Merge Linked List segmentation fault (core dumped) error please help

Arthur13 (7)
This is my code. Everytime I attempted to run it it gives me a segmentation fault and the core dump error. Please help!!!

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#include<iostream>
using namespace std;

struct node
{
  int data;
  struct node *next;
};

class list
{
  struct node *start;
  public:
	void create();
	void show();
	void merge(list,list);
};

void list::create()
{
  struct node *nxt_node, *pre_node;
  int value, no, i;
  start=nxt_node=pre_node=NULL;
  cout<<"\nHow many nodes:";
  cin>>no;
  cout<<"Enter "<<no<<" Eelements:";
  for(i=1;i<=no;i++)
  {
	cin>>value;
	nxt_node=new node;
	nxt_node->data=value;
	nxt_node->next=NULL;

	if (start==NULL)
	start=nxt_node;
	else
	pre_node->next=nxt_node;
	pre_node=nxt_node;
  }
  	
}

void list::show()
{
  struct node *ptr=start;

  
  while(ptr!=NULL)
  {
	cout<<ptr->data<<"->'";
	ptr=ptr->next;
  }
}

void list::merge(list l1,list l2)
{
  struct node *nxt_node, *pre_node, *pptr, *qptr;
  int dat;

  pptr=l1.start;
  qptr=l2.start;
  start=nxt_node=pre_node=NULL;
  while(pptr!=NULL && qptr!=NULL)
  {
	if(pptr->data<=qptr->data)
	{
		dat=pptr->data;
		pptr=pptr->next;
	}
	else
	{
		dat=qptr->data;
		qptr=qptr->next;
	}
  	
	nxt_node=new node;
	nxt_node->data=dat;
	nxt_node->next=NULL;
	
	if(start==NULL)
	{
	  start=nxt_node;
	}	
	else
	{
	  pre_node->next=nxt_node;
	  pre_node=nxt_node;
	}
  }
  if(pptr==NULL)
  {
    while(qptr!=NULL)
    {
	nxt_node=new node;
	nxt_node->data=qptr->data;
	nxt_node->next=NULL;
	
	if(start==NULL)
	   start=nxt_node;
	else
	   pre_node->next=nxt_node;
	   pre_node=nxt_node;
	   qptr=qptr->next;	
    }
  }
  else if (qptr==NULL)
  {
	while(pptr!=NULL)
	{
		nxt_node=new node;
		nxt_node->data=pptr->data;
		nxt_node->next=NULL;
		if(start==NULL)
		   start=nxt_node;
		else
		   pre_node->next=nxt_node;
		   pre_node=nxt_node;
		   pptr=pptr->next;
	}
  }
  
  return;
}

int main()
{
	list l1,l2,l3;
	cout<<"Enter the first list in ascending order.";
	l1.create();
	cout<<"Enter the Second list in ascending order.";
	l2.create();
	cout<<"The first list is:";
	l1.show();
	cout<<"The second list is"<<endl;
	l2.show();
	l3.merge(l1,l2);
	l3.show();


return (0);
}

ne555 (4978)
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//list::merge
start=nxt_node=pre_node=NULL;
//...
	if(start==NULL)
	{
	  start=nxt_node;
	}	
	else
	{
	  pre_node->next=nxt_node; //crash
	  pre_node=nxt_node;
	}
Arthur13 (7)
How would you fix this crash? I have been sitting infront of this thing for to long. My brain is fried.
Smac89 (506)
Have you ever heard of gdb?

(gdb) run
Starting program: /student/mac/JD/Desktop/data 
Reading symbols for shared libraries ++......................... done
Enter the first list in ascending order.
How many nodes:4
Enter 4 Eelements:4 5 6 7
Enter the Second list in ascending order.
How many nodes:4
Enter 4 Eelements:4 5 6 7
The first list is:4->'5->'6->'7->'The second list is

Program received signal EXC_BAD_ACCESS, Could not access memory.
Reason: KERN_INVALID_ADDRESS at address: 0x0000000000000008
0x00000001000009ff in list::merge (this=0x7fff5fbff9f0, l1={start = 0x100100a30}, l2={start = 0x100100a70}) at Node.cc:86
86	            pre_node->next=nxt_node;
Last edited on
Arthur13 (7)
I have not. But I already know where the crash occurs from ne555 answer. But I need to find out how to correct it.
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