Global variable in shared library

This is a discussion on Global variable in shared library within the C Programming forums, part of the General Programming Boards category; If a shared library uses a global variable that is not exported, does each process that uses that library get ...

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    Global variable in shared library

    If a shared library uses a global variable that is not exported, does each process that uses that library get a copy of that variable?

    For example, in the library in the source for one of the object files:

    Code:
    static struct foo * ptr;
    ptr is only used within that file, which is obvious because it is static. So, what I don't understand is if there is only one copy of ptr or if there is one for each process that is using the shared library?

    My initial thought would be that there are indeed multiple copies but I can't seem to get a definitive answer.

    Either way, there's only one copy for all threads that a single process creates, correct? So, I do need to make sure everything is thread safe, but that's another topic.

    Thanks for any help.

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    Quote Originally Posted by krock923 View Post
    If a shared library uses a global variable that is not exported, does each process that uses that library get a copy of that variable?
    Of course. Data is specific to processes, not libraries.

    My initial thought would be that there are indeed multiple copies but I can't seem to get a definitive answer.
    Correct.

    Either way, there's only one copy for all threads that a single process creates, correct?
    Correct.

    Only CODE is shared, not data (well, constant data might be shared)

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    Thanks

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    In practice, what brewbuck says is usually true. However, technically, it depends on how the library is built (linker settings, what segment particular types of data are put into, etc) and on the operating system. With some older systems, which have less emphasis on security than modern systems, a static in a shared library was one way of sharing data between processes.

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    Sigh, what a pain. I'm probably better off just rewriting the thing to get rid of the global, even if it makes the interface less convenient.

    I might be actually better off using C++

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    Quote Originally Posted by krock923 View Post
    Sigh, what a pain. I'm probably better off just rewriting the thing to get rid of the global, even if it makes the interface less convenient.
    I'd agree with that, regardless of whether you're building a shared library or not. Use of globals imply various trade-offs in terms of code reuse (and that directly affects usability of a shared library), ability to use in a multi-threaded environment, ability to call functions recursively, ability to use the same code multiple times within the one program, etc etc. There are certainly some circumstances where a global is necessary but, if possible, it is better to remove the need.

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