Unable to resolve multiple definition error while linking files

Question

I am working on a question from my computer programming course this semester and am unable to resolve a linker error that I am facing. Can someone take a look at the following files and let me know what's wrong. As instructed, I placed guards to ensure multiple definitions don't take place but the guards don't seem to be functioning as I expected because gcc is giving me and error saying that count and booksCatalog are defined multiple times.

Below are the C files and header files used for the question.

book.h

#ifndef BOOK_H
#define BOOK_H

#include <stdio.h>

typedef enum shelf{Shelf1=1,Shelf2=2,Shelf3=3,Shelf4=4} SHELF;

typedef struct book{
    int ID;    
    SHELF shelfNum;
    float price;
} BOOK;


// the function should create a new variable of the type struct book (typdefed as BOOK)
// using the parameters passed to it and return it.
BOOK newBook(int ID, SHELF shelfNum, float price);

// this function should print the values of the members of book1 (passed as parameter)
void printBook(BOOK book1);

#endif

book.c

#include "book.h"

BOOK newBook(int ID, SHELF shelfNum, float price)
{
    // implement this function as per specification in books_def.h
    BOOK newbook = {ID, shelfNum, price};
    return newbook;
}

void printBook(BOOK book1)
{
   // implement this function as per specification in books_def.h
   printf("Book ID : %d\n", book1.ID);
   printf("Book shelf number : %d\n", book1.shelfNum);
   printf("Price of Book: %f\n\n",book1.price);
   return;
}

catalog.h

#ifndef CATALOG_H
#define CATALOG_H

#include "book.h"

#define MAX_SIZE 100

BOOK booksCatalog[MAX_SIZE];
int count;

// This function should take an input parameter of the type struct book (typdefed as BOOK)
// and it to the booksCatalog array at position count. Then, count should be incremented.
void addBookToCatalog(BOOK book1);


// This function should print all the books that are added to booksCatalog array.
// It should use the printBook() function defined in "book_def.h" to print the details of each book.
void printBookCatalog();


// This function should sort the booksCatalog array based on the ID of the book.
void sortBookCatalogOnID();

#endif

catalog.c

#include "catalog.h"

void addBookToCatalog(BOOK book1)
{
    // implement this function as per specification in books_catalog.h
    if (count<MAX_SIZE)
    {
        booksCatalog[count]=book1;
        count++;
    }
    else
        printf("Catalog is full!");
    return;
    
}

void printBookCatalog()
{
    // implement this function as per specification in books_catalog.h
    for (int i=0; i<count; i++)
    {
        printBook(booksCatalog[i]);
    }
    return;
}

void sortBookCatalogOnID()
{
    // implement this function as per specification in books_catalog.h
    int min;
    for (int i=0; i<count-1; i++)
    {
        min=i;
        for (int j=i+1; j<count; j++)
        {
            if ((booksCatalog[j].ID)<(booksCatalog[min].ID))
                min=j;
        }
        BOOK temp = booksCatalog[i];
        booksCatalog[i]=booksCatalog[min];
        booksCatalog[min]=temp;
    }
    return;
}

main.c

#include "catalog.h"
#include "book.h"

int main()
{
    count = 0;
    BOOK book1 = newBook(1847, Shelf3, 8768.95);
    BOOK book2 = newBook(5984, Shelf1, 7845.25);
    BOOK book3 = newBook(6325, Shelf2, 3154.47);
    BOOK book4 = newBook(5843, Shelf2, 1487.51);
    BOOK book5 = newBook(7894, Shelf2, 541.29);
    // printBook(book1); 

    // adding book1 to catalog of books
    addBookToCatalog(book1);
    addBookToCatalog(book2);
    addBookToCatalog(book3);
    addBookToCatalog(book4);
    addBookToCatalog(book5);

    printBookCatalog();

    sortBookCatalogOnID();

    printBookCatalog();

    return 0;
}

Answer 1

The header catalog.h has two tentative definitions of variables

BOOK booksCatalog[MAX_SIZE];
int count;

As the header is included in modules catalog.c and main.c then in fact these two variables are defined twice in each translation unit like

BOOK booksCatalog[MAX_SIZE] = { 0 };
int count = { 0 };

From the C23 Standard (6.9.2 External object definitions)

2 A declaration of an identifier for an object that has file scope without an initializer, and without a storage-class specifier or with the storage-class specifier static, constitutes a tentative definition. If a translation unit contains one or more tentative definitions for an identifier, and the translation unit contains no external definition for that identifier, then the behavior is exactly as if the translation unit contains a file scope declaration of that identifier, with the composite type as of the end of the translation unit, with an initializer equal to {0}.

You should declare the variables without their definitions in the header catalog.h

extern BOOK booksCatalog[MAX_SIZE];
extern int count;

(Note: the variables in these declarations shall not be initialized. Otherwise they be externally defined in the header and the linker again will issue an error because the header is included in two translation units.)

and define them in the module catalog.c like for example

BOOK booksCatalog[MAX_SIZE] = { 0 };
int count = 0;

Pay attention to that in C opposite to C++ to make a file (a namespace in C++) scope external definition of a variable you need to initialize it. In C++ there is no such a notion as "tentative definition".

Answer 2

In your header file catalog.h you have the line

int count;

which is a tentative definition. In this case, this tentative definition will always become an actual definition.

This means the two source files main.c and catalog.c are each providing a definition of the variable count. This violates the one-definition rule, even if the two definitions are identical.

In order to comply with the one-definition rule, only one of the source files should provide a definition. Therefore, I suggest that in the header file catalog.h, you change the line

int count;

to:

extern int count;

That way, this line will be a declaration instead of a tentative definition.

However, now the variable count is declared in main.c and catalog.c, but not defined anywhere. Therefore, in order to comply with the one-definition rule, you must also define the variables in one of the source files. This definition can be a (tentative) definition

int count;

or an explicit definition:

int count = 0;

The variable booksCatalog has the same problem.