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CHAPTER 10 WRAPPING NATIVE LIBRARIES
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For this function, a reasonable wrapper could be the following: void fWrapper(int% i) { int j = i; f(j); i = j; } A native int reference must be passed to call the native function. Since there is no conversion from a tracking reference to a native reference, the argument must be marshaled manually. Since there is a standard conversion from int to int&, a local variable of type int is used as a buffer for the by-reference argument. Before the native function is called, this buffer is initialized with the value passed as the argument i. When the native function returns to the wrapper, the value referred by the argument i is updated with the changes made to the buffer j. As you can see in this sample, in addition to the costs of managed-unmanaged transitions, wrapper libraries often need extra processor cycles for type marshaling. For more complex types (discussed later), this overhead can be significantly higher. You should also be aware that some other .NET languages, including C#, distinguish by-reference arguments and out-only arguments. For a by-reference argument, an initialized variable must be passed, and the called function can modify this value or leave the value untouched. For an out-only argument, an uninitialized variable can be passed, and the called function must modify or initialize its value. By default, a tracking reference is considered to have by-reference semantics. If you want to define an argument with out-only semantics, you have to use the OutAttribute from the namespace System::Runtime::InteropServices, as shown here: void fWrapper([Out] int% i); Argument types of native functions often have the const modifier, as shown in the following sample: void f(int& i1, const int& i2); As discussed in 8, the const modifier is translated to an optional signature modifier. Managed callers that do not understand the const signature modifier can still call an fWrapper function, defined as follows: void fWrapper(int% i1, const int% i2); When the native argument is a pointer to an array, tracking reference arguments are not sufficient. To discuss this case, let s assume that the native SampleCipher class has a constructor that expects arguments to pass the encryption key: namespace NativeLib { class SampleCipher : public CryptoAlgorithm { public: SampleCipher(const unsigned char* pKey, int nKeySizeInBytes); /* ... implementation can be ignored so far ... */ }; }
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CHAPTER 10 WRAPPING NATIVE LIBRARIES
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Mapping const unsigned char* to const unsigned char% would not be sufficient here, because the encryption key passed to the constructor of the native type contains more than one byte. The following code shows a better approach: namespace ManagedWrapper { public ref class SampleCipher { NativeLib::SampleCipher* pWrappedObject; public: SampleCipher(array<Byte>^ key); /* ... */ }; } In this constructor, both native arguments (pKey and nKeySizeInBytes) are mapped to a single argument of a managed array type. This is possible because the size of a managed array can be determined at runtime. The implementation of this constructor depends on the implementation of the native SampleCipher class. If the constructor of the native class internally copies the key that is passed via the argument pKey, you can use a pinned pointer to pass the key: SampleCipher::SampleCipher(array<Byte>^ key) { if (!key) throw gcnew ArgumentNullException("key"); pin_ptr<unsigned char> pp = &key[0]; pWrappedObject = new NativeLib::SampleCipher(pp, key->Length); } However, a pinned pointer cannot be used if the native SampleCipher class is implemented as follows: namespace NativeLib { class SampleCipher : public CryptoAlgorithm { const unsigned char* pKey; const int nKeySizeInBytes; public: SampleCipher(const unsigned char* pKey, int nKeySizeInBytes) : pKey(pKey), nKeySizeInBytes(nKeySizeInBytes) {} /* ... rest of the class can be ignored here ... */ }; }
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CHAPTER 10 WRAPPING NATIVE LIBRARIES
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This constructor expects the client to ensure that the memory for the key is not deallocated and the pointer to the key remains valid until the SampleCipher instance is destroyed. The constructor of the wrapper class does not meet both requirements. Since no tracking handle to the managed array is stored in the managed wrapper class, the GC could clean up the managed array before the native class is destroyed. Even if a tracking handle to the managed array ensures that the memory is not reclaimed, the array could be relocated during garbage collection. In this case, the native pointer would no longer refer to the managed array. To ensure that the key s memory is not reclaimed and that the key s data is not relocated during garbage collection, it should be copied on the native heap. This requires changes in the constructor as well as the destructor of the managed wrapper class. The following code shows how the constructor and the destructor could be implemented: public ref class SampleCipher { unsigned char* pKey; NativeLib::SampleCipher* pWrappedObject; public: SampleCipher(array<Byte>^ key) : pKey(0), pWrappedObject(0) { if (!key) throw gcnew ArgumentNullException("key"); pKey = new unsigned char[key->Length]; if (!pKey) throw gcnew OutOfMemoryException("Allocation on C++ free store failed"); try { Marshal::Copy(key, 0, IntPtr(pKey), key->Length); pWrappedObject = new NativeLib::SampleCipher(pKey, key->Length); if (!pWrappedObject) throw gcnew OutOfMemoryException("Allocation on C++ free store failed"); } catch (Object^) { delete[] pKey; throw; } } ~SampleCipher() { try { delete pWrappedObject;
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