CODEWITHSUNDEEP
cstructlinked-list
user1400312
user1400312

Reputation:

Structs not working as expected in C

I have the following program, with the following output written in C.

I expect after prepending a value for my tail to be 40 and my head 30, but it is not.

Am I making a mistake with memory allocation of the structs or is it something less obvious than that?

I am using the GCC compiler on Mac OSX.

#include <stdio.h>

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

struct LinkedList {
    int size;
    struct LinkedListNode *head;
    struct LinkedListNode *tail;
};

struct LinkedList create_linked_list(){
    struct LinkedList linkedList;

    linkedList.size = 0;
    linkedList.head = NULL;
    linkedList.tail = NULL;

    return linkedList;
}

struct LinkedList prepend(struct LinkedList *linkedList, int data){
    struct LinkedListNode linkedListNode;
    linkedListNode.data = data;
    linkedListNode.next = linkedList->head;

    linkedList->head = &linkedListNode;

    if(linkedList->tail == NULL){
        printf("Setting tail to: %d\n", data);
        linkedList->tail = &linkedListNode;
    }else{
        printf("The tail is: %d\n", linkedList->tail->data);
    }

    linkedList->size = linkedList->size + 1;

    return *linkedList;
}

int main(){
    struct LinkedList linkedList;
    linkedList = create_linked_list();

    linkedList = prepend(&linkedList, 40);

    struct LinkedListNode head = *linkedList.head;
    struct LinkedListNode tail = *linkedList.tail;

    printf("%d\n", head.data);
    printf("%d\n", tail.data);
    printf("%d\n", linkedList.size);

    linkedList = prepend(&linkedList, 30);

    head = *linkedList.head;
    tail = *linkedList.tail;

    printf("%d\n", head.data);
    printf("%d\n", tail.data);
    printf("%d\n", linkedList.size);
}

Output:

Setting tail to: 40
40
40
1
The tail is: 30
30
30
2

Upvotes: 1

Views: 99

Answers (1)

Some programmer dude
Some programmer dude

Reputation: 409136

One major problem is here:

linkedList->head = &linkedListNode;

Here you make linkedList->head point to the location of the local variable linkedListNode. That variable will go out of scope once the function returns, and the pointer will become a stray pointer that you no longer can dereference without having undefined behavior.

The solution to this problem is to allocate the LinkedListNode structure dynamically using malloc.

Upvotes: 4

Related Questions