Two questions on arrays with size defined by variables

I need an array whose size is depending on values that are dynamically
determined by the program. (I suppose those are the "VLAs" that I've
read about here occasionally? - Anyway, however they're named...)

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];
     ...
  }

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];
     ...
  }

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).

Q1: Is this a correct (portable) form?


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?

Janis