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From: David Brown <david.brown@hesbynett.no>
Newsgroups: comp.lang.c
Subject: Re: "A diagram of C23 basic types"
Date: Thu, 3 Apr 2025 09:38:05 +0200
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On 03/04/2025 05:43, Janis Papanagnou wrote:
> On 02.04.2025 09:32, Kaz Kylheku wrote:
>> On 2025-04-02, Janis Papanagnou <janis_papanagnou+ng@hotmail.com> wrote:
>>> On 02.04.2025 07:59, Alexis wrote:
>>>>
>>>> Thought people here might be interested in this image on Jens Gustedt's
>>>> blog, which translates section 6.2.5, "Types", of the C23 standard
>>>> into a graph of inclusions:
>>>>
>>>>    https://gustedt.wordpress.com/2025/03/29/a-diagram-of-c23-basic-types/
>>>
>>> A nice overview. - I have questions on some of these types...
>>>
>>> The _Decimal* types - are these just types with other implicit
>>> encodings, say, BCD encoded, or some such?
>>
>> IEEE 754 defines decimal floating point types now, so that's what
>> that is about. The spec allows for the significand to be encoded
>> using Binary Integer Decimal, or to use Densely Packed Decimal.
> 
> Thanks for the hint and keywords. It seems my BCD guess was not far
> from what these two IEEE formats actually are.
> 
> Does that now mean that every conforming C23 compiler must support
> yet more numeric types, including multiple implementation-versions
> of the whole arithmetic functions and operators necessary?
> 

They are an optional feature (as are the other floating point and 
complex types beyond the basics of float, double and long double).

> I wonder why these variants had been introduced.

I presume that some people want them.  They are in the ISO 60559 
standard, along with things like "interchange" floating point types 
intended to be maximally consistent between different systems.

> 
> In many other languages you have abstractions of numeric types, not
> every implicitly encoding variant revealed an the programming level.
> 

That's often fine within a program, but sometimes you need to exchange 
data with other programs.  In particular, C is the standard language for 
common libraries - being able to reliably and consistently exchange data 
with other languages and other machines is thus very important.