Re: CHAR_BIT is not eight

From	David Brown <david.brown@hesbynett.no>
Newsgroups	comp.lang.c++
Subject	Re: CHAR_BIT is not eight
Date	2022-10-15 13:39 +0200
Organization	A noiseless patient Spider
Message-ID	<tie65c$2n1ei$1@dont-email.me> (permalink)
References	(6 earlier) <20221013223034.00002f0b@reddwarf.jmc.corp> <tibo3h$24fcq$1@dont-email.me> <20221014144746.00000b28@reddwarf.jmc.corp> <tibtfd$1sps$1@gioia.aioe.org> <20221014210120.00005b43@reddwarf.jmc.corp>

Show all headers | View raw

On 14/10/2022 22:01, Mr Flibble wrote:
> On Fri, 14 Oct 2022 14:58:54 -0000 (UTC)
> Muttley@dastardlyhq.com wrote:
> 
>> On Fri, 14 Oct 2022 14:47:46 +0100
>> Mr Flibble <flibble@reddwarf.jmc.corp> wrote:
>>> On Fri, 14 Oct 2022 15:27:13 +0200
>>> David Brown <david.brown@hesbynett.no> wrote:
>>>> Yes, it is always /possible/.  But the complications and effort
>>>> involved is unlikely to be worth the effort, and you'd still end up
>>>> with something taking a massive amount of code space.  (With
>>>> "massive" being relative to the small code memories of such
>>>> systems.)
>>>>
>>>> It's not uncommon in my coding to build up a few lists at the
>>>> startup of the system - lists of "run" functions or software
>>>> timers, etc. Modules might register such callbacks or functions
>>>> when initialised. In "big system" C++, you might just use a vector
>>>> for that - adding your timer objects to your vector as needed.
>>>> It's simple and easy in the code.
>>>>
>>>> But in a resource-constraint embedded system, where efficiency of
>>>> code space, ram space and run-time is important (though run-time
>>>> efficiency is usually less important for startup code), that's not
>>>> what you would do.  You make your timer class have the required
>>>> list link pointers in the class, have each registering function
>>>> declare their timer object with static lifetime, and your
>>>> registration function links them together.
>>>>
>>>> There's no doubt that this takes a bit more effort to write than
>>>> an off-the-shelf std::vector.  But it is vastly simpler than
>>>> faffing around making your own allocator, avoids the use of any
>>>> kind of dynamic memory (using neither the standard heap nor a
>>>> home-made allocation system), and pulls in a tiny fraction of the
>>>> amount of library code.
>>>>
>>>> std::array<> is free - it's just a nice wrapper around a plain
>>>> C-style array.
>>>
>>> List link pointers? Again there is no reason to not use std::list
>>> with a suitable allocator even on such resource constrained hardware
>>> and I disagree that the resultant text size and ram usage would be
>>> any worse than anything you could come up with by hand.
>>
>> If the memory layout is hard coded at boot time why even bother
>> pulling in std::list or any kind of container as you don't need their
>> generic functionality which will almost certainly waste EEPROM space.
>> With embedded development you often literally have to worry about
>> every byte you use both in ROM and RAM and whether the mainloop is
>> going to be fast enough to do its job.
> 
> In C++ you don't pay for what you don't use: in the case of a C++ class
> template only the member functions instantiated will exist in the text
> segment.
> 

There are always /lots/ of other functions that get pulled in from 
libraries.  Remember, for small embedded systems the libraries are not 
shared dynamic libraries that you don't see.

Just for a quick test, I made a minimal C++ main() function and compiled 
for a modern microcontroller.  With an empty main(), it was about 8400 
bytes code of common library code.  Adding a "std::list<int>" object and 
using a couple of pops and pushes adds about 7 KB code to the program.

If you are making a lot of use of the standard containers, that's fine - 
and there's a lot of overlap and sharing in this extra code.  But in 
small systems, this kind of stuff can get significant - microcontrollers 
with 32 KB flash are common, and it's nice to be able to program them in 
C++.  In the C++ /language/, you pay very little (not quite nothing) for 
features that you don't use, but that does not apply equally to the more 
advanced standard library classes.

It's the same in C, of course - call just one little time conversion 
function and you can find half your flash is used for locale handling 
and time zones.

The needs of embedded systems vary enormously - for many, the cost in 
code space or run-time efficiency for using a std::list or std::vector 
is negligible.  But for others, it is very far from negligible.

(It is relatively rare that you have to worry about /every/ byte of code 
or data space, however - though it does happen.)

