C：使用链表将数据添加到结构

Question

I'm new to c and learning about linked lists, I decided to create a library manager to manage my books using linked list but it doesn't seem to be saving the data to the structure using linked list. When someone tries to add a new book the function hits the checkID function to see if the book with the same id already exists but when i do a display information nothing seems to exists in the structure.

void addBook()
{
    int bookId;
    BOOK *head = NULL, *temp;
    temp = (BOOK*) malloc(sizeof(BOOK));

    printf("\n Enter Book Details\n");
    printf("Enter book ISBN: ");
    scanf("%d", &bookId);
    int bInd = checkID(bookId);
    if (bInd == 0)
    {
        printf("Enter book title: ");
        scanf("%s", &temp->chTitle);
        printf("Enter book type (eg. magazine, novel): ");
        scanf("%s", &temp->chType);
        printf("Enter book publisher (eg. UTA): ");
        scanf("%s", &temp->chPublisher);
        printf("Enter book's number of pages: ");
        scanf("%d", &temp->nPages);
        printf("Enter book's price: ");
        scanf("%f", &temp->nPrice);
        printf("Enter year published: ");
        scanf("%d", &temp->nPubYear);
        //temp->next=NULL;
        if (head == NULL)
        {
            head = temp;
            temp->next = NULL;
        }
        else{
            temp->next = head;
            head = temp;
        }
        //BOOK[count].nStatus = IN;
        count++;
    }
    else
    {
        printf("\nSorry another book with that id: Try again!\n" );
        addBookFunction();
    }
}

int checkID(int t)
{
    BOOK *head;
    while (head != NULL)
    {
        if (head->nID == t)
            return 1;
        head = head->next;
    }
    return 0;
}

Answer 1

Don't worry ! It's normal ! You are creating a new book library each time you add a book ! :)

At your place, an easy way to solve it would be to pass your "Library collection of book" to your AddBook method :

void addBook(BOOK *libraryBooks)
int checkID(BOOK *libraryBooks, int t)

Then remove your variable declaration 'head'. Change all 'head' by 'libraryBooks' and pass the variable to checkID.

Above this function you'll have to manage your library.

Answer 2

checkID() and addBook() should probably both accept the head of your linked list as an additional parameter. The head pointer that iterates through your linked list in checkID() looks like it's getting used before it's initialized with a proper value. It may just be lucky happenstance that you haven't hit a null pointer exception already.