Deadlock occurs in Function Scoped Static variables (Thread Unsafe in VC++)
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Thread: Deadlock occurs in Function Scoped Static variables (Thread Unsafe in VC++)

  1. #1
    Join Date
    Feb 2010
    Posts
    1

    Angry Deadlock occurs in Function Scoped Static variables (Thread Unsafe in VC++)

    The question is how function-level statics are constructed when the function is called on multiple threads?

    Problem Description: Deadlock occurs and my application doesn't get terminated. During initialization of local static variable it tries to acquire MSVCR80!_lock and never gets hold on the lock.

    Below is the calls stack and you will see that it will never get hold on the lock _mlock

    (_EXIT_LOCK1); //C:\Program Files\Microsoft Visual Studio 8\VC\crt\src\crt0dat.c

    ntdll!RtlpWaitForCriticalSection+0x132 ntdll!RtlEnterCriticalSection+0x46
    MSVCR80!_lock+0x2e MyDLL!_onexit+0x36 [f:\sp\vctools\crt_bld\self_x86\crt\src\atonexit.c @ 103] MyDLL!atexit+0x9 [f:\sp\vctools\crt_bld\self_x86\crt\src\atonexit.c @ 127] MyDLL!__DllMainCRTStartup+0x7a [f:\sp\vctools\crt_bld\self_x86\crt\src\crtdll.c @ 498] MyDLL!_DllMainCRTStartup+0x1d [f:\sp\vctools\crt_bld\self_x86\crt\src\crtdll.c @ 462] ntdll!LdrpCallInitRoutine+0x14

    // Code snippet below
    Code:
    void main() 
    {
    
        atexit(MyCallBack); 
        exit(0); 
    
    }
    
    void MyCallBack() 
    {
    
    // Waitingforsingleobject() // Waits until all threads are terminated
    
    }
    The EXE call DllMain with DLL_THREAD_DETACH flag and we have an explicit handling as shown below
    Code:
    BOOL APIENTRY DllMain( HANDLE, DWORD dwReason, LPVOID ) 
    {
      if(dwReason == DLL_THREAD_DETACH) 
      { 
        F1();
        F2();
      }
    }
    
    F1()
    {
    
        const static CComBSTR bstrMethod = __ FUNCTION __ ;
    
    }
    
    F2()
    {
    
        const static CComBSTR bstrMethod = __ FUNCTION __ ;
    
    }
    Is it thread safe to have local static initialization within a function. Also I noticed if static variable is once initialized before the exit() of main application I don't see any problem. Can any one please explain what might be issue?

    Note: But when I make static variable as non static the deadlock doesn't occur and problem is solved.

    Also let me know any alternate solution which might help in this situation Eagerly waiting for reply.
    Last edited by cilu; February 26th, 2010 at 08:26 AM. Reason: code tags

  2. #2
    Join Date
    Nov 2003
    Posts
    1,794

    Re: Deadlock occurs in Function Scoped Static variables (Thread Unsafe in VC++)

    >> Is it thread safe to have local static initialization within a function.
    Nope. Local statics are initialized when code execution reaches that point. So the compiler has to generate code like "if this is the first time, do init". This generated code isn't thread safe, unless your compiler explicitly documents support for thread-safe initialization of local statics.

    On a side note, the new C++0x standard will make local static initialization thread-safe.

    I'm guessing that you're having deadlock because you're trying to do too much from within DllMain(). First, all calls into DllMain are serialized by the OS. So blocking inside DllMain can easily deadlock. DllMain calls should do very little - preferably nothing.

    http://msdn.microsoft.com/en-us/libr...8VS.85%29.aspx

    http://blogs.msdn.com/oldnewthing/ar.../28/63880.aspx

    gg

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