Why is (null) being printed in my c code?-CodePudding

I am trying to add strings to a linked list but I have a problem with (null) being printed. Anyone who can help me?

The best I could do is narrow it down to this being the problem:

struct node *head = malloc(sizeof(struct node));
struct node *ptr = malloc(sizeof(struct node));

Here is the code:

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

struct node {
    char *data;
    struct node *link;
};

struct node *add_begin(char *d, struct node *head) {
    struct node *ptr = malloc(sizeof(struct node));
    ptr->data = d;
    ptr->link = head;
    return ptr;
}

void add_end(struct node *point, char *data) {
    struct node *temp = malloc(sizeof(struct node));
    
    temp->data = data;
    temp->link = NULL;
    
    while (point->link != NULL) {
        point = point->link;
    }
    point->link = temp;
}

int main() {
    struct node *head = malloc(sizeof(struct node));
    struct node *ptr = malloc(sizeof(struct node));
    head->link = ptr;
    char *data = "Chocolate Cake";
    head = add_begin(data, head);
    add_end(ptr, data);
    while (head != NULL) {
        printf("%s \n", head->data);
        head = head->link;
    }
}

Output:

Chocolate Cake
(null) 
(null) 
Chocolate Cake

CodePudding user response：

The problem is you allocate dummy nodes, which are uninitialized and happen to have null pointers as data and link. The list should be initially empty, ie: head should be a null pointer.

Note that add_end should also return the head pointer in case an empty list was passed. Passing the arguments in the same order to both functions is highly recommended.

Here is a modified version:

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

struct node {
    char *data;
    struct node *link;
};

struct node *add_begin(struct node *head, char *data) {
    struct node *ptr = malloc(sizeof(*ptr));
    ptr->data = data;
    ptr->link = head;
    return ptr;
}

struct node *add_end(struct node *head, char *data) {
    struct node *ptr = malloc(sizeof(*ptr));

    ptr->data = data;
    ptr->link = NULL;

    if (head == NULL) {
        return ptr;
    } else {
        struct node *temp = head;
        while (temp->link != NULL) {
            temp = temp->link;
        }
        temp->link = ptr;
        return head;
    }
}

int main() {
    struct node *head = NULL;
    char *data = "Chocolate Cake";

    head = add_begin(head, data);
    head = add_end(head, data);

    for (struct node *ptr = head; ptr; ptr = ptr->link) {
        printf("%s\n", ptr->data);
    }
    return 0;
}

Output:

Chocolate Cake
Chocolate Cake