Error including .h files made by me

Hello everyone. I created this library:

Code:

---

//liberr.h
void hello();

---

and the correspondent .c :

Code:

---

//liberr.c
#include <stdio.h>

void hello(){
       
        printf("hello!!\n");
       
}

---

then i made a simple program like this:

Code:

---

//errtest2.c
#include "liberr.h"
#include <stdio.h>

int main(){

        printf("testing...\n");
        hello();
       
        return 0;
}

---

but i receive this error :
"/tmp/ccub0rMj.o: In function `main':
/home/paulo/Desktop/errtest2.c:7: undefined reference to `hello'
collect2: ld returned 1 exit status"

i'm using linux. what is the problem? thanks for your help

In this case you can compile with the following cmd:
gcc errtest2.c liberr.c -o errtest2

but I guess you wish to compile your liberr module separately, in this case:
gcc -c liberr.c -o liberr.o
gcc -c errtest2.c -o errtest2.o
and to link everything:
gcc errtest2.o liberr.o -o errtest2
The two first cmd generate intermediary object code so that liberr is compiled once and for all.

Your fundamental problem is you think the .h file is the library. It's not -- the .c file is the library. The .h is just an attendant to it. You aren't linking with the library itself. The header file is a ghost.

More than likely it's because you aren't linking the liberr.o file to the final executable. Just creating a header file won't tell the linker that you have multiple object files nor will it tell the compiler to compile the other file. If you are using gcc it would look like...

gcc liberr.c errtest2.c -o hello