using extern to have a global variable

Hello,

Compiling using gcc

I am working on a project that has some modules that will share a common handler. So I am using a extern

I have created a cut down version of my project.

However, I get an undefined reference hLibrary in library.c.

Is this the correct way to do this using extern. Or is there a better way to share a global that is safer?

Many thanks,

library.h

Code:

#ifndef LIBRARY_H_INCLUDED
#define LIBRARY_H_INCLUDED

int open_library();

#endif // LIBRARY_H_INCLUDED

library.c

Code:

#include <stdio.h>
#include "library.h"

extern int hLibrary;

int open_library()
{
    printf("The library has been opened");
    
    //Set the value for this handler so it can be
    //used in another module (adapter.c)
    hLibrary = 1010;

    return 1;
}

adapter.h

Code:

#ifndef ADAPTER_H_INCLUDED
#define ADAPTER_H_INCLUDED

int find_adapter();

#endif // ADAPTER_H_INCLUDED

The module implementation file that will use the hLibrary handler.

Code:

#include <stdio.h>

#include "adapter.h"
#include "library.h"

int find_adapter()
{
    printf("\nStarting adapter");
    int hLibrary;

    if(hLibrary == 1010)
    {
        printf("\nAdapter has been started: %d", hLibrary);

        return 1;
    }
    else
    {
        printf("\nCould not find hLibrary");
    }

    return 1;
}

to have a global var you need to declare it outside the function.
extern just notifies compiler that there is such global var somethere else.

if actually - there is no such thing - linker will be not able to find it later.

Why do you need it as global? - pass it as a parameter

Hello,

Thanks for the response.

Working ok now.

Apart from passing as a parameter. Is using extern like this a good style and safe to do?

Thanks,

Steve

Is using extern like this a good style and safe to do?

In my opinion - using globals never a good style

Safe? from what? Multi-thread safe? definitely not.
User-prone safe? definitely not. You have no control over who and where modifies your global data