Struct inside list in C - Display members of a struct existing inside a Linked list

Question

I have a struct named book and a list linked to struct book. I must add books to my book list and then to display the list. I created a function "printBList" to print at screen the information of my list. But i do something wrong when I am trying to print the book.id and the program stops to responding. I believe that the lines "book *b=head->book;" and "printf("book ID%d\n", b->id);" are wrong, but I can not find how I have to write then right. Could someone help me?

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

#define MAXSTRING 100
#define MAXREVIEWS 100

typedef  enum genres{
    fiction, 
    sientific,
    politics
};

typedef struct
{
    char author[MAXSTRING];
    char title[MAXSTRING];
    enum genres genre;
    int id;
    char reviews[MAXREVIEWS][MAXSTRING];
} book;


typedef struct list 
{
    int BookID;
    struct list * next;
    struct book * book;
} BList;


void printBList(BList * head) 
{
    BList * current = head;
    book *b=head->book;

    while (current != NULL) {
        printf("List ID:: %d\n", current->BookID);
        printf("book ID%d\n", b->id);
        //printf("%d\n", current->BookID);        
        current = current->next;
    }
}


int main()
{
    BList * head = NULL;
    head = malloc(sizeof(BList));
    if (head == NULL) {
        return 1;
    }

    book b={"author 1","title 1",1,22,"review 1"};  

    head->next = NULL;
    head = malloc(sizeof(BList));

    head->BookID = 1;
    head->next = malloc(sizeof(BList)); 
    head->next->BookID = 24;
    head->next->book;
    head->next->next = NULL;

    printBList(head);
    return 0;
}

Answer 1

The problem is in the way you print the list, here:

while (current != NULL) {
    printf("List ID:: %d\n", current->BookID);
    printf("book ID%d\n", b->id); // <-- HERE
    current = current->next;
}

You work with current, but you try to print b, which is fixed.

You probably meant to say this:

printf("book ID%d\n", current->book->id);

---

There more issues though:

Moreover, you meant to write:

typedef struct book {
   ..
} book;

You are accessing next of head, before creating the space for head, here:

head->next = NULL;
head = malloc(sizeof(BList));

So change it to:

head = malloc(sizeof(BList));
head->next = NULL;

Moreover, this line:

head->next->book;

does nothing.

Next, you should check your warnings:

Georgioss-MacBook-Pro:~ gsamaras$ gcc -Wall main.c 
main.c:7:1: warning: typedef requires a name [-Wmissing-declarations]
typedef  enum genres{
^~~~~~~
main.c:34:11: warning: incompatible pointer types initializing 'book *' with an
      expression of type 'struct book *' [-Wincompatible-pointer-types]
    book *b=head->book;
          ^ ~~~~~~~~~~
main.c:53:39: warning: suggest braces around initialization of subobject
      [-Wmissing-braces]
    book b={"author 1","title 1",1,22,"review 1"};  
                                      ^~~~~~~~~~
                                      {         }
main.c:61:17: warning: expression result unused [-Wunused-value]
    head->next->book;
    ~~~~~~~~~~  ^~~~
main.c:53:10: warning: unused variable 'b' [-Wunused-variable]
    book b={"author 1","title 1",1,22,"review 1"};  
         ^
5 warnings generated.

When you typedef something, you need to give a synonym, so do this for your enumeration:

typedef  enum genres{
  ..
} genres;

Since reviews is a 2D array, you can do this instead:

book b={"author 1","title 1",1,22, {"review 1"} }; 

Moreover, do not use magic numbers, like 1 in this case, since you have an enum for this purpose.

---

Then, I changed your main() a bit, to add the same book twice, with a different List ID. Putting everything together, we get:

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

#define MAXSTRING 100
#define MAXREVIEWS 100

typedef  enum genres{
    fiction, 
    sientific,
    politics
} genres;

typedef struct book
{
    char author[MAXSTRING];
    char title[MAXSTRING];
    enum genres genre;
    int id;
    char reviews[MAXREVIEWS][MAXSTRING];
} book;


typedef struct list 
{
    int BookID;
    struct list * next;
    struct book * book;
} BList;


void printBList(BList * head) 
{
    BList * current = head;

    while (current != NULL) {
        printf("List ID:: %d\n", current->BookID);
        printf("book ID%d\n", current->book->id);
        current = current->next;
    }
}


int main()
{
    BList * head = NULL;
    head = malloc(sizeof(BList));
    if (head == NULL) {
        return 1;
    }

    book b={"author 1","title 1", sientific ,22,{"review 1"}};  

    head->next = NULL;
    head->BookID = 1;
    head->book = &b;

    head->next = malloc(sizeof(BList)); 
    head->next->BookID = 24;
    head->next->book = &b;
    head->next->next = NULL;

    printBList(head);
    return 0;
}

Output:

Georgioss-MacBook-Pro:~ gsamaras$ gcc -Wall main.c 
Georgioss-MacBook-Pro:~ gsamaras$ ./a.out 
List ID:: 1
book ID22
List ID:: 24
book ID22

---

PS:

1. Don't forget to de-allocate the space you allocated dynamically. Do this with free().
2. It's a good practice to check if malloc() was successful, by checking if its return value was NULL (meaning failure). Read more in How detect malloc failure?