Question

I’ve been unable to complete my homework because I cannot seem to identify the source of this segmentation fault.

I am trying to add nodes to a linked list from a file. I’ve run multiple tests and have narrowed the problem down quite a bit, but, I don’t know what’s actually creating the problem and therefore I create new problems when I try to change other details.

This is my second course, so, hopefully my code isn’t so bad that it can’t be helped.
Here’s the add method:

    bool OrderedList::add (CustomerNode* newEntry)
{
if (newEntry != 0)
{
    CustomerNode * current;
    CustomerNode * previous = NULL;
    if(!head)
        head = newEntry;
    current = head;
  // initialize "current" & "previous" pointers for list traversal
   while(current && *newEntry < *current) // location not yet found (use short-circuit evaluation)
   {
    // move on to next location to check
    previous = current;
    current = current->getNext();
   }

  // insert node at found location (2 cases: at head or not at head)
  //if previous did not acquire a value, then the newEntry was
  //superior to the first in the list. 
  if(previous = NULL)
    head = newEntry;
  else
  {
    previous->setNext(newEntry); //Previous now needs to point to the newEntry
    newEntry->setNext(current); //and the newEntry points to the value stored in current.
  }
}
    return newEntry != 0;  // success or failure
    }

Okay, there is an overloaded operator< included in the program, outside testing does not indicate a problem with the operator, but I will include it as well:

    bool CustomerNode::operator< (const CustomerNode& op2) const
    {
       bool result = true;
       //Variable to carry & return result
       //Initialize to true, and then:
       if (strcmp(op2.lastName, lastName))
        result = false;

        return result;
       }

And here is the backtrace from gdb:

    #0  0x00401647 in CustomerNode::setNext(CustomerNode*) ()
    #1  0x00401860 in OrderedList::add(CustomerNode*) ()
    #2  0x004012b9 in _fu3___ZSt4cout ()
    #3  0x61007535 in _cygwin_exit_return () from /usr/bin/cygwin1.dll
    #4  0x00000001 in ?? ()
    #5  0x800280e8 in ?? ()
    #6  0x00000000 in ?? ()

This was the result of a lot of work of trying to correct a different segfault, and this one was much more surprising. I have no idea how my setNext method is causing a problem, here it is:

void CustomerNode::setNext (CustomerNode* newNext)
{
    //set next to newNext being passed
    next = newNext;
    return;
}

Thanks in advance, I will be glad to post more of the code if it is necessary to identifying this problem.

Answer 1

It’s

if(previous = NULL)

instead of

if(previous == NULL)

This sets previous to NULL and then enters the else branch:

previous->setNext(newEntry); //Previous now needs to point to the newEntry
newEntry->setNext(current);

resulting in undefined behavior.