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| From | Tim Rentsch <txr@alumni.caltech.edu> |
|---|---|
| Newsgroups | comp.lang.c |
| Subject | Re: counting bytes |
| Date | 2015-08-01 10:53 -0700 |
| Organization | A noiseless patient Spider |
| Message-ID | <kfn7fpec2un.fsf@x-alumni2.alumni.caltech.edu> (permalink) |
| References | (12 earlier) <lnwpxug0bb.fsf@kst-u.example.com> <kfnmvylh4r0.fsf@x-alumni2.alumni.caltech.edu> <lna8ulzam2.fsf@kst-u.example.com> <kfn4mkled7c.fsf@x-alumni2.alumni.caltech.edu> <lnwpxhl83j.fsf@kst-u.example.com> |
Keith Thompson <kst-u@mib.org> writes:
> Tim Rentsch <txr@alumni.caltech.edu> writes:
>> Keith Thompson <kst-u@mib.org> writes:
>>> Tim Rentsch <txr@alumni.caltech.edu> writes:
>>>> Keith Thompson <kst-u@mib.org> writes:
>>>>> Tim Rentsch <txr@alumni.caltech.edu> writes:
>>>>>> Keith Thompson <kst-u@mib.org> writes:
>>>>>>> Tim Rentsch <txr@alumni.caltech.edu> writes:
>>>>>
>>>>> [...]
>>>>>
>>>>>>>> It is possible to have a conforming implementation with
>>>>>>>> CHAR_BIT > 8 but still having a type that occupies only eight
>>>>>>>> bits. AFAIK there is no reason such an implementation
>>>>>>>> could not use that type to provide the [u]int8_t types.
>>>>>>>
>>>>>>> I'm skeptical.
>>>>>>
>>>>>> The combination of undefined behavior resulting from trap
>>>>>> representatons and the Standard allowing an extremely wide
>>>>>> range of extensions is enormously powerful. In some sense
>>>>>> it's an academic question since such an implementation will
>>>>>> in all likelihood never be produced. But I think it's
>>>>>> interesting to consider the possibility.
>>>>>
>>>>> Sure -- but I still don't think it's a possibility.
>>>>>
>>>>> Extensions can take advantage of undefined behavior, but your proposed
>>>>> extension (using padding bits and trap representations in size_t to
>>>>> represent fractional values) introduces undefined behavior where it
>>>>> doesn't currently exist. I believe it is therefore non-conforming.
>>>>>
>>>>> I believe that this program:
>>>>>
>>>>> #include <stdio.h>
>>>>> #include <stdint.h>
>>>>>
>>>>> int main(void) {
>>>>> #ifdef UINT8_MAX
>>>>> if (sizeof (uint8_t) < (size_t)1) {
>>>>> puts("I am confused");
>>>>> }
>>>>> else {
>>>>> puts("ok");
>>>>> }
>>>>> #else
>>>>> puts("ok");
>>>>> #endif
>>>>> return 0;
>>>>> }
>>>>>
>>>>> is strictly conforming (subject to the usual caveats about the puts call
>>>>> failing) and must print "ok".
>>>>
>>>> Sure, but that doesn't mean there can't an extension that provides
>>>> these types, as long as they are provided only to (some subset of)
>>>> programs that are not strictly conforming.
>>>
>>> If you enable the extension only in the presence of a #pragma, for
>>> example, then sure, it would be valid. A conforming implementation
>>> could have a "#pragma LANGUAGE FORTRAN" that turns it into a Fortran
>>> compiler.
>>
>> Yes, but a program doesn't need #pragma to put it outside the
>> realm of being strictly conforming. For example, if a source
>> file has '#include <Xstdint.h>' (and assuming of course that it
>> isn't skipped by pre-processors conditionals) then the program is
>> not strictly conforming.
>
> Sure -- for example <Xstdint.h> could contain a #pragma.
