Thread Memory

This is a discussion on Thread Memory within the Windows Programming forums, part of the Platform Specific Boards category; If I create a thread using CreateThread( ), does it share the same memory for global variables as the main ...

  1. #1
    C++ Developer XSquared's Avatar
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    Thread Memory

    If I create a thread using CreateThread( ), does it share the same memory for global variables as the main thread? So if I changed a global variable from the main thread, would the value be changed in the new thread?
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    That Creepy Network Guy DeepBlackMagic's Avatar
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    Yes. If you have a global variable the threads can use it. This is my experience with it. Back in the day when i didnt have a clue how to pass a chunk of data as a void pointer i used a bunch of global variables to control my poorly writen threads =P

  3. #3
    It's full of stars adrianxw's Avatar
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    >>>
    changed a global variable from the main thread, would the value be changed in the new thread?
    <<<

    On a single processor system, yes.

    Not necessarily so in a multiprocessor - the main and worker threads will each load the original value into their processors cache, in a poorly designed application, it is possible for both threads to then modify the value in their own processors cache the final value being written back to the programs main memory depending on whichever thread happened to flush it's cache last.

    *** EDIT ***

    As an aside...

    >>> CreateThread( )

    ... has been the topic of much debate. It is now claimed by MS that the call is safe, however, for many years, it has been implicated in memory leaks. I and everyone I know, always use _beginthread() or _beginthreadex() to spin threads.
    Wave upon wave of demented avengers march cheerfully out of obscurity unto the dream.

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    Guest Sebastiani's Avatar
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    Right. It can 'see' global data just as well as other parts of your program. HOWEVER, you must be extremely careful. What if one part of the program is changing the contents of that data just as the thread get's it's turn? Read about locks, mutexes, semaphores, and critical sections before proceeding!
    Code:
    bool fun(bool value)
    {
        return std::pow(std::exp(1), std::complex<float>(0, 1) 
        * std::complex<float>(std::atan(1)*(1 << (value + 2))))
        .real() > 0;
    }

  5. #5
    Guest Sebastiani's Avatar
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    ... has been the topic of much debate. It is now claimed by MS that the call is safe, however, for many years, it has been implicated in memory leaks. I and everyone I know, always use _beginthread() or _beginthreadex() to spin threads.
    That's an important point. If you are using *any* of the C-runtime functions (and who doesn't??), CreateThread() WILL cause small memory leaks (unless it has indeed been fixed - which I honestly doubt, BTW).
    Code:
    bool fun(bool value)
    {
        return std::pow(std::exp(1), std::complex<float>(0, 1) 
        * std::complex<float>(std::atan(1)*(1 << (value + 2))))
        .real() > 0;
    }

  6. #6
    C++ Developer XSquared's Avatar
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    Alright, thanks. I'll definitely re-think how I'm using my threads in that case, and I'll look into _beginthread( ).
    Naturally I didn't feel inspired enough to read all the links for you, since I already slaved away for long hours under a blistering sun pressing the search button after typing four whole words! - Quzah

    You. Fetch me my copy of the Wall Street Journal. You two, fight to the death - Stewie

  7. #7
    BMJ
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    Banal internet user BMJ's Avatar
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    If you use _beginthread don't forget to add "/MT" to your command line (in project options in MSVC++, set for multithreaded), otherwise you'll come back here asking us why you get an "undeclared identifier" for _beginthread

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