why am i getting for the following code AddressSanitizer: SEGV on unknown address error

Question

'''
#include<stack>
using namespace std;
class Solution {
public:
    bool isValid(string s) {
        stack<char> mystack;
        char c;

        for(int i=0;i<= s.length();i++){


            if(s[i]=='(' || s[i]=='{' || s[i]== '[')
                mystack.push(s[i]);
            if(i!=0){
            switch (s[i]){
               case ')':
                   if(mystack.top()== '(')
                       mystack.pop();
                   else return false;
               case '}':
                   if(mystack.top()== '{')
                       mystack.pop();
                   else return false;
               case ']':
                   if (mystack.top()== '[')
                       mystack.pop();
                   else return false;

           }
            }
            if(i==s.length() && !mystack.empty())
                return false;


        } 
        return true;
    }
};'''

I dont know why but Iam getting this error AddressSanitizer: SEGV on unknown address 0x000000000000 (pc 0x000000383dd4 bp 0x7ffc40c5d970 sp 0x7ffc40c5d860 T0) with hint ** address pointing to zero page** can someone please explain why iam getting this and how to resolve this. THANKS in advance!

Answer 1

This code contained so many bugs. But the cause of the error actually was that inside the if condition of switch case

if( mystack.top() )

here I am accessing the top element of my stack without checking if it even exist or not hence the unknown address. It should have been

if( !mystack.empty() && mystack.top())

This was compiler check if the address you are looking for exist or not if it does than only it will try to access the value.

I asked this as a noob programmer and after months of programming I debugged this myself.