Showing runtime error in printing - c

Question

When i am printing linked list , i get runtime error
#include stdio.h
#include stddef.h
typedef struct
{
int info;
struct DEMO1 *next;
} DEMO1;
void insertatlast(DEMO1 * p, int o);
void printl(DEMO1 * p);
int main()
{
int temp;
DEMO1 *head = NULL;
if (head == NULL)
while (1)
{
printf("ENTER 0 to exit");
scanf("%d", &temp);
if (temp == 0)
break;
insertatlast(head, temp);
}
printl(head);
return 0;
}
void insertatlast(DEMO1 * head, int data)
{
if (head == NULL)
{
DEMO1 *node = (DEMO1 *) (malloc(sizeof(DEMO1)));
node->info = data;
node->next = 0;
head = node;
}
else
{
DEMO1 *temp;
temp = head;
while (temp->next != NULL)
{
temp = temp->next;
}
DEMO1 *node = (DEMO1 *) (malloc(sizeof(DEMO1)));
node->info = data;
node->next = 0;
temp->next = node;
}
}
void printl(DEMO1 * head)
{
DEMO1 *temp;
temp = head;
printf("%d", temp->info);
while (temp != NULL)
{
printf(":) %d\n", temp->info);
temp = temp->next;
}
}

Answer 1

When head is NULL, head is changed within the insertatlast() function only and not in main(). return head to update head in main() also.
DEMO1* insertatlast(DEMO1 * head, int data)
{
if (head == NULL)
{
DEMO1 *node = (DEMO1 *) (malloc(sizeof(DEMO1)));
node->info = data;
node->next = 0;
head = node;
}
else
{
DEMO1 *temp;
temp = head;
while (temp->next != NULL)
{
temp = temp->next;
}
DEMO1 *node = (DEMO1 *) (malloc(sizeof(DEMO1)));
node->info = data;
node->next = 0;
temp->next = node;
}
return head;
}
int main()
{
int temp;
DEMO1 *head = NULL;
if (head == NULL)
while (1)
{
printf("ENTER 0 to exit");
scanf("%d", &temp);
if (temp == 0)
break;
head=insertatlast(head, temp);
}
printl(head);
return 0;
}

Answer 2

When you compile it (after fixing the missing <> in #includes, you can read some warning from your compiler — hopefully you have warnings enabled. Even if they doesn't solve your problem, it is a good habit to take a look at them.
Lines like
DEMO1 *head = NULL;
if (head == NULL)
should suggest you you are doing something wrong. The fact the if-body is executed, tell you alone that head is NULL when you call insertatlast. That's fine, you expect it. What you likely do not expect is that your head is not modified by the call to insertatlast: so when you call printl, head is NULL still, and calling
`printl(NULL);`
is problematic, because of
temp = head;
... temp->info ...
I guess someone told you that if you want to modify a value of a variable in the caller, you have to pass a pointer to that variable, and that since you have already a pointer, you get confused.
The fact is, a pointer is a variable too, and if you need to modify it, you must get the pointer to... a pointer.
DEMO1* head = NULL;
//...
insertatlast(&head, temp);
and your prototype for the insert would be
void insertatlast(DEMO1 **head, int data);
Then, you modify the caller variable (the DEMO1*), access it and so on, by using *head (don't forget that in insertatlast head has type DEMO1**, and so *head has type DEMO1*).
Note
Do not cast malloc return value; in C
DEMO1* node = malloc(sizeof (DEMO1));
suffices.