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If I had a header file named "Header1.h" and another header file named "Header2.h", and include the first header file into the second, Header1 -> Header2, I would have access to Header1's code from Header2 . (For example, if I had defined a struct inside Header1, I could now make variables of that struct type in Header2).

Now if I had a .c file named "Main.c" and if I include "Header2.h" in "Main.c", Header2 -> Main, I would be able to access Header2's code from Main, but I would also be able to access Header1's code from Main as if I have included "Header1.h" inside "Main.c" specifically as well, Header1 -> Main. (For example, I could make a variable of type struct [the one defined in "Header1.h"] inside "Main.c").

Is there a way for me to restrict such access to "Header1.h" from "Main.c" and allow only "Header2.h" to be able to access information from "Header2.h", and perform similar "privatization" of a header file?

Transcribing a comment into the question:

Header1.h:

void printLinkedListImpl(Node* head)
{
    while (head) {
        printf("%d ", head->data);
        head = head->next;
    }
    printf("\n");
}

Header2.h:

typedef struct {
    Node* head;
    int size;
    void (*printLinkedList)(Node*);
} LinkedList;

LinkedList* LinkedListConstructorImpl() {
    LinkedList* linkedList = (LinkedList*)malloc(sizeof(LinkedList));
    linkedList->head = NULL;
    linkedList->size = 0;
    linkedList->printLinkedList = &printLinkedListImpl;
}

Main.c:

void runner() {
    LinkedList* linkedList = LinkedListConstructorImpl();
}
Jonathan Leffler
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starter_dev
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1 Answers1

1

I see two main options:

  1. Header2.h needs the contents of Header1.h, in which case you can't sensibly want to omit the contents of Header1.h.
  2. Header2.h does not need the contents of Header1.h, in which case Header2.h should not include Header1.h.

Write the code using the headers according to which option applies.

You would need a good reason not to decide on one or the other of those two options. However, it might be possible to define the header guard for Header1.h before including Header2.h:

Header1.h

#ifndef HEADER1_H
#define HEADER1_H
…
#endif

Header2.h

#ifndef HEADER2_H
#define HEADER2_H

#include "Header1.h"

…

#endif

Main.c

…
#define HEADER1_H
#include "Header2.h"
…

But, as I stated at the outset, either Header2.h needs the information from Header1.h to compile (in which case this will cause a compilation failure), or it doesn't (in which case Header2.h should not include Header1.h).

After transcribing the comment into the question:

Header files should not define general functions like it appears Header1.h does. A header should declare functions, types, enumerations, macros — and may perhaps define static inline functions — but should not define variables or functions. (See also How do I use extern to share variables between source files?). If you define functions in a header, the header can only be used by one source file, but the whole point of a header is to share information between source files.

If you're not sure how to build programs from multiple source files, that is a different question — one which has been answered many times. For example, Compiling multiple C files in a program, and there are numerous questions and answers for different IDEs that can be found by searching on SO with [c] compile multiple files program.

Jonathan Leffler
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  • **Header1**: `void printLinkedListImpl(Node* head) {}` **Header2**: `typedef struct { Node* head; int size; void (*printLinkedList)(Node*); }LinkedList; LinkedList* LinkedListConstructorImpl() { LinkedList* linkedList = (LinkedList*)malloc(sizeof(LinkedList)); linkedList->head = NULL; linkedList->size = 0; linkedList->printLinkedList = &printLinkedListImpl; }` **Main.c**: `void runner() { LinkedList* linkedList = LinkedListConstructorImpl(); }` – starter_dev Dec 16 '22 at 22:26
  • If header2 needs some stuff from header1, but not other stuff from header1 and you don't want to include that stuff every time you include header2, maybe header1 should actually be header1a and header1b. – Chris Dec 16 '22 at 22:27
  • @Chris what i want is an inability for someone to say in "Main.c" something like: `printLinkedListImpl(arg);` – starter_dev Dec 16 '22 at 22:30
  • @starter_dev — (1) I've transcribed your comment into the question, which is where you should have put the contents of the comment in the first place. (2) You should not be defining functions (other than `static inline` functions, perhaps) in a header. If you define functions in a header, the header can only be used by one source file, but the whole point of a header is to share information between source files. – Jonathan Leffler Dec 16 '22 at 22:35