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CHAPTER 12 ASSEMBLY STARTUP AND RUNTIME INITIALIZATION
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Also notice that the function HelperClass::DoSomething is compiled to native code here. When a native client calls f, and f calls g_helper.DoSomething, no managed code is executed. Therefore, the module constructor is not called. As a consequence, the HelperClass constructor will never be called to initialize g_helper, and f will use an uninitialized g_helper. The output of a native client that just calls f would therefore be the following: f called HelperClass::DoSomething Notice that there is no output from the HelperClass constructor, because it has not been called. If HelperClass::DoSomething had been implemented in a managed object file, the module constructor would have initialized g_helper before DoSomething was called. In this case, the output would be what you expect: f called HelperClass::HelperClass HelperClass::DoSomething The second pitfall that can cause access to uninitialized state is also related to an unprecise separation of the compilation models. It is shown in this source file: // Lib7.cpp // compile with "cl /LD /clr Lib7.cpp" #include <stdio.h> class HelperClass { public: HelperClass(); void DoSomething(); }; HelperClass::HelperClass() { System::Console::WriteLine("HelperClass::HelperClass"); } #pragma managed (push, off) static HelperClass g_helper; void HelperClass::DoSomething() { printf("HelperClass::DoSomething\n"); }
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extern "C" __declspec(dllexport) void f() { printf("f called\n"); g_helper.DoSomething(); } #pragma managed (pop) Compared to the previous source files, this one looks quite convenient. Instead of two CPP files and one header file, only one file is needed here. As in the previous sample, the HelperClass constructor is compiled to managed code, the method HelperClass::DoSomething and the exported function f are compiled to native code. This is achieved via the #pragma managed construct. Since Lib7.cpp is compiled with the /clr switch, g_helper is initialized in the module constructor, not at DLL startup. The fact that g_helper is defined inside a code section that is compiled to native code does not change how it is initialized. When a native client calls f, the native methods printf and HelperClass::DoSomething are called. Since no managed code needs to be executed, the module constructor would not be called, and like before, HelperClass::DoSomething would be called on an uninitialized object g_helper. To avoid access to uninitialized data, follow these rules: If you define a global variable in a CPP file compiled with /clr, define it as a static global variable so that it can only be used in the file in which it is created. static HelperClass g_helper; If the variable is needed in other files, as well, make it available via a getter function. HelperClass& GetHelper() { return g_helper; } Since GetHelper is a managed method, the module constructor will initialize g_helper before calling GetHelper. Therefore, a valid reference to an initialized g_helper can be returned. Similar rules exist for static member variables of native classes as follows: If you define a static member variable of a native class in a CPP file compiled with /clr, make sure that the variable is private. If the variable is needed in other files as well, make it available via a public static getter function.
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To reduce the chance of accessing uninitialized data, you should follow another hint, too: Compile all methods of a class to either managed code or native code. Do not mix compilation models in a class. Had all methods of HelperClass been compiled to managed code, the initialization problem would not have existed. Providing a clear separation between managed code and native code is an important general rule that you should follow.
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Most of the information I have discussed so far applies only to mixed-code DLLs. For DLLs compiled with /clr:pure, the situation is different, because there are two significant restrictions as follows: DLLs built with /clr:pure can contain only managed code. DLLs built with /clr:pure cannot have functions with native calling conventions. The first restriction removes a significant pitfall. Since there is no native code that could be executed, there are no initialization timing problems. All the CRT initialization is done in the module constructor. Due to the second restriction, a powerful option is missing. Since functions with native calling conventions cannot be defined, it is not possible to export functions from a pure-IL DLL. For clients, pure-IL DLLs are only accessible via managed APIs. Therefore, pure-IL DLLs can only be called from managed clients. Consequently, pure-IL DLLs cannot delay-load the CLR.
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In this chapter, I have discussed several implementation details concerning the initialization of the CLR and the CRT in different assembly types. As you have seen, this knowledge is necessary to prevent a range of special problems, most of which are related to the initialization of global variables.
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