Merge double linked list in C language - c

Question

Merge the two linked list in C language.
I tried to merge the two sorted double linked list. When I ran my code with different inputs, sometime the code just crushed with EXC_BAD_ACCESS error. I can't figure out why, the code seemed perfect for me and I use the similar way to merge two single linked list, it worked.
Can someone explain? Thanks!
#include <stdio.h>
#include <stdlib.h>
typedef struct Node
{
struct Node* prior;
struct Node* next;
int value;
}Node,*list;
list create_list()
{
list head = (list)malloc(sizeof(Node));
if(!head) exit(-1);
list tail;
tail=head;
printf("Please enter the length of double linked list:\n");
int len;
scanf("%d",&len);
for(int i=0;i<len;i++)
{
list new = (list)malloc(sizeof(Node));
printf("Please enter the value of node:\n");
int val;
scanf("%d",&val);
new->value=val;
tail->next = new;
new->prior=tail;
tail=new;
}
return head;
}
list merge_list(list a, list b)
{
if(a==NULL||b==NULL) exit(-1);
list p=(list)malloc(sizeof(Node));
list l=p;
while(a&&b)
{
if(a->value<=b->value)
{
p->next = a;
a->prior=p;
p=a;
a=a->next;
}
else
{
p->next = b;
b->prior=p;
p=b;
b=b->next;
}
}
if(a!=NULL)
{
p->next=a;
a->prior=p;
}
if(b!=NULL)
{
p->next=b;
b->prior=p;
}
return l;
}
int main() {
list l = create_list();
l=l->next;
list m = create_list();
m=m->next;
list n =merge_list(l,m);
n=n->next;
while(n)
{
printf("%d\n",n->value);
n=n->next;
}
return 0;
}

Answer 1

The problem is that in create_list you do not initialize new->next with NULL.
From this error it makes no sense in merge_list to compare pointers with NULL.

Answer 2

The most important bug (i.e. no initialization of new->next) has already been addressed by the answer from #alinsoar.
However, there are other bugs in your code that a) cause memory leaks and b) cause the linked list to be incorrect.
In main you have:
list l = create_list();
l=l->next; // Why ......
Why do you "throw away" the first element like that? That's a memory leak! And further it means that l->prio is not NULL as it should be!
I know it's because your create_list inserted a phony node in the start. But don't just fix it by throwing the node away. Fix the function instead.
Do something like this:
list create_list()
{
list head = NULL; // Do not use malloc here - just assign NULL
list tail = NULL;
printf("Please enter the length of double linked list:\n");
int len;
scanf("%d",&len);
for(int i=0;i<len;i++)
{
list new = malloc(sizeof(Node)); // do not cast malloc
new->next = NULL; // set the next pointer
printf("Please enter the value of node:\n");
int val;
scanf("%d",&val);
new->value=val;
// Add the node to the end
new->prior=tail;
if (tail)
{
tail->next = new;
}
else
{
// First element so update head
head = new;
}
tail=new;
}
return head;
}
With this code you don't get an extra element in the start and you can delete the code l=l->next; in main. Similar changes applies to merge_list but I'll leave that to you as an exercise.
In the end your mainshould only be:
int main() {
list l = create_list();
list m = create_list();
list n =merge_list(l,m);
while(n) {
printf("%d\n",n->value);
n=n->next;
}
return 0;
}