Accessing an array out of bounds gives no error, why?


Question

I am assigning values in a C++ program out of the bounds like this:

#include <iostream>
using namespace std;
int main()
{
    int array[2];
    array[0] = 1;
    array[1] = 2;
    array[3] = 3;
    array[4] = 4;
    cout << array[3] << endl;
    cout << array[4] << endl;
    return 0;
}

The program prints 3 and 4. It should be not be possible. I am using g++ 4.3.3

Here is compile and run command

$ g++ -W -Wall errorRange.cpp -o errorRange
$ ./errorRange
3
4

Only when assigning array[3000]=3000 does it give me a segmentation fault.

If gcc doesnt check for array bounds, how can I be sure if my program is correct, as it can lead to some serious issues later?

I replaced above code with

vector<int> vint(2);
vint[0] = 0;
vint[1] = 1;
vint[2] = 2;
vint[5] = 5;
cout << vint[2] << endl;
cout << vint[5] << endl;

and this one is also produces no error.

1
151
8/3/2017 3:23:28 PM

Accepted Answer

Welcome to every C/C++ programmer's bestest friend: Undefined Behavior.

There is a lot that is not specified by the language standard, for a variety of reasons. This is one of them.

In general, whenever you encounter undefined behavior, anything might happen. The application may crash, it may freeze, it may eject your CD-ROM drive or make demons come out of your nose. It may format your harddrive or email all your porn to your grandmother.

It may even, if you are really unlucky, appear to work correctly.

The language simply says what should happen if you access the elements within the bounds of an array. It is left undefined what happens if you go out of bounds. It might seem to work today, on your compiler, but it is not legal C or C++, and there is no guarantee that it'll still work the next time you run the program. Or that it hasn't overwritten essential data even now, and you just haven't encountered the problems, that it is going to cause — yet.

As for why there is no bounds checking, there are a couple aspects to the answer:

  • An array is a leftover from C. C arrays are about as primitive as you can get. Just a sequence of elements with contiguous addresses. There is no bounds checking because it is simply exposing raw memory. Implementing a robust bounds-checking mechanism would have been almost impossible in C.
  • In C++, bounds-checking is possible on class types. But an array is still the plain old C-compatible one. It is not a class. Further, C++ is also built on another rule which makes bounds-checking non-ideal. The C++ guiding principle is "you don't pay for what you don't use". If your code is correct, you don't need bounds-checking, and you shouldn't be forced to pay for the overhead of runtime bounds-checking.
  • So C++ offers the std::vector class template, which allows both. operator[] is designed to be efficient. The language standard does not require that it performs bounds checking (although it does not forbid it either). A vector also has the at() member function which is guaranteed to perform bounds-checking. So in C++, you get the best of both worlds if you use a vector. You get array-like performance without bounds-checking, and you get the ability to use bounds-checked access when you want it.
313
3/22/2017 12:40:50 AM

Using g++, you can add the command line option: -fstack-protector-all.

On your example it resulted in the following:

> g++ -o t -fstack-protector-all t.cc
> ./t
3
4
/bin/bash: line 1: 15450 Segmentation fault      ./t

It doesn't really help you find or solve the problem, but at least the segfault will let you know that something is wrong.


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