Question

This is a class assignment to do ordered insertion into a sorted linked list. I’ve avoided accessing null pointers, and I’ve added some lines of debugging output to narrow the seg fault down to the conditional statement of the while loop itself.

I can add a node to the empty list and add nodes with smaller keys to the head of the list, but I run into a seg fault when adding a key to the end of a non-empty list. As displayed by the debug output, the problematic while loop cycles successfully until it sees cursor->next == NULL. That’s when it throws the seg fault. I’m not trying to access the empty pointer as far as I can tell. I’m really pretty stumped and would appreciate any help. Thanks!

I’ve denoted the troublesome line in the code below.

// Headers
#include <iostream>
#include <iomanip>
#include <string>
using namespace std;

struct Node {
  int id;
  string initials;
  double score;
  Node* next;
};
const int ESC = -9999;
const int ID_LIMIT = 1000000000;

int main(){  
  // initialize variables
  int temp_id = 0;  // temporary input
  Node* Start=NULL; // first node in linked list

  // get input until escape value is entered or input stream fails
  while ( temp_id != ESC && !cin.fail() ){
    cout << "\nPlease enter the student id, or "
         << ESC << " to finish inputing: ";
    cin >> temp_id;
    if ( temp_id >=0 && temp_id < ID_LIMIT ) { 
      // if valid input, continue input & create node
      string temp_inits;
      double temp_score;
      cout << "Please enter the student initials: " ;
      cin >> temp_inits;
      cout << "Please enter the student's test score: " ;
      cin >> temp_score;

      // create new node with input values
      Node* temp = new Node;
      temp->id = temp_id;
      temp->initials = temp_inits;
      temp->score = temp_score;
      temp->next = NULL;

      // TASK 4: SORTED INPUT
      if ( Start == NULL ) {  // if first node to be input
        Start = temp;         // point the first pointer to it
      }else{                  // otherwise
        if( temp->id < Start->id ){  // if new node should go before Start,
          temp->next = Start;        // move start after temp
          Start = temp;              // and reassign Start
        }else{
          Node* cursor = Start; // parse list for proper place or end
          cout << "\nDEBUG: while ( cursor-id:" << cursor->id ;
          cout << " < temp-id:" << temp->id;
          cout << " && cursor-next:" << cursor->next << " != NULL )";
// I THINK THE NEXT LINE IS THE CULPRIT
          while ( cursor->id < temp->id && cursor->next != NULL ) {
            cout << "\nDEBUG: { old cursor:" << cursor->id ;
            cursor = cursor->next;
            cout << ", new cursor:" << cursor->id << " }";
            cout << "\nDEBUG: while ( cursor-id:" << cursor->id ;
            cout << " < temp-id:" << temp->id;
            cout << " && cursor-next:" << cursor->next << " != NULL )";
          }
          cout << "\nDEBUG: insert temp-id:" << temp->id 
               << " after cursor-id:" << cursor->id ;
          temp->next = cursor->next;
          cursor->next = temp;  // inject new node into list.
          cursor = temp->next;
          cout << " before id " << cursor->id;
        }
      }
      cout << "Node with id=" << temp->id << ", initials=" << temp->initials 
           << ", and test score=" << temp->score << " added to the list.\n";

    }else{ // if invalid input & not escape value, print error
      if ( temp_id != ESC ) cout << "!!! Invalid input !!!\n";
    }   
  }

  return 0;
}

Output:

Please enter the student id, or -9999 to finish inputing: 654
Please enter the student initials: abc
Please enter the student's test score: 99.9
Node with id=654, initials=abc, and test score=99.9 added to the list.

Please enter the student id, or -9999 to finish inputing: 312
Please enter the student initials: qwe
Please enter the student's test score: 54.8
Node with id=312, initials=qwe, and test score=54.8 added to the list.

Please enter the student id, or -9999 to finish inputing: 987
Please enter the student initials: rty
Please enter the student's test score: 87.5

DEBUG: while ( cursor-id:312 < temp-id:987 && cursor-next:0x1c26040 != NULL )
DEBUG: { old cursor:312, new cursor:654 }
DEBUG: while ( cursor-id:654 < temp-id:987 && cursor-next:0 != NULL )
Segmentation fault

I have also tried the loop with ( ... cursor->next != 0 ) and ( ... cursor->next )

Answer 1

The problem is after you break out of the while() loop:

  1>    temp->next = cursor->next;
  2>    cursor->next = temp;  // inject new node into list.
  3>    cursor = temp->next;
  4>    cout << " before id " << cursor->id;

You reach the above lines with cursor->next == null (end of the list), and
temp->next is also null. Line 1 therefore does nothing in this case. Line 2 copies temp’s address into cursor->next. At this point we have:

temp: temp’s original address, temp->next == NULL
cursor: cursor’s original address, cursor->next == temp’s original address

Then you copy temp->next into cursor (uhoh!), but temp->next is null, so cursor now is null.
So, when you go to execute line 4 and get cursor->id, it tries to read “null->id” and crashes.