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From: Thiago Adams <thiago.adams@gmail.com>
Newsgroups: comp.lang.c
Subject: Re: integer divided by zero
Date: Mon, 28 Apr 2025 11:05:11 -0300
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On 25/04/2025 17:31, Keith Thompson wrote:
> Thiago Adams <thiago.adams@gmail.com> writes:
>> Em 4/25/2025 4:05 PM, Keith Thompson escreveu:
>>> Thiago Adams <thiago.adams@gmail.com> writes:
>>>> Does anyone know of any platform where integer division by zero
>>>> returns a number, or in other words, where it's not treated as an
>>>> error? I'm asking because division by zero is undefined behaviour, but
>>>> I think division by a constant zero should be a constraint instead.
>>> Division by a constant zero is a constraint violation in a context
>>> that requires a constant expression.
>>
>> Consider this sample
>>
>> int main(){
>>      int a[1/0];
>> }
>>
>> 1/0 does not have a value in compile time,
>> So I believe compilers are making "a" a VLA because 1/0 is
>> not constant.
> 
> 1/0 is not a constant expression.
> 
> A conforming compiler that supports VLAs (C99, or optionally C11 or
> later) would make `a` a VLA, with undefined behavior at runtime when
> 1/0 is evaluated.  For a conforming compiler that doesn't support
> VLAs (C89/C90, or optionally C11 or later) the declaration is a
> constraint violation.
> 
>> (But what old c89 compilers where doing in this case?)
>>
>> This sample is a motivation to make integer divided by
>> zero a constrain.
> 
> I consider it motivation not to write code like that.
> 
> Sure, I wouldn't mind if using / or % with a right operand
> that's a constant expression with value zero (either integer or
> floating-point) were a constraint violation, but some compilers
> are going to warn about it anyway, and I doubt that such a language
> change would catch many errors.
> 

Another interesting example

enum {A = ((-2147483647-1) / -1)};

In this case it is a constant expression.
The resulting value is -2147483648 of type int.

The mathematically correct value is 2147483648 that does not fit
in a integer.