6.31.17 Microsoft Windows Function Attributes

The following attributes are available on Microsoft Windows and Symbian OS targets.

dllexport

On Microsoft Windows targets and Symbian OS targets the dllexport attribute causes the compiler to provide a global pointer to a pointer in a DLL, so that it can be referenced with the dllimport attribute. On Microsoft Windows targets, the pointer name is formed by combining _imp__ and the function or variable name.

You can use __declspec(dllexport) as a synonym for __attribute__ ((dllexport)) for compatibility with other compilers.

On systems that support the visibility attribute, this attribute also implies “default” visibility. It is an error to explicitly specify any other visibility.

GCC’s default behavior is to emit all inline functions with the dllexport attribute. Since this can cause object file-size bloat, you can use -fno-keep-inline-dllexport, which tells GCC to ignore the attribute for inlined functions unless the -fkeep-inline-functions flag is used instead.

The attribute is ignored for undefined symbols.

When applied to C++ classes, the attribute marks defined non-inlined member functions and static data members as exports. Static consts initialized in-class are not marked unless they are also defined out-of-class.

For Microsoft Windows targets there are alternative methods for including the symbol in the DLL’s export table such as using a .def file with an EXPORTS section or, with GNU ld, using the --export-all linker flag.

dllimport

On Microsoft Windows and Symbian OS targets, the dllimport attribute causes the compiler to reference a function or variable via a global pointer to a pointer that is set up by the DLL exporting the symbol. The attribute implies extern. On Microsoft Windows targets, the pointer name is formed by combining _imp__ and the function or variable name.

You can use __declspec(dllimport) as a synonym for __attribute__ ((dllimport)) for compatibility with other compilers.

On systems that support the visibility attribute, this attribute also implies “default” visibility. It is an error to explicitly specify any other visibility.

Currently, the attribute is ignored for inlined functions. If the attribute is applied to a symbol definition, an error is reported. If a symbol previously declared dllimport is later defined, the attribute is ignored in subsequent references, and a warning is emitted. The attribute is also overridden by a subsequent declaration as dllexport.

When applied to C++ classes, the attribute marks non-inlined member functions and static data members as imports. However, the attribute is ignored for virtual methods to allow creation of vtables using thunks.

On the SH Symbian OS target the dllimport attribute also has another affect—it can cause the vtable and run-time type information for a class to be exported. This happens when the class has a dllimported constructor or a non-inline, non-pure virtual function and, for either of those two conditions, the class also has an inline constructor or destructor and has a key function that is defined in the current translation unit.

For Microsoft Windows targets the use of the dllimport attribute on functions is not necessary, but provides a small performance benefit by eliminating a thunk in the DLL. The use of the dllimport attribute on imported variables can be avoided by passing the --enable-auto-import switch to the GNU linker. As with functions, using the attribute for a variable eliminates a thunk in the DLL.

One drawback to using this attribute is that a pointer to a variable marked as dllimport cannot be used as a constant address. However, a pointer to a function with the dllimport attribute can be used as a constant initializer; in this case, the address of a stub function in the import lib is referenced. On Microsoft Windows targets, the attribute can be disabled for functions by setting the -mnop-fun-dllimport flag.