Linked list program facing segmentation fault

Question

I was writing a C++ program to implement a linked list. On compilation it's not giving any error but in the output windows it goes blank and program ended with

list1.exe has encountered a problem and needs to close.

Debugger response: Program received signal SIGSEGV, Segmentation fault.

Maybe it's because of memory leakage, but I'm not able to figure out the exact bug and how can we fix that. Please what's wrong in the prog and what should be fixed?

Below is the code

  //Program to implement linked list

  #include <iostream>
  #include <cstdlib>

  using namespace std;

  class Node
  {
      int data;
      Node * next;

   public:
      Node (){}
      int getdata(){return data ;}
      void setdata(int a){data=a;}
      void setnext(Node* c){next=c;}
      Node* getnext(){return next;}
  };

  class linkedlist
  {
      Node* head;

  public:
      linkedlist(){head=NULL;}
      void print ();
      void push_back(int data);
  };

  void linkedlist::push_back(int data)
  {
      Node* newnode= new Node();
      if(newnode!=NULL)
      {
          newnode->setdata(data);
          newnode->setnext(NULL);
      }
      Node* ptr= head;

      if(ptr==NULL) 
          {head=newnode;}
      while ((ptr->getnext())!=NULL)
      {
          ptr=ptr->getnext();
      }
      ptr->setnext(newnode);
  }

  void linkedlist::print()
  {
      Node* ptr=head;
      if(ptr==NULL)
          {cout<<"null"; return;}

      while(ptr!=NULL)
      {
          cout<<(ptr->getdata())<<" ";
          ptr=ptr->getnext();
      }
  }

  int main()
  {
     linkedlist list;
      list.push_back(30);
      list.push_back(35);
      list.print();
      return 0;
  }

hi luchian..i am sorry actually i am new in this..can you please tell how should i use a debugger..i am developing this in COdeblocks IDE... — assasinC
– assasinC, Commented Jul 10, 2012 at 6:48

Asherah · Accepted Answer · 2012-07-09 12:50:13Z

4

The main issue is here:

if(ptr==NULL) {head=newnode;}
while ((ptr->getnext())!=NULL)
{
    ptr=ptr->getnext();
}
ptr->setnext(newnode);

There's probably meant to be a return; in the if (ptr == NULL) part; as it stands, it sets head = newnode, but then continues to try to access ptr->getnext(), which causes the segfault.

Some answers have suggested setting ptr = head = newnode, but note that the bottom line is ptr->setnext(newnode)—this would cause head->getnext() == head. Infinite list!

For your interest, here's your code:

Formatted nice;
Cleaned up not to use using namespace std; (see the C++ FAQ on this);
Takes advantage of const-correctness;
etc.

Enjoy!

#include <iostream>
#include <stdexcept>

class Node {
    int data;
    Node *next;

public:
    Node(): next(NULL) {}

    int getdata() const {
        return data;
    }

    void setdata(int a) {
        data = a;
    }

    Node *getnext() const {
        return next;
    }

    void setnext(Node *c) {
        next = c;
    }
};

class linkedlist {
    Node* head;

public:
    linkedlist(): head(NULL) {} 

    void print() const {
        Node *ptr = head;

        if (ptr == NULL) {
            std::cout << "null";
            return;
        }

        while (ptr != NULL) {
            std::cout << ptr->getdata() << " ";
            ptr = ptr->getnext();
        }
    }

    void push_back(int data) {
        Node *newnode = new Node();

        if (newnode == NULL) {
            throw std::runtime_error("out of memory!");
        }

        newnode->setdata(data);

        Node *ptr = head;

        if (ptr == NULL) {
            head = newnode;
            return;
        }

        while ((ptr->getnext()) != NULL) {
            ptr = ptr->getnext();
        }

        ptr->setnext(newnode);
    }
};

int main() {
    linkedlist list;
    list.push_back(30);
    list.push_back(35);
    list.print();
    return 0;
}

edited Jul 9, 2012 at 12:50

answered Jul 9, 2012 at 12:37

Asherah

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2 Comments

ereOn Over a year ago

+1: Because you put a lot of efforts in your answer to a very poorly written question.

assasinC Over a year ago

@len: thanks alot man!!! yes you were right! i forgot put a return statement..but i didn't change anything else..and prog is working fine..grt man!!!

Igor R. · Accepted Answer · 2012-07-09 12:30:38Z

0

In the following line: while ((ptr->getnext())!=NULL) ptr is NULL

answered Jul 9, 2012 at 12:30

Igor R.

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1 Comment

assasinC Over a year ago

@Len: One more help please...if i want to use this prog in future also...like making a header file and then including that file in future progs..can you please write and tell steps i need to do to achieve this..would appreciate a lot!

Synxis · Accepted Answer · 2012-07-09 12:48:17Z

0

The push_back code is incorrect, and I've seen some other parts of your code that can be improved:

#include <iostream>
#include<cstdlib>

using namespace std;

class Node
{
      int data;
      Node * next;

   public:
      Node(int d = 0) : data(d), next(NULL) {}

      int getdata() { return data; }
      void setdata(int a) { data = a; }

      void setnext(Node* c) { next = c; }
      Node* getnext() { return next; }
};

class linkedlist
{
      Node* head;

   public:
      linkedlist() : head(NULL) {}
      void print ();
      void push_back(int data);
};

void linkedlist::push_back(int data)
{
   Node* newnode = new Node(data);

   if(head == NULL)
   {
      head = newnode;
   }
   else
   {
      Node* last = head;
      while(last->getnext() != NULL)
         last = last->getnext();
      last->setnext(newnode);
   }
}

void linkedlist::print()
{
   Node* ptr = head;
   if(!ptr)
   {
      cout << "null";
      return;
   }

   while(ptr != NULL)
   {
      cout << ptr->getdata() << " ";
      ptr=ptr->getnext();
   }
}

int main()
{
   linkedlist list;
   list.push_back(30);
   list.push_back(35);
   list.print();
   return 0;
}

There are still some points to be improved...

edited Jul 9, 2012 at 12:48

answered Jul 9, 2012 at 12:33

Synxis

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1 Comment

Asherah Over a year ago

Have you tried this? As with @Razvan's answer, yours will cause an infinite loop when it ties head->next to itself.

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