Yes, certainly it could, but my point is it doesn't have to. In
fact the file for <Xstding.h> could be empty and the program
would still be not strictly conforming. Or the source text might
have, eg, 'enum { not_strictly_conforming_eh = 50000 };', and the
program would not be strictly conforming. For that matter, even
these two lines
#include <stdint.h>
typedef uint8_t Octet;
are enough to ensure the program is not strictly conforming. Any
of these methods, or all of them, could be used to allow the
undersized types extension.
>>> Types smaller than CHAR_BIT bits are permitted as an extension
>>> only in the sense that just about *anything* is permitted as an
>>> extension.
>>>
>>> Using such a type to define uint8_t on a system where CHAR_BIT > 8
>>> would not, I think, be conforming.
>>
>> I understand the gut reaction. Can you cite a passage in the
>> Standard that supports this assertion? I don't know of one.
>
> I don't think there's a single passage I can cite.
>
> In the absence of a #pragma, or a non-standard #include, or something
> along those lines, I don't think it's permitted. In the presence of
> such a thing, just about *anything* is permitted.
I agree on the second sentence, but I'm not predicating the
suggestion on #pragma or something like it. Why do you
think, in the case of the first sentence, that an extension
like what I'm describing is not permitted? AFAICT the Standard
allows it.
>>> In addition, I think it would be a bad idea; it would break some
>>> assumptions that C programmers can reasonably depend on, such as
>>> that any object's representation can be treated as an array of
>>> unsigned char -- or that sizeof yields an integer.
>>
>> Perhaps, but I think that depends on how the extension is
>> presented. If it were presented in a way that such types
>> were provided only when there is some explicit indication
>> that they should be (eg, like the #include mentioned above),
>> why is that bad?
>
> I find it counterintuitive. If an implementation defines uint8_t
> in a way that violates the expectations I have for a type with
> that name (such as that sizeof (uint8_t) == 1, or at least that
> it's not between 0 and 1), I think that's a bad thing. Making it
> depend on a non-standard #pragma, #include, or whatever can
> probably make it a conforming extension, but IMHO it would be far
> better not to change the existing meanings of uint8_t, sizeof, and
> size_t.
I think I understand your reaction. I agree that having types
with a size that is greater than zero but less than one is a
little weird. However, given the existence of a type that
satisfies all the requirements of [u]int8_t except the (indirect)
requirement that sizes are a multiple of 1, and since the whole
point of providing undersized types is to provide types whose
sizes are not always multiples of 1, at some level it seems like
the presence of the extension virtually requires [u]int8_t be
provided (this is assuming of course that there is an 8-bit
undersized type, etc).
> Sub-byte types are an interesting idea for an extension, but using
> a distinctive syntax would be less confusing. Permitting bit
> fields outside structs might be a good starting point.
Allowing arbitrary widths is possible but complicated. In most
cases I expect implementations would provide just those widths
that evenly divide CHAR_BIT. Using a syntax that makes it
obvious that something unusual is going on is often a good idea,
although I don't think it needs to be anything high-powered --
just __unsigned_1, __unsigned_2, __unsigned_4, etc, and the
corresponding __signed_NN counterparts, should suffice.
