Stack smashing detected : terminated Aborted (core dumped)

Question

My array is of size 3x3 means i have values of index from 0 to 2 only. but when i traverse using for loop then why it is picking value of a[3][-3] on value of a[2][0] ???

and what is that error when i try a[3][3] it should give garbage value so why this error

*** stack smashing detected ***: terminated

#include <bits/stdc++.h>
#include <iostream>
#define vi vector<int>
using namespace std;

int main()
{
    char a[3][3];
    int pos, row, col;
    cin >> pos;
    memset(a, 48, sizeof(a));
    //row = pos / 3;
    //col = pos % 3 - 1;
    a[3][3] = 'X';
    //a[3][-3] = 'X';
    for (char *b : a)
    {
        for (size_t i = 0; i < 3; i++)
        {
            cout << b[i] << " ";
        }
        cout << endl;
    }
}

for a[3][-3] result will be output:

0 0 0
0 0 0
X 0 0

for a[3][3] result will be output:

0 0 0
0 0 0
0 0 0
*** stack smashing detected ***: terminated
Aborted (core dumped)

You said it yourself: index values in an array of size 3 can only range between 0 and 2. So how can you possibly expect `a[3][3] = 'X'` to be valid? The result of doing something like that is *undefined behavior* - you *might* get a garbage value, but you might also cause a crash, or data corruption, or worse. Just don't do it. — Nate Eldredge, Jun 24 '20 at 13:35
"*it should give garbage value*" No, it's undefined behavior. The behavior you're seeing here is probably the best that could happen. The worst is when it seems like it works until you release your code to the world and *then* it blows up! — Kevin, Jun 24 '20 at 13:37
While `a[3][-3]` has undefined behaviour, there is an explanation for the result: it is three `char`s backwards from the non-existent first element of the non-existent fourth array, which happens to be the first element of the third array. — molbdnilo, Jun 24 '20 at 13:58

cigien · Accepted Answer · 2020-06-24T13:52:51.417

3

For an array like a[3][3], indexing into a with [3][3], or [3][-3] invokes undefined behavior. Anything can happen, including printing out garbage values. The program is not even guaranteed to print a value, it could simply crash.

The only valid indexes for either bound is 0, 1, and 2.

Note that even if you index into a correctly, reading from a is also undefined behavior, since you have not initialized a. You can do that like this:

char a[3][3]{};

and now reading from, say a[2][0] is fine.

edited Jun 24 '20 at 13:52

answered Jun 24 '20 at 13:34

cigien

57,834
11
73
112

it will always give value of a[3][-3] when traverse on a[2][0] and also a[3][-1] on a[2][2] always why?? – Kuldeep Soni Jun 24 '20 at 13:44
I'm not sure what you mean. Are you talking about the `for` loops? Does this happen even without calls to `a[3][3]` or `a[3][-3]`? – cigien Jun 24 '20 at 13:48
yes it is happen even without for loop, or direct calling of a[2][0] it will always pick value of a[3][-3] – Kuldeep Soni Jun 24 '20 at 13:50
Updated the answer, you need to initialize `a`. There is no `a[3][-3]`, so ignore whatever that happens to print. – cigien Jun 24 '20 at 13:53
`a[2][0]` is `2*3 + 0 = 6` bytes into `a` and `a[3][-3]` is also `3*3 - 3 = 6` bytes into `a`. – Kevin Jun 24 '20 at 13:54
@Kevin I don't think that's correct. You can't do arithmetic like that to figure out the offset. Each index needs to be valid. – cigien Jun 24 '20 at 13:56
i initialized that array with your given code then also it is giving same result – Kuldeep Soni Jun 24 '20 at 13:58
@cigien it's likely still undefined behavior (I think that's worth a separate SO question). I'm just explaining why they're probably giving the same value. – Kevin Jun 24 '20 at 13:58
@Kevin Aah, I see, that's ok I guess. – cigien Jun 24 '20 at 13:59
No problem, happy to help. Consider [accepting](https://stackoverflow.com/help/someone-answers) the answer if it solves your problem. – cigien Jun 24 '20 at 14:00
@cigien Thankyou so much for helping – Kuldeep Soni Jun 24 '20 at 14:02
But why it is giving me that error on a[3][3] *** stack smashing detected ***: terminated – Kuldeep Soni Jun 24 '20 at 14:11
1

As I said, it's UB, so *anything* can happen. – cigien Jun 24 '20 at 14:16

score 0 · Answer 2 · answered Jun 24 '20 at 14:07

0

This is because of wrong arithmetic as @Kevin answer in comment that

a[2][0] = 2*3 + 0 = 6 bytes

and also

a[3][-3] = 3*3 - 3 = 6bytes

so both pointing to same value.

answered Jun 24 '20 at 14:07

Kuldeep Soni

43
1
6

Stack smashing detected : terminated Aborted (core dumped)

2 Answers2