C++ read/write to array out of bounds

Question

There's probably already an answer to this somewhere, but I can't find it.

As noted in this question: Accessing an array out of bounds gives no error, why? , C++ does not enforce array bounds, instead opting to provide undefined behavior. What I'm concerned about is the power of this behavior.

So suppose I write some simple program:

#include <iostream>
int main() {
    int* a = new int[1];
    long large_number = 9223372036854775807l; //2**63 - 1
    for (long i = 0l; i < large_number; i++) {
        std::cout << i << " " << a[i] << std::endl;
    }
    return 0;
}

This will continue to print the next 32-bit number stored on my system (assuming 32 bit-sized ints, obviously). When I run this on my machine, the program segfaults when i is around 30,000, which I'm guessing is around the size of the memory allocated for my program. This brings me to my question, which is three-fold:

1. What's preventing me from continuing to read (not write) values outside this range? Is this prevention of reading system-specific? Compiler-specific?

2. If I was clever with how I manipulate my pointer, could I read or write values outside of the scope of my program (without, obviously, having direct/normal access to these values)?

3. I'm running all of this on a virtual machine. Can I access read/write memory values on my host machine? (If (2) is a no, then this is a no as well).

Note that I'm running g++ 5.3.1, no c++11, on a ubuntu virtualbox with a windows host machine.

Also, I recognize this question could be considered a security issue (reading/writing memory). I'm certainly not intending anything malicious, but if this is a problem, let me know and I will be glad to close the question.

EDIT: The following question appears related and interesting: Accessing outside the memory allocated by the program. (Accessing other app's memory) There doesn't seem to be a consensus on whether or not a program can read outside of it's virtual memory space though.

Answer 1

1. The operating system. The operating system mapped only a certain range of virtual addresses onto physical addresses. After reaching the end of the mapped virtual address ranges, an attempt to access a nonexistent virtual address generates a SIGSEGV .

2. No. At least not on any modern operating system.

3. Not unless you can find a bug in your host operating system's virtual machine, and exploit it.