Linked list without malloc

November 9, 2023

I’m starting to learn C myself, and based on the book "Programming in C (4th Edition)", the author defined a linked list as following:

struct entry
{
    int value;
    struct entry *next;
};

struct entry *createNewLinkList(void)
{
    struct entry n1, n2, n3;
    struct entry *list_pointer = &n1;

    n1.value = 100;
    n1.next = &n2;
    n2.value = 200;
    n2.next = &n3;
    n3.value = 300;
    n3.next = NULL;
    return list_pointer;
}

void print_linked_list(struct entry *linked_list)
{
    struct entry *current = linked_list;
    while (current != NULL)
    {
        printf("%d\n", current->value);
        current = current->next;
    }
    printf("\n");
}

int main(void)
{
    struct entry *linked_list = createNewLinkList(); //DEBUG: This line is fine linked_list: {value:100, next:0x000000016fdff200}
    printf("Original list \n");
    print_linked_list(linked_list);   //DEBUG: it is wrong after entering this line linked_list:{value:1876947504, next:0x8b7a000100003f48}
}

I don’t understand why the value of linked_list just changed to some address.

Is will work if I create a linked list with malloc like this:

struct entry *createNewLinkList(void)
{
    struct entry *n1 = (struct entry *)malloc(sizeof(struct entry));
    struct entry *n2 = (struct entry *)malloc(sizeof(struct entry));
    struct entry *n3 = (struct entry *)malloc(sizeof(struct entry));

    n1->value = 100;
    n1->next = n2;

    n2->value = 200;
    n2->next = n3;

    n3->value = 300;
    n3->next = NULL;
    return n1;
}

Thank you in advance,

>Solution :

The program in the book (if it is indeed the code from the book) has undefined behavior.

The function createNewLinkList returns a pointer to a local object with automatic storage duration (n1) that will not be alive (along with other local variables that are chained through pointers) after exiting the function

struct entry *createNewLinkList(void)
{
    struct entry n1, n2, n3;
    struct entry *list_pointer = &n1;

    n1.value = 100;
    n1.next = &n2;
    n2.value = 200;
    n2.next = &n3;
    n3.value = 300;
    n3.next = NULL;
    return list_pointer;
}

So dereferencing the pointer (and other pointers stored in the data member next of the structure) within the function print_linked_list

void print_linked_list(struct entry *linked_list)
{
    struct entry *current = linked_list;
    while (current != NULL)
    {
        printf("%d\n", current->value);
        current = current->next;
    }
    printf("\n");
}

invokes undefined behavior.

The program could be correct if the variables n1, n2, n3 had static storage duration that is if they would be declared like