C++ Segmentation Fault if integer is input after printed, but ok otherwise

Question

The following source code compiles and runs fine

#include <cstdio>
#include <iostream>

using namespace std;

int main() {
    int t;
    cout << t << endl; // the seg fault / rte is for this
    cout << t+10 << endl;
    int mat[0];
    int mat2[t+10];
    for (int i = 0 ; i<100 ; i++) {
        //cout << mat[i] << endl;
    }

    cout << sizeof(mat)/sizeof(int) << endl;
    cout << sizeof(mat2)/sizeof(int) << endl;

    //cin >> t;


    return 0;
}

But if I uncomment the cin >> t it causes a Segmentation Fault.

#include <cstdio>
#include <iostream>

using namespace std;

int main() {
    int t;
    cout << t << endl; // the seg fault / rte is for this
    cout << t+10 << endl;
    int mat[0];
    int mat2[t+10];
    for (int i = 0 ; i<100 ; i++) {
        //cout << mat[i] << endl;
    }

    cout << sizeof(mat)/sizeof(int) << endl;
    cout << sizeof(mat2)/sizeof(int) << endl;

    cin >> t;


    return 0;
}

What is happening in it?

**UB** using a unintialized variable (line 2 and 3 of the main), the second `cout` too. — NetVipeC, Sep 16 '14 at 17:10
Compile with all warnings and debug info (e.g. `g++ -Wall -g`). Then **use a debugger** (`gdb`) — Basile Starynkevitch, Sep 16 '14 at 17:11
Please note that array declarations without constant expressions is a GCC extension. You should use C++ container classes instead. Also, make sure you initialize variables. You also seem to think you have 100 elements in `mat`, when you declare it with 0. — crashmstr, Sep 16 '14 at 17:12
I have intentionally done that since if I do not use the `cin` at the end the array is successfully initialized with `t`, I am trying to understand this behavior. Since this is not an isolated case. The same behavior was observed on a Windows PC. — Unrealist, Sep 16 '14 at 17:15
For a more detailed explanation, see [this question](http://stackoverflow.com/questions/11962457/why-is-using-an-uninitialized-variable-undefined-behavior-in-c). — Beta, Sep 16 '14 at 17:19
@Unrealist The undefined behavior (and the segfault) happens here (to be precise): `int mat2[t+10];` — πάντα ῥεῖ, Sep 16 '14 at 17:26
@Beta, Thanks. A totally unknown face of this language has exposed itself with this to me :) — Unrealist, Sep 16 '14 at 17:36

score 1 · Answer 1 · answered Sep 16 '14 at 18:29

I found the problem

#include 
#include 

using namespace std;

int main() {
    int t;
    cout << t << endl; // the seg fault / rte is for this
    cout << t+10 << endl;
    int mat[0];
    int mat2[t+10]; // <=== HERE IS THE PROBLEM
    for (int i = 0 ; i<100 ; i++) {
        //cout << mat[i] << endl;
    }

    cout << sizeof(mat)/sizeof(int) << endl;
    cout << sizeof(mat2)/sizeof(int) << endl;

    cin >> t;


    return 0;
}

You are creating an array of variable size, there is no problem with this, but you are defining the size based on an uninitialized variable. Uninitialized local variables on C/C++, has NO default value, it can have ANY value it get from stack garbage

To test this, I compiled the program with strange (but possible) values for t to get Segementation Faults:

int main() {
    int t = -22;
    ...
    int mat[0];
    int mat2[t+10];

int main() {
    int t = 0xdeadbeef;
    ...
    int mat[0];
    int mat2[t+10];

You can get any of the values, and it would fail at random, my be using cin >> t changes certain variables on compiler optimization leading to change stack values or something like that, but it is totally unpredictable you can consider it almost random

I'm pretty sure you did not understand the core question. I know it's Undefined behavior and all of that you said is true. But the original question has 2 code samples, the first of which runs without a Segmentation Fault (on multiple environments) while the other does not. Usually problems caused by uninitialized variables persists through the code but in this case the problems seemed to disappear when a certain section of code was missing. Anyway, I found the possible cause here http://stackoverflow.com/questions/11962457/why-is-using-an-uninitialized-variable-undefined-behavior-in-c — Unrealist, Sep 16 '14 at 18:37

C++ Segmentation Fault if integer is input after printed, but ok otherwise

1 Answers1