Back to comp.lang.c | Previous | Next — Previous in thread | Next in thread | Find similar | Unroll thread
counting bytes bilsch01 <king621@comcast.net> - 2015-06-06 04:58 -0700
Re: counting bytes Bartc <bc@freeuk.com> - 2015-06-06 13:05 +0100
Re: counting bytes bilsch01 <king621@comcast.net> - 2015-06-06 05:24 -0700
Re: counting bytes Malcolm McLean <malcolm.mclean5@btinternet.com> - 2015-06-06 05:41 -0700
Re: counting bytes Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-06-06 14:00 +0100
Re: counting bytes Bartc <bc@freeuk.com> - 2015-06-06 13:52 +0100
Re: counting bytes Lew Pitcher <lew.pitcher@digitalfreehold.ca> - 2015-06-06 10:27 -0400
Re: counting bytes jt@toerring.de (Jens Thoms Toerring) - 2015-06-06 22:31 +0000
THANKS EVERYBODY FOR THE INFO bilsch01 <king621@comcast.net> - 2015-06-06 23:35 -0700
Re: THANKS EVERYBODY FOR THE INFO Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-06-07 10:08 +0100
Re: THANKS EVERYBODY FOR THE INFO Malcolm McLean <malcolm.mclean5@btinternet.com> - 2015-06-07 09:22 -0700
Re: THANKS EVERYBODY FOR THE INFO Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-06-07 18:07 +0100
Re: THANKS EVERYBODY FOR THE INFO Malcolm McLean <malcolm.mclean5@btinternet.com> - 2015-06-08 04:03 -0700
Re: THANKS EVERYBODY FOR THE INFO Tim Rentsch <txr@alumni.caltech.edu> - 2015-07-11 08:12 -0700
Re: THANKS EVERYBODY FOR THE INFO Keith Thompson <kst-u@mib.org> - 2015-06-07 08:01 -0700
Re: counting bytes gordonb.3gcai@burditt.org (Gordon Burditt) - 2015-06-07 05:02 -0500
Re: counting bytes David Brown <david.brown@hesbynett.no> - 2015-06-07 15:32 +0200
Re: counting bytes Keith Thompson <kst-u@mib.org> - 2015-06-07 12:48 -0700
Re: counting bytes David Brown <david.brown@hesbynett.no> - 2015-06-07 23:05 +0200
Re: counting bytes Richard Heathfield <rjh@cpax.org.uk> - 2015-06-07 22:17 +0100
Re: counting bytes David Brown <david.brown@hesbynett.no> - 2015-06-07 23:49 +0200
Re: counting bytes Richard Heathfield <rjh@cpax.org.uk> - 2015-06-07 23:37 +0100
Re: counting bytes Tim Rentsch <txr@alumni.caltech.edu> - 2015-07-11 08:25 -0700
Re: counting bytes Tim Rentsch <txr@alumni.caltech.edu> - 2015-07-11 08:21 -0700
Re: counting bytes Martin Shobe <martin.shobe@yahoo.com> - 2015-07-11 13:54 -0500
Re: counting bytes Tim Rentsch <txr@alumni.caltech.edu> - 2015-07-20 06:33 -0700
Re: counting bytes Martin Shobe <martin.shobe@yahoo.com> - 2015-07-20 09:51 -0500
Re: counting bytes Tim Rentsch <txr@alumni.caltech.edu> - 2015-07-24 09:49 -0700
Re: counting bytes Martin Shobe <martin.shobe@yahoo.com> - 2015-07-24 12:26 -0500
Re: counting bytes Tim Rentsch <txr@alumni.caltech.edu> - 2015-07-30 11:16 -0700
Re: counting bytes Martin Shobe <martin.shobe@yahoo.com> - 2015-07-30 23:15 -0500
Re: counting bytes Tim Rentsch <txr@alumni.caltech.edu> - 2015-08-02 06:07 -0700
Re: counting bytes Richard Damon <Richard@Damon-Family.org> - 2015-08-02 14:31 -0400
Re: counting bytes Tim Rentsch <txr@alumni.caltech.edu> - 2015-08-09 05:31 -0700
Re: counting bytes Richard Damon <Richard@Damon-Family.org> - 2015-08-09 17:09 -0400
Re: counting bytes Tim Rentsch <txr@alumni.caltech.edu> - 2015-09-05 09:58 -0700
