Accessing non-existing array element

Question

int main(int argc, char const *argv[])
{

    int anArray[5];

    anArray[0] = 54;
    anArray[1] = 54;
    anArray[2] = 54;
    anArray[3] = 54;
    anArray[4] = 54;
    anArray[5] = 54;
    anArray[6] = 54;
    anArray[7] = 54;


    printf ("%i\n", anArray[7]);

    return 0;
}

This prints 54.

How does this even work? We say that C arrays are not dynamic. Why should this even compile? or even if it compiles, it should throw a seg fault.

I have defined an array of 5 elements, then I accessed elements 5,6,7. Why is it possible to assign a value to, for example, anArray[5]?

Please note that I have a c++ background and I haven't used this kind of array for a long time.

"undefined behaviour"

Mitch Wheat
– Mitch Wheat

2013-12-07 11:49:50 +00:00
Commented Dec 7, 2013 at 11:49 — Mitch Wheat
– Mitch Wheat, Commented Dec 7, 2013 at 11:49

John Zwinck · Accepted Answer · 2013-12-07 11:51:39Z

3

You are scribbling into memory that you don't own, so anything could happen. You got lucky and the computer let you write and then read the value in that location. But it's just luck: the behavior is undefined.

Note that the exact same thing applies to C++ (since you mentioned it), not only with C-style arrays but also with std::vector::operator[] and std::array in C++11. In C++ you can use vec.at(idx) instead of vec[idx] to do bounds checking always.

answered Dec 7, 2013 at 11:51

John Zwinck

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2 Comments

khajvah Over a year ago

Thanks for the answer but is it possible to bound-check like .at() does in case of std::vector or force the compiler to throw an error on run/compile time?

John Zwinck Over a year ago

In C? Not exactly. You could try writing a function that does it, but given that arrays degrade to pointers, you'd have a hard time knowing the true size of the array in all cases to check it. I suggest using tools like valgrind to catch such errors during testing.

Luchian Grigore · Accepted Answer · 2013-12-07 11:51:55Z

2

The language itself doesn't say the runtime or the compiler has to check you're actually accessing elements inside the bounds of the array. The compiler could emit a warning, but that's it. You are responsible for accessing valid elements. Not doing so results in undefined behavior, which means anything can happen, including appearing to work.

answered Dec 7, 2013 at 11:51

Luchian Grigore

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Comments

Octoshape · Accepted Answer · 2013-12-07 11:51:27Z

0

You're basically reading into memory to places where you don't know what's there. This can be a useful thing in C (if you really know what you're doing) but also can get you hours of frustrating debugging because it is undefined behaviour what's going to happen there.

From wikipedia:

Many programming languages, such as C, never perform automatic bounds checking to raise speed. However, this leaves many off-by-one errors and buffer overflows uncaught. Many programmers believe these languages sacrifice too much for rapid execution.

answered Dec 7, 2013 at 11:51

Octoshape

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Comments

Kaustav Ray · Accepted Answer · 2013-12-07 11:54:39Z

0

No compiler error: as no compiler error related issues.

Run time error: because of undefined behavior and you are lucky that the memory location you

were trying to access was free at that time !

answered Dec 7, 2013 at 11:54

Kaustav Ray

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