I'm getting into infinite while loop (linked lists c) - linked-list

Question

I'm stuck in a while loop (the one that prints "no" ):
#include<stdlib.h>
#include<stdio.h>
typedef struct bill {
int value;
struct bill *next;
} list;
void printlist(list *head) {
list *temp = head;
while (temp != NULL)
printf("%d ",temp->value), temp = temp->next;
}
void insert_at_end(list *head, int value) {
list *current, *first;
first = malloc(sizeof(list));
if (head == NULL)
head = first;
else {
current = head;
while (current->next != NULL) {
current = current->next;
printf("no");
}
}
}
void main() {
list *head = NULL;
head = malloc(sizeof(list));
for (int i = 0; i < 10; i++)
insert_at_end(head, i);
printlist(head);
}
I'm not sure why, but current never gets to NULL. I watched numerous videos and they all do the same:
while (current != NULL)
current = current->next
...which should just get to NULL at one point, but it doesn't in my case.

Answer 1

There are these issues:
When you do head = malloc(sizeof(list)), the members of that node are still undefined, including its next member. As a consequence, when you get into the else block in the insert_at_end function, the loop will access this undefined next pointer and bring about undefined behaviour.
Similar to the previous problem, also first does not get initialised with a value and a next member.
In the else block in the insert_at_end function there is no code that attaches the new node to the list.
In the if block in the insert_at_end function, the value of head is altered. But this just changes the value of a local variable -- the caller will not see this change. For that to happen you should alter the function parameter so that it is a pointer to the head pointer.
The creation of a head node in the main program seems unwaranted -- you should start with an empty list, i.e. with head equal to NULL.
void main is not correct. It should be int main, and the appropriate value should be returned by it.
Here is the correction to the two functions that have issues:
// This function accepts a pointer to a head-pointer, so the head-pointer
// can be changed and the caller will receive that change.
void insert_at_end(list **head, int value){
list *current, *first;
first = malloc(sizeof(list));
first->value = value; // This was missing
first->next = NULL; // This was missing
if (*head == NULL) {
*head = first;
} else {
current = *head;
while (current->next != NULL) {
current = current->next;
}
current->next = first; // This was missing
}
}
// The main method has an int return type
int main() {
list *head = NULL;
// Do not create a node here.
for (int i = 0; i < 10; i++) {
insert_at_end(&head, i); // pass pointer to head-pointer
}
printlist(head);
return 0;
}