Importing Data Using __declspec(dllimport)
The latest version of this topic can be found at 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:
// 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:
// project.def
LIBRARY project
EXPORTS
ulDataInDll CONSTANT
and access it outside the DLL:
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:
__declspec(dllexport) ULONG ulDataInDLL;