Re: Two questions on arrays with size defined by variables

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?

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

 

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