Re: counting bytes Keith Thompson <kst-u@mib.org> - 2015-09-05 19:29 -0700
Re: counting bytes Tim Rentsch <txr@alumni.caltech.edu> - 2015-09-06 14:38 -0700
Re: counting bytes Keith Thompson <kst-u@mib.org> - 2015-09-06 17:46 -0700
Re: counting bytes Tim Rentsch <txr@alumni.caltech.edu> - 2015-09-07 23:50 -0700
Re: counting bytes Richard Damon <Richard@Damon-Family.org> - 2015-09-06 22:00 -0400
Re: counting bytes Tim Rentsch <txr@alumni.caltech.edu> - 2015-09-07 23:24 -0700
Re: counting bytes Richard Damon <Richard@Damon-Family.org> - 2015-07-24 20:03 -0400
Re: counting bytes Martin Shobe <martin.shobe@yahoo.com> - 2015-07-24 22:13 -0500
Re: counting bytes Malcolm McLean <malcolm.mclean5@btinternet.com> - 2015-07-25 02:22 -0700
Re: counting bytes Martin Shobe <martin.shobe@yahoo.com> - 2015-07-25 11:39 -0500
Re: counting bytes Malcolm McLean <malcolm.mclean5@btinternet.com> - 2015-07-25 09:48 -0700
Re: counting bytes Martin Shobe <martin.shobe@yahoo.com> - 2015-07-25 14:04 -0500
Re: counting bytes Malcolm McLean <malcolm.mclean5@btinternet.com> - 2015-07-25 14:22 -0700
Re: counting bytes Richard Damon <Richard@Damon-Family.org> - 2015-07-25 11:07 -0400
Re: counting bytes James Kuyper <jameskuyper@verizon.net> - 2015-07-25 12:43 -0400
Re: counting bytes Richard Damon <Richard@Damon-Family.org> - 2015-07-25 13:32 -0400
Re: counting bytes glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2015-07-25 20:14 +0000
Re: counting bytes James Kuyper <jameskuyper@verizon.net> - 2015-07-25 17:24 -0400
Re: counting bytes Malcolm McLean <malcolm.mclean5@btinternet.com> - 2015-07-25 14:42 -0700
Re: counting bytes glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2015-07-25 23:02 +0000
Re: counting bytes James Kuyper <jameskuyper@verizon.net> - 2015-07-25 19:17 -0400
Re: counting bytes Malcolm McLean <malcolm.mclean5@btinternet.com> - 2015-07-25 16:37 -0700
Re: counting bytes glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2015-07-26 06:10 +0000
Re: counting bytes James Kuyper <jameskuyper@verizon.net> - 2015-07-26 08:31 -0400
Re: counting bytes Bartc <bc@freeuk.com> - 2015-07-30 21:32 +0100
Re: counting bytes raltbos@xs4all.nl (Richard Bos) - 2015-07-31 13:25 +0000
Re: counting bytes James Kuyper <jameskuyper@verizon.net> - 2015-07-31 11:31 -0400
Re: counting bytes Keith Thompson <kst-u@mib.org> - 2015-07-31 11:30 -0700
Re: counting bytes James Kuyper <jameskuyper@verizon.net> - 2015-07-31 14:29 -0400
Re: counting bytes Keith Thompson <kst-u@mib.org> - 2015-07-31 12:14 -0700
Re: counting bytes David Kleinecke <dkleinecke@gmail.com> - 2015-07-31 17:39 -0700
Re: counting bytes James Kuyper <jameskuyper@verizon.net> - 2015-07-31 23:32 -0400
Re: counting bytes David Kleinecke <dkleinecke@gmail.com> - 2015-08-01 16:41 -0700
Re: counting bytes James Kuyper <jameskuyper@verizon.net> - 2015-08-01 20:23 -0400
Re: counting bytes Malcolm McLean <malcolm.mclean5@btinternet.com> - 2015-07-31 11:01 -0700
Re: counting bytes Richard Damon <Richard@Damon-Family.org> - 2015-07-25 19:42 -0400
Re: counting bytes Tim Rentsch <txr@alumni.caltech.edu> - 2015-07-30 12:16 -0700
Re: counting bytes Bartc <bc@freeuk.com> - 2015-07-30 22:04 +0100
