Insertion in a doubly linked list in C - c

Question

My program keeps on crashing. I think there's a problem in my logic. Please help! Thanks!
struct node{
int data;
struct node *prev,*next;
} *head = NULL;
void insert(){
struct node* temp = (struct node*) malloc (sizeof (struct node*));
int value;
printf("Enter an element");
scanf("%d",&value);
temp -> data = value;
temp -> next = NULL;
temp -> prev = NULL;
if(value < head -> data || head == NULL){
temp -> next = head -> next;
head = temp;
return;
}
while(head->next != NULL && value > head -> next -> data)
head = head -> next;
temp -> next = head ->next->prev;
head -> next = temp -> prev;
while (head -> prev != NULL)
head = head -> prev;
}

Answer 1

Try this:
#include <stdio.h>
#include <stdlib.h>
struct node{
int data;
struct node *prev,*next;
} *head = 0;
void insert(int value){
struct node* temp = (struct node*) malloc (sizeof (struct node));
//int value;
//printf("Enter an element");
//scanf("%d",&value);
temp -> data = value;
temp -> next = 0;
temp -> prev = 0;
if(head == 0) {
// no head, make temp the head
head = temp;
} else if(value > head->data) {
// put temp in front of the head
temp->next = head;
head->prev = temp;
head = temp;
} else {
// put temp after the head
struct node *this = head;
struct node *prev = 0;
while(this && this->data > value) {
prev = this;
this = this->next;
}
temp->next = this;
temp->prev = prev;
prev->next = temp;
if(this) {
this->prev = temp;
}
}
}
int main() {
insert(1);
insert(3);
insert(4);
struct node *t = 0;
for(t = head; t; t = t->next) {
printf("%d\n", t->data);
}
}
This one compiles and sorts in descending value. Note I commented out your scanf
and made insert take a parameter.

Answer 2

the space you malloc is not enough.
change
struct node* temp = (struct node*) malloc (sizeof (struct node*));
to
struct node* temp = (struct node*) malloc (sizeof (struct node));

Answer 3

As a start, think about these two lines and what happens if head == NULL:
if(value < head->data || head == NULL){
temp->next = head->next;
Specifically what is head->next?

Answer 4

There are two core problems I see.
You are not allocating sufficient space for a node struct. sizeof(struct node *) returns the size of a node pointer, not a node. This:
struct node* temp = (struct node*) malloc (sizeof (struct node*));`
... should be this:
struct node* temp = (struct node*) malloc (sizeof (struct node));
Be careful with pointers:
if (value < head -> data || head == NULL)
... what is the value of head->data if you haven't initialized head? (e.g. it is still NULL). Your program will fetch data from address NULL with an offset for field data. Address NULL is not valid / is part of protected memory space, your OS won't like this :).
You should first check if head is NULL. If it is, then you can't reference fields of head as they are invalid (in any situation). Be careful with multi-part conditional expressions to check for struct pointer NULL presence prior to subsequent parts that reference struct fields, also.