Thread: Why is this an infinite loop?

Originally Posted by Caleb Duncan

I guess I just don't understand at all as to why this loops infinitely.

It should not loop infinitely: you set tail->back to head, then you compare temp->back to head, where temp == tail. Hence the condition is false even before the first iteration of the loop. Consequently, either you are misinterpreting your output, or the code that you posted is not what you actually compiled and ran (and indeed it is not so because the code that you posted contains a syntax error).

Here's proof:

Code:

#include <stdlib.h>
#include <stdio.h>

struct node {
    struct node* back;
    int data;
    struct node* next;
};

struct node *head;
struct node *tail;
struct node *temp;
 
int main(void) {
    head = (struct node*)malloc(sizeof(struct node));
    tail = (struct node*)malloc(sizeof(struct node));
    tail->back = head;
    head->next = tail;
    temp = tail;
    while (temp->back != head) {
        temp = temp->back;
    }

    printf("Hello world!\n");
    return 0;
}

The above prints "Hello world" for me, clearly showing that there is no infinite loop.

A few comments:

• You desperately need to indent your code properly.
• void main should be int main(void) as in my code example.
• Avoid global variables: head, tail and temp should have been declared in main.
• There is no need to cast the return value of malloc. I recommend:
  
  Code:

  head = malloc(sizeof(*head));

• Check that malloc does not return a null pointer before using the pointer.
• free what you malloc when done.

Originally Posted by Caleb Duncan

The intentions of this code is for temp to become the node "in front of the head node". How would I fix this?

temp is tail though. You basically want to move the tail to be the head? Or are you trying to insert a new node as the head?