Linked List access violation - c++

Question

I get this error during execution.
You see I had a free(temp) before the
cout<<
statements.I removed them.I thought it was because of bad dereferencing turns out its something more to it.
This is my program:
#include "stdafx.h"
#include <stdlib.h>
#include <stdio.h>
#include <math.h>
#include <iostream>
using namespace std;
struct node{
int data;
node* next;
};
node* head=NULL;
node* current=NULL;
void insert_node()
{
int num=0;
cout<<"\nEnter the value of the node to insert\n:";
cin>>num;
if(head==NULL)
{
head=(node*)malloc(sizeof(*head));
//current=(node*)malloc(sizeof(*current));
head->data=num;
head->next=NULL;
current=head;
cout<<"Created list\n";
}
else
{
node* temp=(node*)malloc(sizeof(*temp));
temp->data=num;
temp->next=NULL;
current->next=temp;
current=temp;
cout<<"Added element\n";
free(temp);
cout<<"dereferenced element\n";
}
}
void delete_node()
{
if(head!=NULL && head->next==NULL )//only one node
{
current=head=NULL;
cout<<"Deleted Head\n";
}
else if(head!=NULL && head->next!=NULL)//>= 2 nodes
{
node* temp;
//temp=NULL;
temp=head;
while(temp->next!=current)
{
temp=temp->next;
}
temp->next=NULL;
current=temp;
cout<<"Deleted last element\n";
// free(temp);
cout<<"Dereferenced temp\n";
}
else
{
cout<<"delete was not performed";
}
}
void list_linked_list()
{
node* temp=(node*)malloc(sizeof(* temp));
temp=head;
while(temp!=NULL)
{
cout<<temp->data<<"->";
temp=temp->next;
}
cout<<"displayed list\n";
//free(temp);
cout<<"dereferenced temp";
}
void search_node()
{
cout<<"\nenter a number to search";
int search=0,found=0;
cin>>search;
node* temp=(node*)malloc(sizeof(* temp));
temp=head;
while(temp!=NULL)
{
if(temp->data==search)
found=1;
}
if(found==1)
cout<<"found\n";
else
cout<<"not found\n";
//free(temp);
cout<<"dereferenced temp";
}
void main()
{
int n=0;
k:
cout<<"Linked List operations: \n1. insert \n2. delete \n3. search\n 4. view List \n5. Exit";
cin>>n;
switch(n)
{
case 1: insert_node();break;
case 2: delete_node();break;
case 3: search_node();break;
case 4: list_linked_list();break;
case 5: exit(0);break;
default: cout<<" Please enter valid number between 1 and 5";
break;
}
goto k;
}
I dont think i misunderstood linked list concept.
Im pretty clear on it.I think there a mistake with the pointer.
Thank you.
EDIT: NEW CODE:
struct node{
int data;
struct node* next;
};
struct node* head=NULL;
struct node* current=NULL;
void insert_node()
{
int num=0;
cout<<"\nEnter the value of the node to insert\n:";
cin>>num;
if(head==NULL)
{
head->data=num;
head->next=NULL;
current=head;
cout<<"Created list\n";
}
else
{
struct node* temp=(node*)malloc(sizeof(node));
temp->data=num;
temp->next=NULL;
current->next=temp;
current=temp;
cout<<"Added element\n";
cout<<"dereferenced element\n";
}
}
void delete_node()
{
if(head!=NULL && head->next==NULL )//only one node
{
current=head=NULL; //Am I supposed to do anything else here??
cout<<"Deleted Head\n";
}
else
if(head!=NULL && head->next!=NULL)//>= 2 nodes
{
struct node* temp=(node*)malloc(sizeof(node));;
//temp=NULL;
temp=head;
while(temp->next!=current)
{
temp=temp->next;
}
temp->next=NULL;
current=temp;
cout<<"Deleted last element\n";
free(temp->next);
cout<<"Dereferenced temp\n";
}
else
{
cout<<"delete was not performed";
}
}
void list_linked_list()
{
node* temp=(node*)malloc(sizeof(node));
temp=head;
while(temp!=NULL)
{
cout<<temp->data<<"->";
temp=temp->next;
}
cout<<"displayed list\n";
//free(temp); //should I free temp?
cout<<"dereferenced temp";
}
void search_node()
{
cout<<"\nenter a number to search";
int search=0,found=0;
cin>>search;
node* temp=(node*)malloc(sizeof(node));
temp=head;
while(temp!=NULL)
{
if(temp->data==search)
found=1;
else
temp=temp->next;
}
if(found==1)
cout<<"found\n";
else
cout<<"not found\n";
free(temp); //shoudl I free temp?
cout<<"dereferenced temp";
}

Answer 1

There are multiple problems in your code:
You are free()ing a node in your insert function which is not what you want. So remove the line free(temp) from your insert function.
You do want to free the node when you delete an element from your linked list. So uncomment the line: free(temp);. But this is not the correct current node that you want to free(). Here temp is your new current whereas you want to free() your old current which temp->next. So your free() statement should be: free(temp->next); in delete_node() function (Not free(temp); ).
Return value of main should an int.
If you are using C++ there are better ways to implement linked lists. You may want to use new and delete instead of malloc and free. Use C++ headers instead of C headers.
If you do use C, then don't cast the value returned by malloc in C.
You are using goto as a replacement for a loop which is unnecessary when you can simply use for(;;) { } or while(1) { }.

Answer 2

In insert function's else part you free new node just after adding it in Linked List that causes Undefined behaviour at runtime:
else
{
node* temp=(node*)malloc(sizeof(*temp));
temp->data=num;
temp->next=NULL;
current->next=temp;
current=temp;
cout<<"Added element\n";
free(temp); <------"Bug"
cout<<"dereferenced element\n";
}
Note: You can't access a node for which memory is deallocated (free()), doing this is an illegal operation. You should free memory for node when you are done with the program (and don't need access that memory again).