Thread

CHAR_BIT is not eight Frederick Virchanza Gotham <cauldwell.thomas@gmail.com> - 2022-10-12 15:57 -0700
  Re: CHAR_BIT is not eight "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2022-10-12 16:01 -0700
  Re: CHAR_BIT is not eight Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2022-10-12 17:16 -0700
    Re: CHAR_BIT is not eight David Brown <david.brown@hesbynett.no> - 2022-10-13 11:38 +0200
    Re: CHAR_BIT is not eight Michael S <already5chosen@yahoo.com> - 2022-11-16 06:24 -0800
      Re: CHAR_BIT is not eight David Brown <david.brown@hesbynett.no> - 2022-11-17 11:04 +0100
        Re: CHAR_BIT is not eight Muttley@dastardlyhq.com - 2022-11-17 16:40 +0000
          Re: CHAR_BIT is not eight Richard Damon <Richard@Damon-Family.org> - 2022-11-17 23:23 -0500
            Re: CHAR_BIT is not eight David Brown <david.brown@hesbynett.no> - 2022-11-18 08:16 +0100
              Re: CHAR_BIT is not eight Muttley@dastardlyhq.com - 2022-11-18 16:33 +0000
                Re: CHAR_BIT is not eight David Brown <david.brown@hesbynett.no> - 2022-11-18 18:54 +0100
            Re: CHAR_BIT is not eight Michael S <already5chosen@yahoo.com> - 2022-11-18 03:47 -0800
              Re: CHAR_BIT is not eight scott@slp53.sl.home (Scott Lurndal) - 2022-11-18 14:52 +0000
            Re: CHAR_BIT is not eight Muttley@dastardlyhq.com - 2022-11-18 16:32 +0000
              Re: CHAR_BIT is not eight David Brown <david.brown@hesbynett.no> - 2022-11-18 19:05 +0100
                Re: CHAR_BIT is not eight scott@slp53.sl.home (Scott Lurndal) - 2022-11-18 18:16 +0000
  Re: CHAR_BIT is not eight Juha Nieminen <nospam@thanks.invalid> - 2022-10-13 08:02 +0000
    Re: CHAR_BIT is not eight Muttley@dastardlyhq.com - 2022-10-13 08:08 +0000
      Re: CHAR_BIT is not eight Bonita Montero <Bonita.Montero@gmail.com> - 2022-10-15 11:35 +0200
        Re: CHAR_BIT is not eight Frederick Virchanza Gotham <cauldwell.thomas@gmail.com> - 2022-10-15 02:53 -0700
          Re: CHAR_BIT is not eight Bonita Montero <Bonita.Montero@gmail.com> - 2022-10-15 11:57 +0200
            Re: CHAR_BIT is not eight Frederick Virchanza Gotham <cauldwell.thomas@gmail.com> - 2022-10-15 03:05 -0700
            Re: CHAR_BIT is not eight Juha Nieminen <nospam@thanks.invalid> - 2022-10-17 06:18 +0000
            Re: CHAR_BIT is not eight Lynn McGuire <lynnmcguire5@gmail.com> - 2022-10-17 15:29 -0500
    Re: CHAR_BIT is not eight Frederick Virchanza Gotham <cauldwell.thomas@gmail.com> - 2022-10-13 02:06 -0700
      Re: CHAR_BIT is not eight David Brown <david.brown@hesbynett.no> - 2022-10-13 11:42 +0200
        Re: CHAR_BIT is not eight Muttley@dastardlyhq.com - 2022-10-13 15:36 +0000
          Re: CHAR_BIT is not eight David Brown <david.brown@hesbynett.no> - 2022-10-13 23:06 +0200
            Re: CHAR_BIT is not eight Mr Flibble <flibble@reddwarf.jmc.corp> - 2022-10-13 22:30 +0100
              Re: CHAR_BIT is not eight David Brown <david.brown@hesbynett.no> - 2022-10-14 15:27 +0200
                Re: CHAR_BIT is not eight Mr Flibble <flibble@reddwarf.jmc.corp> - 2022-10-14 14:47 +0100
                Re: CHAR_BIT is not eight Muttley@dastardlyhq.com - 2022-10-14 14:58 +0000
                Re: CHAR_BIT is not eight Mr Flibble <flibble@reddwarf.jmc.corp> - 2022-10-14 21:01 +0100
                Re: CHAR_BIT is not eight Muttley@dastardlyhq.com - 2022-10-15 10:28 +0000
                Re: CHAR_BIT is not eight David Brown <david.brown@hesbynett.no> - 2022-10-15 13:39 +0200
                Re: CHAR_BIT is not eight Mr Flibble <flibble@reddwarf.jmc.corp> - 2022-10-15 12:45 +0100
                Re: CHAR_BIT is not eight Muttley@dastardlyhq.com - 2022-10-15 15:18 +0000
