Importing Data Using __declspec(dllimport)

In the case of data, using __declspec(dllimport) is a convenience item that removes a layer of indirection. When you import data from a DLL, you still have to go through the import address table. Before __declspec(dllimport), this meant you had to remember to do an extra level of indirection when accessing data exported from the DLL:

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// project.h #ifdef _DLL // If accessing the data from inside the DLL ULONG ulDataInDll; #else // If accessing the data from outside the DLL ULONG *ulDataInDll; #endif

You would then export the data in your .DEF file:

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// project.def LIBRARY project EXPORTS ulDataInDll CONSTANT

and access it outside the DLL:

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if (*ulDataInDll == 0L) { // Do stuff here }

When you mark the data as __declspec(dllimport), the compiler automatically generates the indirection code for you. You no longer have to worry about the steps above. As stated previously, do not use __declspec(dllimport) declaration on the data when building the DLL. Functions within the DLL do not use the import address table to access the data object; therefore, you will not have the extra level of indirection present.

To export the data automatically from the DLL, use this declaration:

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__declspec(dllexport) ULONG ulDataInDLL;

See Also

Concepts