Re: counting bytes Tim Rentsch <txr@alumni.caltech.edu> - 2015-08-02 06:10 -0700
Re: counting bytes David Kleinecke <dkleinecke@gmail.com> - 2015-07-25 19:10 -0700
Re: counting bytes Richard Damon <Richard@Damon-Family.org> - 2015-07-25 22:16 -0400
Re: counting bytes David Kleinecke <dkleinecke@gmail.com> - 2015-07-25 19:46 -0700
Re: counting bytes Tim Rentsch <txr@alumni.caltech.edu> - 2015-07-30 11:52 -0700
Re: counting bytes raltbos@xs4all.nl (Richard Bos) - 2015-07-31 08:51 +0000
Re: counting bytes Tim Rentsch <txr@alumni.caltech.edu> - 2015-08-02 06:19 -0700
Re: counting bytes Richard Damon <Richard@Damon-Family.org> - 2015-08-01 09:39 -0400
Re: counting bytes Keith Thompson <kst-u@mib.org> - 2015-08-01 13:06 -0700
Re: counting bytes Richard Damon <Richard@Damon-Family.org> - 2015-08-01 20:43 -0400
Re: counting bytes Tim Rentsch <txr@alumni.caltech.edu> - 2015-08-02 06:52 -0700
Re: counting bytes Richard Damon <Richard@Damon-Family.org> - 2015-08-02 15:14 -0400
Re: counting bytes Tim Rentsch <txr@alumni.caltech.edu> - 2015-08-09 07:54 -0700
Re: counting bytes Richard Damon <Richard@Damon-Family.org> - 2015-08-09 16:43 -0400
Re: counting bytes Tim Rentsch <txr@alumni.caltech.edu> - 2015-09-05 10:48 -0700
Re: counting bytes Richard Damon <Richard@Damon-Family.org> - 2015-07-11 17:50 -0400
Re: counting bytes Tim Rentsch <txr@alumni.caltech.edu> - 2015-07-20 06:36 -0700
Re: counting bytes raltbos@xs4all.nl (Richard Bos) - 2015-07-20 15:14 +0000
Re: counting bytes Tim Rentsch <txr@alumni.caltech.edu> - 2015-07-24 09:38 -0700
Re: counting bytes raltbos@xs4all.nl (Richard Bos) - 2015-07-25 19:12 +0000
Re: counting bytes Tim Rentsch <txr@alumni.caltech.edu> - 2015-07-30 12:18 -0700
Re: counting bytes Keith Thompson <kst-u@mib.org> - 2015-07-11 17:03 -0700
Re: counting bytes glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2015-07-12 00:15 +0000
Re: counting bytes Keith Thompson <kst-u@mib.org> - 2015-07-11 17:42 -0700
Re: counting bytes glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2015-07-12 02:46 +0000
Re: counting bytes James Kuyper <jameskuyper@verizon.net> - 2015-07-12 12:33 -0400
Re: counting bytes Tim Rentsch <txr@alumni.caltech.edu> - 2015-07-20 06:48 -0700
Re: counting bytes Keith Thompson <kst-u@mib.org> - 2015-07-20 11:18 -0700
Re: counting bytes Tim Rentsch <txr@alumni.caltech.edu> - 2015-07-24 09:58 -0700
Re: counting bytes Keith Thompson <kst-u@mib.org> - 2015-07-24 11:14 -0700
Re: counting bytes Tim Rentsch <txr@alumni.caltech.edu> - 2015-07-30 11:02 -0700
Re: counting bytes Keith Thompson <kst-u@mib.org> - 2015-07-30 13:10 -0700
Re: counting bytes Tim Rentsch <txr@alumni.caltech.edu> - 2015-08-01 10:53 -0700
Re: counting bytes Keith Thompson <kst-u@mib.org> - 2015-06-07 14:46 -0700
Re: counting bytes David Brown <david.brown@hesbynett.no> - 2015-06-08 09:05 +0200
Re: counting bytes Siri Cruz <chine.bleu@yahoo.com> - 2015-06-06 05:06 -0700
Re: counting bytes bartekltg <bartekltg@gmail.com> - 2015-07-26 00:58 +0200
Re: counting bytes bartekltg <bartekltg@gmail.com> - 2015-07-26 01:00 +0200
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