                Re: CHAR_BIT is not eight Mr Flibble <flibble@reddwarf.jmc.corp> - 2022-10-15 16:24 +0100
                Re: CHAR_BIT is not eight Muttley@dastardlyhq.com - 2022-10-15 15:34 +0000
                Re: CHAR_BIT is not eight Mr Flibble <flibble@reddwarf.jmc.corp> - 2022-10-15 16:39 +0100
                Re: CHAR_BIT is not eight Muttley@dastardlyhq.com - 2022-10-15 15:53 +0000
                Re: CHAR_BIT is not eight Mr Flibble <flibble@reddwarf.jmc.corp> - 2022-10-15 16:55 +0100
                Re: CHAR_BIT is not eight Muttley@dastardlyhq.com - 2022-10-15 15:57 +0000
                Re: CHAR_BIT is not eight Mr Flibble <flibble@reddwarf.jmc.corp> - 2022-10-15 16:59 +0100
                Re: CHAR_BIT is not eight Muttley@dastardlyhq.com - 2022-10-17 15:27 +0000
                Re: CHAR_BIT is not eight "daniel...@gmail.com" <danielaparker@gmail.com> - 2022-10-17 09:04 -0700
                Re: CHAR_BIT is not eight Muttley@dastardlyhq.com - 2022-10-17 16:13 +0000
                Re: CHAR_BIT is not eight red floyd <no.spam.here@its.invalid> - 2022-10-17 09:44 -0700
                Re: CHAR_BIT is not eight Mr Flibble <flibble@reddwarf.jmc.corp> - 2022-10-17 17:47 +0100
                Re: CHAR_BIT is not eight Manfred <noname@add.invalid> - 2022-10-18 01:10 +0200
                Re: CHAR_BIT is not eight "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2022-10-17 16:22 -0700
                Re: CHAR_BIT is not eight Paul N <gw7rib@aol.com> - 2022-10-18 05:13 -0700
                Re: CHAR_BIT is not eight Muttley@dastardlyhq.com - 2022-10-18 15:04 +0000
                Re: CHAR_BIT is not eight Mr Flibble <flibble@reddwarf.jmc.corp> - 2022-10-18 17:40 +0100
                Re: CHAR_BIT is not eight "daniel...@gmail.com" <danielaparker@gmail.com> - 2022-10-18 12:14 -0700
                Re: CHAR_BIT is not eight Muttley@dastardlyhq.com - 2022-10-19 15:12 +0000
                Re: CHAR_BIT is not eight scott@slp53.sl.home (Scott Lurndal) - 2022-10-19 15:35 +0000
                Re: CHAR_BIT is not eight Muttley@dastardlyhq.com - 2022-10-20 16:16 +0000
                Re: CHAR_BIT is not eight Bonita Montero <Bonita.Montero@gmail.com> - 2022-10-15 20:13 +0200
                Re: CHAR_BIT is not eight Bonita Montero <Bonita.Montero@gmail.com> - 2022-10-15 20:11 +0200
                Re: CHAR_BIT is not eight Mr Flibble <flibble@reddwarf.jmc.corp> - 2022-10-15 20:03 +0100
                Re: CHAR_BIT is not eight Bonita Montero <Bonita.Montero@gmail.com> - 2022-10-16 07:51 +0200
                Re: CHAR_BIT is not eight David Brown <david.brown@hesbynett.no> - 2022-10-16 17:03 +0200
                Re: CHAR_BIT is not eight Mr Flibble <flibble@reddwarf.jmc.corp> - 2022-10-16 16:34 +0100
                Re: CHAR_BIT is not eight David Brown <david.brown@hesbynett.no> - 2022-10-16 18:51 +0200
                Re: CHAR_BIT is not eight Mr Flibble <flibble@reddwarf.jmc.corp> - 2022-10-16 18:11 +0100
                Re: CHAR_BIT is not eight Mr Flibble <flibble@reddwarf.jmc.corp> - 2022-10-16 19:18 +0100
                Re: CHAR_BIT is not eight David Brown <david.brown@hesbynett.no> - 2022-10-16 22:02 +0200
                Re: CHAR_BIT is not eight Mr Flibble <flibble@reddwarf.jmc.corp> - 2022-10-16 21:19 +0100
                Re: CHAR_BIT is not eight scott@slp53.sl.home (Scott Lurndal) - 2022-10-16 21:24 +0000
                Re: CHAR_BIT is not eight "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2022-10-16 14:38 -0700
                Re: CHAR_BIT is not eight "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2022-10-16 14:48 -0700
