Re: Two questions on arrays with size defined by variables

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.

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.

Then, with above setting, I also tried
    arr[99] = "foobar";
 To my astonishment the compiler did not only accept that but it also
operated without runtime error; but I assume it's an error that may
severely corrupt the memory. - Q2: Is my suspicion correct?

Yes. Since the beginning of times the language itself does not check/enforce array boundaries. When you violate the boundary, the behavior is undefined, meaning that compilers can do anything (including implementing array boundary checks, where possible). But for obvious performance reasons C compilers do not normally enforce array boundaries in "production" compilation modes.
--
Best regards,
Andrey