6.32.8 Microsoft Windows Variable Attributes

You can use these attributes on Microsoft Windows targets. x86 Variable Attributes for additional Windows compatibility attributes available on all x86 targets.

The dllimport and dllexport attributes are described in Microsoft Windows Function Attributes .
The selectany attribute causes an initialized global variable to have link-once semantics. When multiple definitions of the variable are encountered by the linker, the first is selected and the remainder are discarded. Following usage by the Microsoft compiler, the linker is told not to warn about size or content differences of the multiple definitions.

Although the primary usage of this attribute is for POD types, the attribute can also be applied to global C++ objects that are initialized by a constructor. In this case, the static initialization and destruction code for the object is emitted in each translation defining the object, but the calls to the constructor and destructor are protected by a link-once guard variable.

The selectany attribute is only available on Microsoft Windows targets. You can use __declspec (selectany) as a synonym for __attribute__ ((selectany)) for compatibility with other compilers.

On Microsoft Windows, in addition to putting variable definitions in a named section, the section can also be shared among all running copies of an executable or DLL. For example, this small program defines shared data by putting it in a named section shared and marking the section shareable:
          int foo __attribute__((section ("shared"), shared)) = 0;
            /* Read and write foo.  All running
               copies see the same value.
            return 0;

You may only use the shared attribute along with section attribute with a fully-initialized global definition because of the way linkers work. See section attribute for more information.

The shared attribute is only available on Microsoft Windows.