Thread: using extern to have a global variable

  1. #1
    UK2
    Join Date
    Sep 2003
    Posts
    112

    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;
    }

  2. #2
    Hurry Slowly vart's Avatar
    Join Date
    Oct 2006
    Location
    Rishon LeZion, Israel
    Posts
    6,794
    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
    All problems in computer science can be solved by another level of indirection,
    except for the problem of too many layers of indirection.
    David J. Wheeler

  3. #3
    UK2
    Join Date
    Sep 2003
    Posts
    112
    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

  4. #4
    Hurry Slowly vart's Avatar
    Join Date
    Oct 2006
    Location
    Rishon LeZion, Israel
    Posts
    6,794
    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
    All problems in computer science can be solved by another level of indirection,
    except for the problem of too many layers of indirection.
    David J. Wheeler

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