Question

I have the following C program. It works when I include the following line, otherwise it gives a segmentation fault:

printf("head(%p), last(%p), newnode(%p)\n", head, last, newnode);

Any idea what’s the issue here?

Here is my whole program. It’s a basic circular queue example.

#include "stdio.h"
#include "malloc.h"

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

typedef struct node NODE;

void display(NODE *);

int main(void) {

    NODE *head, *last, *newnode = NULL;
    int i = 5;

    for ( ; i > 0; i--) {
        newnode = (NODE *) malloc(sizeof(NODE));
        newnode->data = i*10;
        newnode->next = NULL;

        //printf("head(%p), last(%p), newnode(%p)\n", head, last, newnode);


        if (head == NULL) {
            head = newnode;
            last = newnode;
        } else {
           last->next = newnode;
           last = newnode;
        }

        last->next = head;
    }

    display(head);

    return 1;
}

void display(NODE *head) {

    NODE *temp = NULL;
    temp = head;

    printf("Elements --> ");
    do {
            printf("%d ", temp->data);
            temp = temp->next;
    } while (temp != head);

    printf("\n");

}

Answer 1

You need to explicitly initialize head and last to NULL also.

In your declaration:

NODE *head, *last, *newnode = NULL;

only newnode is being initialized to NULL. Consequently later in your if/else you’re testing/assigning to random memory.

Do something like this:

NODE *head, *last, *newnode;
head = last = newnode = NULL;

You can’t assume that pointers are automatically initialized to NULL, even though it may be the case on certain systems. Variables declared as static are an exception. Always initialize variables in C to reasonable values, especially pointers.

When you access garbage memory you’re invoking undefined behavior. When you do this you may observe inconsistent and confusing results. In your case, adding the printf seemed to fix the problem. The reason that this affected the behavior of your code is implementation dependent and – once you’ve introduced undefined behavior – beyond your control.