viste <mateusz@x.invalid> - 2025-06-12 11:05 +0200
  Re: Memory protection between compilation units? Tim Rentsch <tr.17687@z991.linuxsc.com> - 2025-06-12 06:05 -0700
    Re: Memory protection between compilation units? Bonita Montero <Bonita.Montero@gmail.com> - 2025-06-13 08:03 +0200
      Re: Memory protection between compilation units? wij <wyniijj5@gmail.com> - 2025-06-14 02:16 +0800
        Re: Memory protection between compilation units? Bonita Montero <Bonita.Montero@gmail.com> - 2025-06-13 20:43 +0200
        Re: Memory protection between compilation units? Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2025-06-13 12:32 -0700
          Re: Memory protection between compilation units? Tim Rentsch <tr.17687@z991.linuxsc.com> - 2025-06-13 15:48 -0700
    Re: Memory protection between compilation units? Mateusz Viste <mateusz@x.invalid> - 2025-06-13 08:59 +0200
      Re: Memory protection between compilation units? Tim Rentsch <tr.17687@z991.linuxsc.com> - 2025-06-13 16:31 -0700
        Re: Memory protection between compilation units? Mateusz Viste <mateusz@not.gonna.tell> - 2025-06-14 22:22 +0000
          Re: Memory protection between compilation units? Tim Rentsch <tr.17687@z991.linuxsc.com> - 2025-07-01 09:54 -0700
  Re: Memory protection between compilation units? Bonita Montero <Bonita.Montero@gmail.com> - 2025-06-13 08:00 +0200
    Re: Memory protection between compilation units? Mateusz Viste <mateusz@x.invalid> - 2025-06-13 08:47 +0200
      Re: Memory protection between compilation units? Kaz Kylheku <643-408-1753@kylheku.com> - 2025-06-13 16:19 +0000
  Re: Memory protection between compilation units? Louis Krupp <lkrupp@invalid.pssw.com.invalid> - 2025-06-16 06:29 -0600
    Re: Memory protection between compilation units? Mateusz Viste <mateusz@x.invalid> - 2025-06-16 15:01 +0200
  Re: Memory protection between compilation units? olcott <polcott333@gmail.com> - 2025-06-21 15:49 -0500

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