Problem with including

[fallow swift]

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[broken moat]

Do not define functions in the header guards, unless it's inline functions

And use header guards to prevent repeated includes

If you have a main.h it already suggests something might be not quite right

[quasi iris]

If providing an inline definition, also be aware you must define it in a .c file too

static functions are also ok in headers

[fallow swift]

What Are the File Types in C/C++

Source Files: `.c|.cpp|.cxx|...`

• function definitions
• global variable definitions
• anything used only in that file
  Each file is compiled independently, and in parallel. Don't #include them, but compile each.

Header Files: `.h|.hpp|.tpp|...`

• forward-declarations/prototypes
• type declarations
• templates
  Headers are #included into other source/header files. You will need include guards to prevent redefinition errors.

Translation Units and Lesser Known File Extensions

Each source file becomes a translation unit during translation (expanding macros, remapping unicode characters, etc.)
Sometimes .ipp or .tpp for C++ are used for internal headers with function definitions.

See Also

📄 How do header and source files in C work? 📄 What are 'Include Guards' and #pragma once?

Common Errors

⛔ Redefinition of ... (compiler) ⛔ Multiple definition of ... (linker) ⛔ Splitting templates into source/header

[warm quiver]

it has a pretty easy fix,

animal.h:
#ifndef ANIMAL_H
#define ANIMAL_H

void some_fn(){

}

#endif

lion.h:
#include "animal.h"
#ifndef LION_H
#define LION_H

void some_lion_fn(){
some_fn();
}

#endif

main.c:
#include "animal.h"
#include "lion.h"

int main(){

return 0;

}

#ifndef LION_H
#define LION_H

// code

#endif

you should always do this in a header file, this will not redefine fucntions if files are included multiple times

ifndef expands to if not defined