Segmentation fault after main exit - c++

Question

I Have tried executing the following code in gdb, but with gdb I don`t see any segmentation fault but without gdb If I run the following code in standalone mode segmentation fault occurs. The code is related to range sum query implemented using segment tree.
#include <iostream>
#include <vector>
using namespace std;
class segmentTree
{
private:
vector<int> a;
void constructUtil(vector<int>& , int , int , int );
int queryUtil(int , int , int , int , int );
void updateUtil(int , int , int , int , int );
public:
segmentTree(vector<int> );
int query(int , int );
void update(int , int );
~segmentTree();
};
segmentTree::segmentTree(vector<int> v)
{
int n = v.size();
a.resize((2*n) - 1);
constructUtil(v, 0 , n - 1, 0);
}
segmentTree::~segmentTree()
{
a.clear();
}
void segmentTree::constructUtil(vector<int>& v, int start, int end, int i)
{
if(start == end)
{
a[i] = v[start];
}
else
{
int mid = start + ((end - start) >> 1);
constructUtil(v, start, mid, ((2*i) + 1));
constructUtil(v, mid + 1, end, ((2*i) + 2));
a[i] = a[(2*i) + 1] + a[(2*i) + 2];
}
}
int segmentTree::queryUtil(int ss, int se, int rs, int re, int i)
{
if(se < rs || re < ss)
{
return 0;
}
else if(rs <= ss && se <= re)
{
return a[i];
}
else
{
int sm = ss + ((se - ss) >> 1);
return queryUtil(ss, sm, rs, re, 2*i + 1) + queryUtil(sm + 1, se, rs, re, 2*i + 2);
}
}
int segmentTree::query(int l, int r)
{
int n = ((a.size() + 1) >> 1);
if(l < 0 || r > n-1)
{
return 0;
}
return queryUtil(0, n-1, l , r, 0);
}
void segmentTree::updateUtil(int ss, int se, int i, int si, int x)
{
if(ss > i || se < i)
{
return ;
}
else if(ss == se)
{
a[si] = x;
}
else
{
int sm = ss + ((se - ss) >> 1);
updateUtil(ss, sm, i, (2*si) + 1, x);
updateUtil(sm + 1, se, i, (2*si) + 2, x);
a[si] = a[(2*si) + 1] + a[(2*si) + 2];
}
}
void segmentTree::update(int i, int x)
{
int n = ((a.size() + 1) >> 1);
if(i < 0 || i > n-1)
{
return ;
}
else
{
updateUtil(0, n-1, i, 0, x);
}
}
int main()
{
int arr[] = {1, 3, 5, 7, 9, 11};
int n = sizeof(arr)/sizeof(arr[0]);
vector<int> v(arr, arr + n);
segmentTree st(v);
// Print sum of values in array from index 1 to 3
cout << "Sum of values in given range = " << st.query(1, 3) << endl;
// Update: set arr[1] = 10 and update corresponding
// segment tree nodes
st.update(1, 10);
// Find sum after the value is updated
cout << "Updated sum of values in given range = " << st.query(1, 3) << endl;
return 0;
}

Answer 1

Consider segmentTree::constructUtil with v.size() == 3. Then in the inital call to constructUtil you have start == 0 and end == 2.
Thus we get mid = 1.
In the second recursive call we are then passing start = 1, end = 2 and i = 2. start != end and so the else is executed.
However in the else block a[(2*i)+2] is accessed (by the way, no need for the parantheses there). This index will be 6.
But if you look at the size of a, it was given as 2*n-1. 2*3-1 = 5, so 6 is clearly out-of-bounds.
I don't know what your intentions with the code are, but that there is undefined behavior. You can easily catch it by either using something like valgrind, by replacing a[...] with a.at(...) for debug purposes, by stepping through the code with gdb and actually following all the variables (there does not need to be a segmentation fault for your program to have undefined behavior) or by entering debug std::cout statements with the variable content everywhere that could cause the issue.