Doubly linked list in C- delete node function - c

Question

Doubly linked list nodes are created at the main function. Ender and header defined. Breaks at the delete node function- ender is null.
What's the best way to free the memory of the last and first input, i.e.: delete: 233,A and 888,F?
#include <stdafx.h>
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <conio.h>
typedef struct record {
int idnumber;
char initial;
struct record *prevStudent;
struct record *nextStudent;
} STUDENT;
STUDENT *header = NULL; //pointer to the start of linked list
STUDENT *ender = NULL; //pointer to the end of the linked list
void Makenode(int x, char y);
void deletenode();
int main() {
Makenode(233, 'A');
Makenode(456, 'H');
Makenode(746, 'G');
Makenode(888, 'F');
deletenode();
fflush(stdin);
getchar();
return 0;
}
void Makenode(int x, char y) {
STUDENT *ptr;
ptr = (STUDENT *)malloc(sizeof(STUDENT));
if (ptr != NULL) {
ptr->idnumber = x;
ptr->initial = y;
ptr->nextStudent = header;
ptr->prevStudent = NULL;
if (header == NULL)
ender = ptr;
else
header->prevStudent = ptr;
header = ptr;
} else {
printf("Memory not allocated\n");
}
}
void deletenode() {
//delete the first and the last node of the linked list
STUDENT *p = header, *q = ender;
char c;
printf("Are you sure you want to delete Y/N:\n");
fflush(stdin); c=getchar();
while (c == 'Y' || c == 'y') {
ender=ender->nextStudent;
header=header->prevStudent;
free(p); free(q);
}
}

Answer 1

Your delete function leaves the linked list in an ilegal state. At all times (except temporarily inside your insert and delete functions), the following must be true:
If the header is null, the ender must also be null and the list is empty.
If a node p has a non-null link to p->next, then p->next->prev == p.
Likewise, if a node p has a non-null link to p->prev, then p->prev->next == p.
The header has no previous node; the ender has no next node.
These are the invariants of your linked list.
If you check your code for deleting:
void deletenode()
{
STUDENT *p = header, *q = ender;
ender=ender->nextStudent;
header=header->prevStudent;
free(p); free(q);
}
you can see that you just set the header and ender to NULL, because that's what ender->nextStudent and header->prevStudent are. But even reversing that won't help, because you must update the adjacent nodes' links.
Here are two functions - one for each task - that work:
void delete_first()
{
STUDENT *p = header;
if (p) {
if (p->nextStudent == NULL) {
header = ender = NULL;
} else {
p->nextStudent->prevStudent = NULL;
header = p->nextStudent;
}
free(p);
}
}
void delete_last()
{
STUDENT *p = ender;
if (p) {
if (p->prevStudent == NULL) {
header = ender = NULL;
} else {
p->prevStudent->nextStudent = NULL;
ender = p->prevStudent;
}
free(p);
}
}