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Writeable static data for ROM-resident kernel-mode DLLs (that is, those declared in the rombuild.oby le with the keywords variant, device or extension) is appended to the kernel s static data. Initialization of variant and extension data occurs at kernel boot time, while initialization of device data occurs at device driver load time. Writeable static data for RAM-resident device drivers is placed in an extra kernel data chunk which is mapped with supervisor-only access. It is important to note that since the kernel itself loads extensions, they are never unloaded. Therefore, the destructors for any globally constructed objects will never be called.
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Entry points
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Each type of kernel DLL has a unique set of characteristics that de ne how and when the kernel loads them during boot; these are de ned by the form of the DLL s entry point. Symbian provides three different entry points, in three different libraries, and you select one by choosing which of these libraries to link against. The tool chain will automatically link in the appropriate library depending on the value of the targettype eld in the DLL s MMP le: Targettype VAR KEXT PDD LDD Library EVAR.LIB EEXT.LIB EDEV.LIB EDEV.LIB DLL type Variant kernel extension Kernel extension Physical device driver DLL Logical device driver DLL
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The main entry point for all kernel DLLs is named _E32Dll, and its address, represented by TRomImageHeader::iEntryPoint, is obtained from the image header. This in turn invokes the DLL speci c entry point _E32Dll_Body, the behavior of which depends on the type of kernel DLL being loaded. Note: There is a fourth library, EKLL.LIB, which is imported when specifying the KDLL keyword for shared library DLLs. Kernel DLLs of this type contain no static data or initialization code, so contain a simple stub entry point. Before I describe in detail how the kernel controls the initialization of kernel DLLs are during boot, let s take a look at how each library entry point handles construction and destruction of its global C++ objects.
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12.1.6.1 Construction of C++ objects Variant extensions EVAR.LIB Since the kernel loads the variant extension once, the variant entry point only constructs the DLL s global C++ objects. Destructors are never called:
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GLDEF_C TInt _E32Dll_Body(TInt aReason) // // Call variant and ASIC global constructors // { if (aReason==KModuleEntryReasonVariantInit0) { TUint i=1; while (__CTOR_LIST__[i]) (*__CTOR_LIST__[i++])(); AsicInitialise(); return 0; } return KErrGeneral; }
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Kernel extensions EEXT.LIB As with the variant extension, the kernel loads an extension once during boot, so it only constructs the DLLs global C++ objects and never calls their destructors:
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GLDEF_C TInt _E32Dll_Body(TInt aReason) // // Call extension global constructors // { if (aReason==KModuleEntryReasonExtensionInit1) { TUint i=1; while (__CTOR_LIST__[i]) (*__CTOR_LIST__[i++])(); } return KernelModuleEntry(aReason); }
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Device drivers EDEV.LIB The kernel loads and unloads device drivers dynamically, so it constructs global C++ objects when loading the DLL, and destroys them when unloading it:
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GLDEF_C TInt _E32Dll_Body(TInt aReason) // // Call global constructors or destructors //
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{ if (aReason==KModuleEntryReasonProcessDetach) { TUint i=1; while (__DTOR_LIST__[i]) (*__DTOR_LIST__[i++])(); return KErrNone; } if (aReason==KModuleEntryReasonExtensionInit1 || aReason==KModuleEntryReasonProcessAttach) { TUint i=1; while (__CTOR_LIST__[i]) (*__CTOR_LIST__[i++])(); } return KernelModuleEntry(aReason); }
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12.1.6.2 Calling entry points
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As the previous code shows, the kernel invokes _E32Dll with a reason code, which it uses to control how DLLs are loaded. Each reason code is passed during a particular stage in the boot process.
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KModuleEntryReasonVariantInit0 Before initializing the variant, the kernel initializes the .data sections for all kernel extensions and passes the reason code KModuleEntryReasonVariantInit0 to all extension entry points. Typically, only the variant extension handles this reason code and, as we have already seen, this is responsible for constructing global C++ objects before invoking AsicInitialise() to initialize the ASSP. In 1, Introducing EKA2, I pointed out that the base port might be split into an ASSP and a variant. Under this model, the generic ASSP class forms a standard kernel extension that exports its constructor (at the very least). The ASSP class must be initialized at the same time as the variant, so it also exports the function AsicInitialise() to allow its global C++ constructors to be called. After it has initialized both the variant and the ASSP extensions, the kernel obtains a pointer to the Asic derived variant speci c class by calling the variant s rst exported function:
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EXPORT_C Asic* VariantInitialise()
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This class is described in 5, Kernel Services. At the end of this process, all of the extensions .data sections are initialized, and the variant and ASSP extensions are constructed and ready for the kernel to use.
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