Definitions and Declarations (C)

Microsoft Specific

The DLL interface refers to all items (functions and data) that are known to be exported by some program in the system; that is, all items that are declared as dllimport or dllexport. All declarations included in the DLL interface must specify either the dllimport or dllexport attribute. However, the definition can specify only the dllexport attribute. For example, the following function definition generates a compiler error:

#define DllImport   __declspec( dllimport )
#define DllExport   __declspec( dllexport )

DllImport int func()    /* Error; dllimport prohibited in */
                        /* definition. */
{
   return 1;
}

This code also generates an error:

#define DllImport   __declspec( dllimport )
#define DllExport   __declspec( dllexport )

DllImport int i = 10;      /* Error; this is a definition. */

However, this is correct syntax:

#define DllImport   __declspec( dllimport )
#define DllExport   __declspec( dllexport )

DllExport int i = 10;      /* Okay: this is an export definition. */

The use of dllexport implies a definition, while dllimport implies a declaration. You must use the extern keyword with dllexport to force a declaration; otherwise, a definition is implied.

#define DllImport   __declspec( dllimport )
#define DllExport   __declspec( dllexport )

extern DllImport int k;   /* These are correct and imply */
Dllimport int j;          /* a declaration. */

END Microsoft Specific

See also

DLL Import and Export Functions