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all the functions exported from the library is wasteful of space) Instead, Symbian OS only uses link by ordinal, which has signi cant implications for binary compatibility Ordinals must not be changed between one release of a DLL and another, otherwise code which originally used the old DLL will not be able to locate the functions it needs in the new version of the DLL I ll discuss binary compatibility further in 18
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While EXE components have separate data areas for program code, read-only data and writable data, DLLs do not have the latter This has the following consequence: Symbian OS DLLs do not support writable global data So why is there no writable data section for Symbian DLLs The reason is that any code which refers to global data must use an address to do so, rather than an offset from a pointer When code is loaded, it must either use a xed address to somewhere in the DLL in order to locate the data, or it must use a relocation value for the data, if it is moved to a new address Furthermore, because DLLs are shared between processes, every process in which it loads must use the same address for the global data2 Thus, each DLL that supported writable static data would need a section of RAM (a chunk , the basic unit of system memory) allocated for it within every process that loaded it, just for static data The smallest size of a chunk is 4 KB which comes to a signi cant overhead when you consider the number of DLLs that a typical application on Symbian OS might use (often over 50), and the fact that the DLL would typically waste most of this memory, since it is unlikely to declare exactly 4 KB worth of static data This restriction means that you cannot use static member variables, such as those used to implement the singleton pattern (which allows only one instance of a class to be instantiated and used, and is useful for implementing a single controller type object) This can be inconvenient, particularly when porting code which makes uses of this idiom, or indeed any other which requires global data Here s an example of a simple task manager where I ve included just the minimum amount of code needed to illustrate the use of a singleton
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// TaskManagerh // Header file class CTask; // Defined elsewhere
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2 If the required address for the data has already been occupied when a DLL comes to load, the DLL will not be usable This is quite possible, because the data is placed in a chunk which means that its address must start on a megabyte boundary, of which there are few A workaround would be to copy the program code for the DLL, and adjust the copy to use a different address for the static data, but the overhead would be unacceptably high
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class CTaskManager : public CBase { public: IMPORT_C static CTaskManager* TaskManagerL(); IMPORT_C static void DestroyTaskManager(); public: IMPORT_C void AddTaskL(CTask* aTask); // Omitted for clarity private: CTaskManager(); // Private - destroy the singleton through DestroyTaskManager() CTaskManager(); void ConstructL(); private: CTaskManager(const CTaskManager&); // Not implemented CTaskManager& operator =(const CTaskManager&);// Prevents copying private: static CTaskManager* iTaskManager; // The singleton instance private: RPointerArray<CTask>* iTasks; }; // TaskManagercpp // Implementation // Initializes the static data CTaskManager* CTaskManager::iTaskManager = NULL; EXPORT_C CTaskManager* CTaskManager::TaskManagerL() { if (!iTaskManager) {// Construct the singleton object on first use CTaskManager* taskManager = new (ELeave) CTaskManager(); CleanupStack::PushL(taskManager); taskManager->ConstructL(); CleanupStack::Pop(taskManager); iTaskManager = iTaskManager; } return (iTaskManager); } EXPORT_C void CTaskManager::DestroyTaskManager() { delete iTaskManager; iTaskManager = NULL; } // The use of dynamic arrays is discussed in 7 void CTaskManager::ConstructL() {// Creates the underlying array iTasks = new (ELeave) RPointerArray<CTask>(4); } // Exported function through which clients manipulate the array EXPORT_C void CTaskManager::AddTaskL(CTask* aTask) { User::LeaveIfError(iTasks->Append(aTask)); }
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