Why does a variable length array work in this C code?

Question

#include "stdio.h"

int main()
    {
        int n;
        printf("Enter n:\n");
        scanf("%d",&n);

        int arr[n][n];
        arr[3][3] = 4;
        printf("%d",arr[3][3]);
        getchar();
        return 0;
    }

Not used int arr[n], where n is a variable illegal in C? I am trying to understand what is happening here. Apparently, the code works on mine clang LLVM compilerand on IDEOne and on Codeblocks. I think that the compiler just makes my task easier by making automatic memory allocation. But another amazing fact: when I try to set nto 1, 2 or 3, it still works.

Answer 1

C C99. : ++
.
, c99 ++ .