Path: ...!weretis.net!feeder9.news.weretis.net!news.nk.ca!rocksolid2!i2pn2.org!.POSTED!not-for-mail
From: Richard Damon <richard@damon-family.org>
Newsgroups: comp.lang.c
Subject: Re: Struct Error
Date: Wed, 29 Jan 2025 07:32:26 -0500
Organization: i2pn2 (i2pn.org)
Message-ID: <3cd9101a43de105e6aaa74614d05fcba5b8c093c@i2pn2.org>
References: <vmr5gg$137jo$1@dont-email.me> <vms4km$19srg$1@dont-email.me>
 <vmt74h$1jac0$1@dont-email.me> <20250124163740.00006281@yahoo.com>
 <vn61ho$1pf2$1@dont-email.me> <86jzad28d5.fsf@linuxsc.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8; format=flowed
Content-Transfer-Encoding: 7bit
Injection-Date: Wed, 29 Jan 2025 12:32:27 -0000 (UTC)
Injection-Info: i2pn2.org;
	logging-data="1698168"; mail-complaints-to="usenet@i2pn2.org";
	posting-account="diqKR1lalukngNWEqoq9/uFtbkm5U+w3w6FQ0yesrXg";
User-Agent: Mozilla Thunderbird
X-Spam-Checker-Version: SpamAssassin 4.0.0
In-Reply-To: <86jzad28d5.fsf@linuxsc.com>
Content-Language: en-US
Bytes: 4232
Lines: 87

On 1/29/25 5:59 AM, Tim Rentsch wrote:
> bart <bc@freeuk.com> writes:
> 
>> On 24/01/2025 14:37, Michael S wrote:
>>
>>> On Thu, 23 Jan 2025 10:54:10 +0000
>>> bart <bc@freeuk.com> wrote:
>>>
>>>> On 23/01/2025 01:05, James Kuyper wrote:
>>>>
>>>>> On 2025-01-22, bart <bc@freeuk.com> wrote:
>>>>>
>>>>>> Gcc 14.1 gives me an error compiling this code:
>>>>>>
>>>>>>       struct vector;
>>>>>>       struct scenet;
>>>>>>
>>>>>>       struct vector {
>>>>>>           double x;
>>>>>>           double y;
>>>>>>           double z;
>>>>>>       };
>>>>>>
>>>>>>       struct scenet {
>>>>>>           struct vector center;
>>>>>>           double radius;
>>>>>>           struct scenet (*child)[];
>>>>>>       };
>>>>>
>>>>> 6.7.6.2p2:  "The element type shall not be an incomplete or
>>>>> function type."
>>>>>
>>>>> I have many draft versions of the C standard.  n2912.pdf, dated
>>>>> 2022-06-08, says in 6.7.2.1.p3 about struct types that "... the
>>>>> type is incomplete144) until immediately after the closing brace
>>>>> of the list defining the content, and complete thereafter."
>>>>>
>>>>> Therefore, struct scenet is not a complete type until the closing
>>>>> brace of it's declaration.
>>>>
>>>> Wouldn't this also be the case here:
>>>>
>>>>       struct scenet *child;
>>>>      };
>>>
>>> Just to point out if it was not said already:  the problem is not
>>> related specifically to recursive structures.  It applies to arrays
>>> of incomplete types in all circumstances.
>>>
>>> struct bar;
>>> struct bar (*bag)[]; // error
>>> typedef struct bar (*bat)[]; // error
>>
>> I don't think anyone has yet explained why that is an error (other
>> than C says it is), but not this:
>>
>>    struct bar *ptr;
>>
>> This is a pointer to an incomplete type.  Attempts to do ++ptr
>> for example will fail later on if that struct has not yet been
>> defined.
>>
>> So why not the same for the pointer-to-array versions?
> 
> The question you should be asking is why did the original C
> standards body make the rule they did?

My guess is that it makes the simplest implementation of a C compiler 
much more complicated. While I don't think it has been explicited 
stated, one goal the original language, and apparently kept by the 
Standards Comittee, has been to make the language fairly simple to 
proceess to get working code. To optimize to make fast, might take more 
work, but to make your first complier for a system should be straight 
forward. I believe a C compiler can still be done with a single pass 
through the source code, with limited look ahead, and only the final 
"link" step needs to be able to handle large chunks of the program.

Allowing the pointer to array time to be based on an incomplete type 
might make this goal harder.

> 
> The answer might be because this exception to a simple and
> general rule is almost never useful, and never necessary.
> 
> Considering that it has been 35 years since that original rule
> was made, and 2025 is the first time the question has come up,
> the indications are that the original decision was a good one.