                Re: CHAR_BIT is not eight Mr Flibble <flibble@reddwarf.jmc.corp> - 2022-10-16 22:39 +0100
                Re: CHAR_BIT is not eight scott@slp53.sl.home (Scott Lurndal) - 2022-10-16 23:49 +0000
                Re: CHAR_BIT is not eight David Brown <david.brown@hesbynett.no> - 2022-10-17 09:54 +0200
                Re: CHAR_BIT is not eight Mr Flibble <flibble@reddwarf.jmc.corp> - 2022-10-17 17:31 +0100
                Re: CHAR_BIT is not eight David Brown <david.brown@hesbynett.no> - 2022-10-17 19:56 +0200
                Re: CHAR_BIT is not eight Muttley@dastardlyhq.com - 2022-10-17 15:29 +0000
                Re: CHAR_BIT is not eight Mr Flibble <flibble@reddwarf.jmc.corp> - 2022-10-17 17:33 +0100
                Re: CHAR_BIT is not eight Frederick Virchanza Gotham <cauldwell.thomas@gmail.com> - 2022-10-15 11:55 -0700
                Re: CHAR_BIT is not eight "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2022-10-15 13:06 -0700
                Re: CHAR_BIT is not eight Frederick Virchanza Gotham <cauldwell.thomas@gmail.com> - 2022-10-15 15:42 -0700
                Re: CHAR_BIT is not eight David Brown <david.brown@hesbynett.no> - 2022-10-16 19:10 +0200
                Re: CHAR_BIT is not eight Vir Campestris <vir.campestris@invalid.invalid> - 2022-10-16 21:28 +0100
                Re: CHAR_BIT is not eight David Brown <david.brown@hesbynett.no> - 2022-10-17 10:09 +0200
                Re: CHAR_BIT is not eight "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2022-10-16 13:35 -0700
                Re: CHAR_BIT is not eight "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2022-10-16 13:36 -0700
      Re: CHAR_BIT is not eight Tim Rentsch <tr.17687@z991.linuxsc.com> - 2022-11-02 17:46 -0700
    Re: CHAR_BIT is not eight Bo Persson <bo@bo-persson.se> - 2022-10-13 11:10 +0200
      Re: CHAR_BIT is not eight Juha Nieminen <nospam@thanks.invalid> - 2022-10-13 10:49 +0000
        Re: CHAR_BIT is not eight Juha Nieminen <nospam@thanks.invalid> - 2022-10-13 12:05 +0000
          Re: CHAR_BIT is not eight Frederick Virchanza Gotham <cauldwell.thomas@gmail.com> - 2022-10-13 06:34 -0700
          Re: CHAR_BIT is not eight Ben Bacarisse <ben.usenet@bsb.me.uk> - 2022-10-13 15:24 +0100
        Re: CHAR_BIT is not eight David Brown <david.brown@hesbynett.no> - 2022-10-13 15:59 +0200
          Re: CHAR_BIT is not eight Juha Nieminen <nospam@thanks.invalid> - 2022-10-14 05:54 +0000
            Re: CHAR_BIT is not eight Paavo Helde <eesnimi@osa.pri.ee> - 2022-10-14 09:16 +0300
              Re: CHAR_BIT is not eight Tim Rentsch <tr.17687@z991.linuxsc.com> - 2022-11-16 05:41 -0800
                Re: CHAR_BIT is not eight Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2022-11-16 10:38 -0800
                Re: CHAR_BIT is not eight Tim Rentsch <tr.17687@z991.linuxsc.com> - 2022-12-05 11:42 -0800
                Re: CHAR_BIT is not eight Öö Tiib <ootiib@hot.ee> - 2022-12-06 01:03 -0800
                Re: CHAR_BIT is not eight Tim Rentsch <tr.17687@z991.linuxsc.com> - 2022-12-28 20:49 -0800
            Re: CHAR_BIT is not eight Frederick Virchanza Gotham <cauldwell.thomas@gmail.com> - 2022-10-14 00:17 -0700
              Re: CHAR_BIT is not eight David Brown <david.brown@hesbynett.no> - 2022-10-14 15:33 +0200
              Re: CHAR_BIT is not eight Vir Campestris <vir.campestris@invalid.invalid> - 2022-10-16 21:37 +0100
                Re: CHAR_BIT is not eight Lynn McGuire <lynnmcguire5@gmail.com> - 2022-10-17 15:24 -0500
  Re: CHAR_BIT is not eight Bonita Montero <Bonita.Montero@gmail.com> - 2022-10-15 11:34 +0200

csiph-web