Path: ...!eternal-september.org!feeder3.eternal-september.org!news.eternal-september.org!eternal-september.org!.POSTED!not-for-mail
From: Michael S <already5chosen@yahoo.com>
Newsgroups: comp.lang.c
Subject: Re: Two questions on arrays with size defined by variables
Date: Sun, 9 Feb 2025 12:39:18 +0200
Organization: A noiseless patient Spider
Lines: 105
Message-ID: <20250209123918.0000754f@yahoo.com>
References: <vo9mns$gsd7$1@dont-email.me>
	<vo9nn3$gtph$1@dont-email.me>
	<vo9u0u$i0n8$1@dont-email.me>
MIME-Version: 1.0
Content-Type: text/plain; charset=US-ASCII
Content-Transfer-Encoding: 7bit
Injection-Date: Sun, 09 Feb 2025 11:39:19 +0100 (CET)
Injection-Info: dont-email.me; posting-host="8e2d9f7cf4b47825ab2c863bbd44b8cd";
	logging-data="566868"; mail-complaints-to="abuse@eternal-september.org";	posting-account="U2FsdGVkX19ACOdRt15QcCJB5PfSgT3Y/bvn4N6JttA="
Cancel-Lock: sha1:m5aqjQ4f2mbF6fnNTjUnOlqWSuA=
X-Newsreader: Claws Mail 3.19.1 (GTK+ 2.24.33; x86_64-w64-mingw32)
Bytes: 4186

On Sun, 9 Feb 2025 10:54:36 +0100
Janis Papanagnou <janis_papanagnou+ng@hotmail.com> wrote:

> On 09.02.2025 09:06, Andrey Tarasevich wrote:
> > On Sat 2/8/2025 11:50 PM, Janis Papanagnou wrote:  
> >> I've found examples on the Net where the arrays have been defined
> >> in a function context and the size passed as parameter
> >>
> >>    f(int n) {
> >>       char * arr[n];
> >>       ...
> >>    }  
> > 
> > Yes, that would be a VLA.
> >   
> >> That reminded me on other languages where you'd need at least a
> >> block context for dynamically sized arrays, like
> >>
> >>    int n = 5;
> >>    {
> >>       char * arr[n];
> >>       ...
> >>    }  
> > 
> > But a function body is in itself a block. Inside a function body
> > you are already in "a block context".
> >   
> >> Anyway. I tried it without function or block context
> >>
> >>    int n = 5;
> >>    char * arr[n];
> >>    ...
> >>
> >> and it seemed to work seamlessly like that (with GNU cc,
> >> -std=C99).  
> > 
> > You mean you did this at file scope? No, VLAs are illegal at file
> > scope. And I was unable to repeat this feat in GCC.  
> 
> Oh, sorry, no; above I had just written an excerpt. - Actually I had
> those two examples above within a main() function. - Sorry again for
> my inaccuracy.
> 
> What I meant was (with surrounding context) that I knew (from _other_
> languages) a syntax like
> 
> main ()
> {
>   int n = 5;
> 
>   {
>      char * arr[n];
>      ...
>   }
> }
> 
> And in "C" (C99) I tried it *without* the _inner block_
> 
> main ()
> {
>   int n = 5;
>   char * arr[n];
>   ...
> }
> 
> and it seemed to work that way. (In those other languages that wasn't
> possible.)
> 
> >   
> >> Q1: Is this a correct (portable) form?  
> > 
> > VLA objects have to be declared locally. However, keep in mind that
> > support for local declarations of VLA _objects_ is now optional
> > (i.e. not portable). Support for variably-modified _types_
> > themselves (VLA types) is mandatory. But you are not guaranteed to
> > be able to declare an actual VLA variable.  
> 
> I fear I don't understand what you're saying here. - By "now" do you
> mean newer versions of the C standards? That you can rely only, say,
> rely on it with C99 but maybe not before and not in later C standards
> conforming compilers?
>

Yes, theoretically.
In practice, I am not sure that there exists fully conforming C17 or
especially C23 compiler that does not support VLA. But there exists one
important almost-C17 compiler that does not support VLA.

There is another problem in your code - it assigns string literal to
non-const char*. It is legal, as far as 'C' Standard is concerned, but
makes very little practical sense, because any attempt to assign to
string literal through resulting pointer is UB. And not just a
theoretical UB, but a real-world UB.

> For my purpose it would be okay to know whether with the C99 version
> (that I used) it's okay, or whether that's some GNU specific extension
> or some such.
> 
> Janis
> 
> >   
> >> [...]  
> 
>