Seg Fault at return statement in function

Question

My program is supposed to convert a prompt from infix to postfix. So far, through a debugger and various other methods, I have located the exact point at which I segfault, but don't understand why.

Here's my code:

Here's itop.h:

using namespace std;
#include <cstdlib>
#include <iostream>

class sNode{
    public:
       char data;
       sNode *next;
};

class stack{
    public:
        sNode *head;
        void push (char);
        sNode pop();
        int rank(char);
        stack()
        {
        cout << "Initiliazing stack." << endl;
        }
};

This is my itop.cpp file:

    #include "itop.h"

    void stack::push (char a)
    {
            // cout << "Pushing " << a << endl;
            sNode *sn;
            sn = new sNode;
            sn->data = a;
            sn->next = head;
            head = sn;
    }

    sNode stack::pop()
    {
            // cout << "Popping stack." << endl;
            sNode *sn;
            sn = head;
            head = head->next;
            return *sn;
    }

    int stack::rank(char x)
    {
            int num = 0;
            // cout << "Checking rank." << endl;
            if(x == '\0')
            {
                    num = 1;
                    // cout << "Checking for null" << endl;
                    return num;
            }
            else if(x == '+' || x == '-')
            {
                    num = 2;
                    // cout << "Checking if + or -" << endl;
                    return num;
                    // cout << "After return." << endl;
            }
            else if(x == '*' || x == '/')
            {
                    num = 3;
                    // cout << "Checking for * or /" << endl;
                    return num;
            }
            else
                    cout << "Error! Input not valid!" << endl;
    }

And here's main.cpp:

using namespace std;
#include <iostream>
#include <cstdlib>
#include <cstring>
#include "itop.h"

int main()
{
char *temp1;            //Instantiating variables.
char *temp2;
temp1 = new char[20];
temp2 = new char [20];
stack s;
do              //Checking commands.
{
    cout << "infix_to_postfix> ";
    cin >> temp1;
    if(strcmp(temp1, "quit") == 0)
    {
        return 0;
    }
    if(strcmp(temp1, "convert") != 0)
    {
        cout << "Error! Invalid command." << endl;
    }
    cin >> temp2;
    if(strcmp(temp1, "convert") == 0)
    {
        for(int i=0; i<sizeof(temp2); i++)
        {
            if(isdigit(temp2[i]))
            {
                cout << atoi(&temp2[i]);
            }
            else if(s.rank(temp2[i]) < s.rank(s.head->data))
            {
                sNode temp = s.pop();
                cout << temp.data;
            }
            else
            {
                s.push(temp2[i]);
            }
        }
    }
    else
    {
        cout << "Error! Command not supported." << endl;
    }
}while(strcmp(temp1, "quit") != 0);

return 0;
}

The function is called at

else if(s.rank(temp2[i]) < s.rank(s.head->data))

And the problem is in here:

            else if(x == '+' || x == '-')
            {
                    num = 2;
                    // cout << "Checking if + or -" << endl;
                    return num;
                    // cout << "After return." << endl;
            }

Specifically right before return num, I get "Segmentation fault (core dumped)" error message. I have used gdb and all I know is that right after "Checking if + or -" I see "$1 = 2". I'm not quite sure what that means, but it is what I want to return.

Thank you for your help.

Answer 1

There are many mistakes in your code. Your stack implementation is wrong. push() for example only sets head over and over again. This results in your stack class being able to ever only hold one element. next is never set to anything, so it contains random garbage. Further down, you have this:

for(int i=0; i<sizeof(temp2); i++)

sizeof(temp2) does not give you the amount of characters of the string temp2 points to. It gives you the size of the pointer temp2 itself. Furthermore, you end up reading s.head from an empty stack, which will be a pointer to random garbage. At that point, all bets are off of course. You can't expect anything else than a crash and burn.

Answer 2

Fix 1:Write a proper constructor.

    stack()
    {
    head=NULL;
    cout << "Initiliazing stack." << endl;
    } 

Fix 2:Write an extra method to check if stack is empty.

int stack::empty()
{
    if(head == NULL)
      return true;
    else
      return false;
}

Fix 3:Check if stack empty before using the stack data.

else if(!s.empty() && s.rank(temp2[i]) < s.rank(s.head->data))
{
 ...
}

Fix 4: Fix the rest of the code logic.