CODEWITHSUNDEEP
cpointersnullsegmentation-fault
Jay Mody
Jay Mody

Reputation: 4073

Segmentation fault: 11 when trying to print linked list

I'm fairly new to C and I know I'm probably doing something wrong with pointers but I just can't seem pinpoint what I'm doing wrong.

Here's the structs and functions for the linked list:

// node structure
struct Node {
    int val;
    struct Node* next;
};

// singly linked list structure
struct LinkedList {
    int size;
    struct Node* head;
} LinkedList = {0, NULL};


// insert at head function
void InsertHead(struct LinkedList* list, int val) {
    struct Node* node = malloc(sizeof(struct Node));
    node->val = val;
    node->next = list->head;
    list->head = node;
    list->size++;
}

// print values in list
void PrintList(struct LinkedList* list) {
    struct Node* node = list->head;

    while (node != NULL) {
        printf("%d, ", node->val);
        node = node->next;
    }
    printf("\n");
}

When I try to call PrintList with the following code:

// main function
int main() {
    struct LinkedList* mylist = malloc(sizeof(LinkedList));
    InsertHead(mylist, 4);
    InsertHead(mylist, 3);
    InsertHead(mylist, 1);

    // printf("%d, ", mylist->head->val);
    // printf("%d, ", mylist->head->next->val);
    // printf("%d, ", mylist->head->next->next->val);
    // printf("\n");
    PrintList(mylist);

    return 0;
}

I get the error Segmentation fault: 11

When I run remove the call to PrintList function and uncomment the printf statements, I get the desired output:

1,3,4,

What am I missing here?

Upvotes: 0

Views: 113

Answers (1)

hmakholm left over Monica
hmakholm left over Monica

Reputation: 23342

You never intialize the struct LinkedList you allocate at the top of main().

So when you're walking the list to print it, after the three elements you inserted explicitly, the next field of the last one will contain whichever garbage was in the head field of the original LinkedList when it was allocated.

To fix this, you can either allocate it with calloc instead (which explicitly zeros out the allocated memory befor it's given to you), or write a helper function that both allocates and explicitly initializes a struct LinkedList.

Upvotes: 1

Related Questions