How can I link multiple c-files together using headers? - c

Question

I work on a project that has multiple c files. Each c file has its own header. Now I want to put all c files together.
As a preperation I have tried the following thing:
This would be my example c-code (function.c):
#include <stdio.h>
#include "function.h"
void output()
{
printf("Thats a text\n");
}
Thats the associated header file (function.h):
//header function.h
#ifndef FUNCTION_H_
#define FUNCTION_H_
#endif // FUNCTION_H_
And thats my main.c:
#include "function.h"
int main()
{
output();
return 0;
}
I would expect the following output:
"Thats a text"
But I only receive following error:
undefined reference to 'output'
What am I doing wrong here?
Thanks a lot!

Answer 1

You need the prototype for output function in your header so that it's visible in other module(s).
//header function.h
#ifndef FUNCTION_H_
#define FUNCTION_H_
void output(void);
#endif // FUNCTION_H_
And you need to link the module (source file function.c) in order to actually provide the definition of output that your main module uses.
For example, you can directly compile them together with:
gcc main.c function.c -o my_out
You may also want to look at Makefiles as well.

Answer 2

Your header should be
//header function.h
#ifndef FUNCTION_H_
#define FUNCTION_H_
void output();
#endif // FUNCTION_H_
compile like this:
(actual flags may depend on compiler used)
cc -c main.c
(creates main.o)
cc -c function.c
(creates function.o)
cc main.o function.o
(creates a.out or whatever your system default is)
...or as someone else mentioned:
cc main.c function.c
(does it all)