Case not executing - c++

Question

I have the following code. The problem is that if I choose an option, for example option #2 to sort the elements the program it's closing and I don't understand why... I'm using Eclipse IDE. The functions do the following:
a) A function to insert a data in a list that does not allow duplicate values (if the value does
not exist, pasting takes place at the beginning of the list). Payload elements to be integer.
b) A function to insert a data in a list so the list remains sorted ascending. Payload elements
to be integer
c) A function to determine the number of elements divisible by a z value received as
parameter.
d) A function to determine the number of elements greater than the information from the
first node of the list.
e) A function that determines the number of occurrences of a given value in the list.
#include<iostream>
using namespace std;
char menu()
{ char choice;
cout<<"********Choose an option********* \n" ;
cout<<"1. duplicate check\n";
cout<<"2. Sort the elements \n";
cout<<"3. Determine elements divisible by a value given Z \n";
cout<<"4. Determine the number of elements greater than the first node\n";
cout<<"5. Determine the number of occurrences in a list\n";
cout<<"6. -----Exit-----\n";
cin>>choice;
return choice;
}
struct node
{
int value;
node* prev;
node* next;
};
typedef node* pnode;
int nro=0;
void showlist(pnode start, int div)
{
nro=0;
if(start!=NULL)
{
while(start!=NULL)
{
if(start->value%div==0)
{
nro++;
cout<<start->value<<" ";
}
start=start->next;
}
}
}
void checkvalue(pnode start, int nr)
{
nro=0;
if(start!=NULL)
{
while(start!=NULL)
{
if(start->value==nr)
{
nro++;
}
start=start->next;
}
}
}
bool checkduplicates(pnode start, int val)
{
if(start==NULL) return false;
else
{
while(start!=NULL)
{
if(start->value==val) return true;
start=start->next;
return false;
}
}
}
void sort(pnode start)
{
pnode p=start;
pnode q=NULL;
while(p->next!=NULL)
{
q=p->next;
while(q!=NULL)
{
if(p->value > q->value)
{
int aux;
aux=p->value;
p->value=q->value;
q->value=aux;
}
q=q->next;
}
p=p->next;
}
}
void showbig(pnode start)
{
pnode q=start->next;
while(q!=NULL)
{
if(q->value > start->value)
{
cout<<q->value<<" ";
}
q=q->next;
}
}
int main()
{
pnode start=NULL;
pnode end=NULL;
pnode aux=NULL;
char choice;
int number;
do{
choice=menu();
switch(choice)
{
case '1':
int z;
cout<<"Value: ";
cin>>z;
if(start==NULL)
{
start = new node;
start->value=z;
start->next=NULL;
start->prev=NULL;
end=start;
aux=start;
}
else
{
aux= new node;
aux->value=z;
aux->next=start;
start->prev=aux;
aux->prev=NULL;
start=aux;
}
if (!checkduplicates(start,z))
{cout<<"Duplicate value. Cannot insert.\n";
break;}
break;
case '2':
sort(start);
break;
case '3':
cout<<"Value: ";
cin>>z;
showlist(start,z);
if(nro==0) cout<<"No values found.\n";
else cout<<"\n";
break;
case '4':
showbig(start);
cout<<"\n";
break;
case '5':
cout<<"Value: ";
cin>>z;
checkvalue(start,z);
if(nro==0) cout<<"No values found.\n";
else cout<<nro<<"\n";
break;
default: cout<<"Exit \n";
}
} while (choice !='6');
return 0;
}

Answer 1

sort(start);
is failing because start is NULL which means p is NULL
pnode p = start;
And you try to dereference a NULL pointer here
while (p->next != NULL)