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From: Janis Papanagnou <janis_papanagnou+ng@hotmail.com>
Newsgroups: comp.lang.c
Subject: Re: Two questions on arrays with size defined by variables
Date: Mon, 10 Feb 2025 03:01:41 +0100
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In-Reply-To: <vob4od$qdjb$2@dont-email.me>

On 09.02.2025 21:55, James Kuyper wrote:
> On 2/9/25 12:53, Janis Papanagnou wrote:
>> On 09.02.2025 18:46, Janis Papanagnou wrote:
>>> [...] my test-wise - deliberately
>>> wrong! - assignment to 'a[99]' produced also no compiler complaints,
>>
>> BTW, it produced also no core dump or any other runtime error.
>> (But it is obviously severely wrong code anyway.)
> 
> Accessing an array beyond it's bounds has undefined behavior, whether or
> not it is a VLA. "undefined behavior" means that the C standard imposes
> no requirements on the behavior. In particular, it does not require a
> diagnostic message, nor a core dump, nor any other kind of runtime error.

Yes. (There's reasons why I prefer programming languages with checks.)

> [...]
> In this particular case. however, it would be trivial to detect the
> violation, but none of the compilers I've tested do so.

Yes, because it's a special case with a constant defined. But the
value may come from "outside", undetectable by the compiler and only
detectable during runtime if there's additional information maintained
(stored and checked).

> When such code is accepted by an implementation, what is most likely to
> happen is that the compiler will generate code that creates an array
> containing "foobar", and which attempts to write a pointer to the first
> element of that array to the location where a[99] should be, if a had at
> least 100 elements. Depending upon what that piece of memory is being
> used for, the results could be catastrophic, or completely innocuous, or
> somewhere in-between.

Yes. That matches exactly my fears or expectations here [with "C"].

Janis