How to properly split a C program in files and include then?

Question

I organized my program splitting every entity in its own file. Which is something like this.

main.c

#include "student.h"
#include "subject.h"
#include "classroom.h"

#define PI 3.14
int sum(int a, int b);

---

student.h

typedef struct st student;

student.c

#include "student.h"

---

subject.h

typedef struct sb subject;

subject.c

#include "subject.h"

---

classroom.h

typedef struct cr classroom;

classroom.c

#include "classroom.h"

---

My problem is, inside classroom I need student and subject. How should I include this? should I include this inside classroom.h or classroom.c?

#include "student.h"
#include "subject.h"

Second, I have things on main.c that are used by all of then like sum() and PI

How it is the right way including implementation in the header or including header in the implementation file? and should I include the header or implementation files?

If I throw everything on a single file it compiles just fine, but I'm not doing this right it does not compile.

Answer 1

First. One thing that is important to know about .h (header) files. They should have the following.

// In the top of the file
#ifndef NAME_OF_FILE_H
#define NAME_OF_FILE_H

// Your header code goes here

// In the end of the file
#endif

Why put this? If you were to include your header file, say header.h, in multiple other files, file1.c, file2.c, you would basically be repeating the code, meaning the code in header.h will be placed in both files during compilation process.

Having those pre processor instructions assure you that the code in header.h will only exist once in the program.

Now. Where do you place your #includes? Well, I assume the student.h and the subject.h file will declare things that are implemented in the student.c and subject.c files. So if the classroom.h file uses things declared in the previous two headers then you need to place #include "student.h" and #include "subject.h" in classroom.h.

If only the classroom.c also uses things declared in the headers, place the includes only here but not in the classroom.h.

Finally, if both files use things declared in the headers, place #include "student.h" and #include "subject.h" in both files.

Basically, you place the includes in the files that need a resource defined (but not implemented) in a header. By having you header surrounded by the code above you can basically place the includes in many files and never have repeated code during the compilation process.

About the sum() and PI. Same thing. Make a header with the code above and included it where those things are needed.

Answer 2

So the organisation of "x.h" and "x.c" (or "x.cpp") is a fairly standard way to do things. But some things don't fit in there, and sometimes you need a "constants.h" or some other name for things like PI and SpeedOfLight.

Something like sum would fit nicely into a "utils.h" - whether you have enough to make it worth having a "utils.c"

Your .c file should include all header files that it needs (but no more).

For example:

#ifndef X_H__
#define X_H__
 ...  
 all the contents of x.h goes here 
 ... 
#endif

And header files should include all things THEY need themselves. So for example, if "x.h" needs "a.h" and "b.h", there should be #include "a.h" and #include "b.h" in "x.h" so you don't have to remember that:

If I include "x.h", I must put "a.h" and "b.h" before it.

At the same time, don't add more includes than you actually need...

Answer 3

Your question is mostly about style.
People can give you general opinions, at best.

I think that each file must have a concrete job to do, or well to define a concrete object and its associated operations.
The "job" must be described in an easy way:

• File for general purposes (simple) routines and/or macros and/or constants.
• File for string handle.
• File for integer arithmetic.
• File for I/O operations.
• File for user interface and/or interaction.
• and so on...

The more you can separate different types of jobs, the better is the organization of your headers.

• If you have doubts, just ask yourself if such or that functions can be applied to other programs, different in nature to your present project. If you have a list of macros/functions/data that you probably are going to use in very different programs, independently of a set of another ones, then it is very likely that the former have to be grouped in the same header file.

An "object" must be described typically by a struct,
and concrete and well understood "operations" (that is, functions) acting explicitly on that struct.

Finally, you could write one or two master files in order to collect and relate all the files you project needs.

In general, I intent to keep the main.c file as short as possible,
as an orchestra director controlling the rest of the program.

By writting clear and nice comments in each file,
the documentation lets you handle a project with multiple files spreaded anywhere.

You must explain:

• what is the purpose of the file,
• what are the data and functions there defined,
• how to use the functions,
• what results can be expected,
• which is the meaning of these results,
• and so on.

If you try to do this exercise of explaining someone else what your header file is supposed to do, and how, you'll quickly see if something has no sense there.

Answer 4

You should include the header file in the implementation file.

Generally you should include as few header files in a header file as possible. Only include the headers that you need for types and such. In the implementation file you can then include all the headers you need for the functions you have in your header file.

So in your case it depends on what you need student and subject for. If you only need it in the implementation file you include it there and if you need it in the header you can include it there and it will be included in the implementation file when you include the header that corresponds to the implementation file.