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.NET Forum / Languages / Managed C++ / October 2007DllMain
Hello everyone, From MSDN, http://msdn2.microsoft.com/en-us/library/ms682583.aspx parameter fdwReason of DllMain has four values, DLL_PROCESS_ATTACH DLL_PROCESS_DETACH DLL_THREAD_ATTACH DLL_THREAD_DETACH I think we could simply understand what MSDN described about the four values in the following way after some experiment (I have read the big table and want to extract some brief and simple information to understand to remember), DLL_PROCESS_ATTACH: will be called only once when the process loads the DLL for the 1st time DLL_PROCESS_DETACH: will be called only once when the process unloads the DLL (when process stops) DLL_THREAD_ATTACH: will be called every time when a thread inside the current process loads the DLL DLL_THREAD_DETACH: will be called every time when a thread inside the current process unloads the DLL Is that correct understanding? thanks in advance, George >Is that correct understanding? Yes that's essentially it - to the best of my understanding of course. Dave Hi David, I think my previous understanding has some mistakes. I think if a process calls LoadLibrary again for the DLL after the DLL is already loaded, DllMain will be invoked again with DLL_PROCESS_ATTACH value, right? regards, George > >Is that correct understanding? > > Yes that's essentially it - to the best of my understanding of course. > > Dave >I think my previous understanding has some mistakes. I think if a process >calls LoadLibrary again for the DLL after the DLL is already loaded, DllMain >will be invoked again with DLL_PROCESS_ATTACH value, right? I wouldn't have thought so - but I've never investigated it in that much details. The MSDN documentation could be construed as saying it would do that, but I think it really means its only done once - or if the process has unloaded the DLL and then re-loaded it (which is the same thing really). Dave Thanks Dave, I have such questions after reading Ben's answer, > If LoadLibrary and FreeLibrary are used, then DLL_PROCESS_ATTACH is called > each time the use count changes from 0 to 1 and DLL_PROCESS_DETACH is > called each time the use count changes from 1 to 0. This could happen multiple > times in a single run. regards, George > >I think my previous understanding has some mistakes. I think if a process > >calls LoadLibrary again for the DLL after the DLL is already loaded, DllMain [quoted text clipped - 7 lines] > > Dave >I have such questions after reading Ben's answer, > >> If LoadLibrary and FreeLibrary are used, then DLL_PROCESS_ATTACH is called >> each time the use count changes from 0 to 1 and DLL_PROCESS_DETACH is >> called each time the use count changes from 1 to 0. This could happen multiple >> times in a single run. Yes it's reasonable that if a process unloads a DLL and then re-loads it, it's not really different to the initial load situation, so you would expect the process attach notification. Dave I agree, thanks Dave. regards, George > >I have such questions after reading Ben's answer, > > [quoted text clipped - 8 lines] > > Dave George, DLL_THREAD_ATTACH will be called for every already loaded DLL when a new thread is created. (This allows the DLL to setup things like thread local storage). There is no requirement for the thread to load the DLL. Similarly for DLL_THREAD_DETACH - every loaded DLL is called when the thread exits. Bob > Hello everyone, > [quoted text clipped - 29 lines] > thanks in advance, > George Thanks Bob, It is interesting and after reading MSDN reference, I think each time a new thread is created, DllMain will be invoked (as callback function) with DLL_THREAD_ATTACH value (support DLL is loaded before creating thread) -- even if the thread does not use the DLL at all, right? regards, George > George, > DLL_THREAD_ATTACH will be called for every already loaded DLL when a new [quoted text clipped - 36 lines] > > thanks in advance, > > George > Hello everyone, > [quoted text clipped - 20 lines] > DLL_PROCESS_DETACH: will be called only once when the process unloads the > DLL (when process stops) No, these are wrong (others have explained the THREAD messages). If LoadLibrary and FreeLibrary are used, then DLL_PROCESS_ATTACH is called each time the use count changes from 0 to 1 and DLL_PROCESS_DETACH is called each time the use count changes from 1 to 0. This could happen multiple times in a single run. Thanks Ben, I think you mean if we have already load the library, if we call LoadLibrary again and again, the DllMain will not be called again and again since the reference counter is already >1, right? But if the process unloads the DLL (make reference counter to 0), then LoadLibrary again, the DllMain will be called again with DLL_PROCESS_ATTACH value, right? regards, George > > Hello everyone, > > [quoted text clipped - 27 lines] > each time the use count changes from 1 to 0. This could happen multiple > times in a single run. > Thanks Ben, > [quoted text clipped - 7 lines] > DLL_PROCESS_ATTACH > value, right? I think that is correct, yes. > regards, > George [quoted text clipped - 34 lines] >> each time the use count changes from 1 to 0. This could happen multiple >> times in a single run. Thanks for your confirmation, Ben! regards, George > > Thanks Ben, > > [quoted text clipped - 48 lines] > >> each time the use count changes from 1 to 0. This could happen multiple > >> times in a single run. Free MagazinesGet these publications absolutely FREE for up to 12 months. There are no hidden fees and no obligation. Simply choose a title, complete the application form and submit it. Read more ...
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