global variables in shared libraries

From: martin f krafft (madduck_at_debian.org)
Date: 01/21/04

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    Date: Wed, 21 Jan 2004 14:13:07 +0100
To: debian users <debian-user@lists.debian.org>

    Hi there,

    Please excuse the OTness of this post. Since I am writing a library
    to be included in Debian, I feel that I should not be slaughtered
    for bothering you and hoping for your time and knowledge.

    I have a question about shared libraries. Even though I did search
    the web, I could not find a reliable answer. I am writing a shared
    library with the intention to make it reentrant and thread-safe, if
    these concepts even apply.

    My question is about global variables -- I know they are bad
    programming style, but when it comes to debugging or subsystem
    initialisation in C, they are sometimes really necessary.

    If I use a global variable in a shared library, is it shared among
    all instances using that library? Here, I would say no because
    a process' memory space is separate from another's.

    However, what happens when I have a threaded process us the library?

    I have come up with two ways to deal without global variables in C:

    The first is to expect the user to instantiate a struct, e.g.
    a struct global_settings and pass it along to each function of the
    API. This is cumbersome but would work, although I could not be
    assured that it's always the same instantiation -- the user could
    create two objects and pass them alternatingly, thereby messing with
    the integrity of the global settings and the library code. It is
    questionable whether I should guard against such a case, but I feel
    like it ;^>.

    The other approach seems cleaner: I declare "extern" variables in
    the interface and expect the user to instantiate them, while I use
    them freely in the code. The problem here is that the user would
    have to do so on a global level in his/her code, so wwe'd have the
    same problem again, albeit the responsibility would be shifted from
    the library to the user code. I am not sure this is preferable.

    I'd be interested to hear your opinions, and how you deal with these
    challenges. 

    -- 
Please do not CC me when replying to lists; I read them!
 
 .''`.     martin f. krafft <madduck@debian.org>
: :'  :    proud Debian developer, admin, and user
`. `'`
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