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<title>Using the GNU Compiler Collection (GCC): Microsoft Windows Function Attributes</title>
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<a name="Microsoft-Windows-Function-Attributes"></a>
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<div class="header">
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<p>
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Next: <a href="MIPS-Function-Attributes.html#MIPS-Function-Attributes" accesskey="n" rel="next">MIPS Function Attributes</a>, Previous: <a href="MicroBlaze-Function-Attributes.html#MicroBlaze-Function-Attributes" accesskey="p" rel="prev">MicroBlaze Function Attributes</a>, Up: <a href="Function-Attributes.html#Function-Attributes" accesskey="u" rel="up">Function Attributes</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Option-Index.html#Option-Index" title="Index" rel="index">Index</a>]</p>
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<hr>
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<a name="Microsoft-Windows-Function-Attributes-1"></a>
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<h4 class="subsection">6.31.17 Microsoft Windows Function Attributes</h4>
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<p>The following attributes are available on Microsoft Windows and Symbian OS
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targets.
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</p>
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<dl compact="compact">
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<dt><code>dllexport</code></dt>
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<dd><a name="index-dllexport-function-attribute"></a>
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<a name="index-_005f_005fdeclspec_0028dllexport_0029"></a>
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<p>On Microsoft Windows targets and Symbian OS targets the
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<code>dllexport</code> attribute causes the compiler to provide a global
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pointer to a pointer in a DLL, so that it can be referenced with the
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<code>dllimport</code> attribute. On Microsoft Windows targets, the pointer
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name is formed by combining <code>_imp__</code> and the function or variable
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name.
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</p>
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<p>You can use <code>__declspec(dllexport)</code> as a synonym for
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<code>__attribute__ ((dllexport))</code> for compatibility with other
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compilers.
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</p>
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<p>On systems that support the <code>visibility</code> attribute, this
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attribute also implies “default” visibility. It is an error to
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explicitly specify any other visibility.
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</p>
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<p>GCC’s default behavior is to emit all inline functions with the
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<code>dllexport</code> attribute. Since this can cause object file-size bloat,
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you can use <samp>-fno-keep-inline-dllexport</samp>, which tells GCC to
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ignore the attribute for inlined functions unless the
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<samp>-fkeep-inline-functions</samp> flag is used instead.
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</p>
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<p>The attribute is ignored for undefined symbols.
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</p>
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<p>When applied to C++ classes, the attribute marks defined non-inlined
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member functions and static data members as exports. Static consts
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initialized in-class are not marked unless they are also defined
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out-of-class.
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</p>
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<p>For Microsoft Windows targets there are alternative methods for
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including the symbol in the DLL’s export table such as using a
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<samp>.def</samp> file with an <code>EXPORTS</code> section or, with GNU ld, using
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the <samp>--export-all</samp> linker flag.
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</p>
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</dd>
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<dt><code>dllimport</code></dt>
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<dd><a name="index-dllimport-function-attribute"></a>
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<a name="index-_005f_005fdeclspec_0028dllimport_0029"></a>
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<p>On Microsoft Windows and Symbian OS targets, the <code>dllimport</code>
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attribute causes the compiler to reference a function or variable via
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a global pointer to a pointer that is set up by the DLL exporting the
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symbol. The attribute implies <code>extern</code>. On Microsoft Windows
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targets, the pointer name is formed by combining <code>_imp__</code> and the
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function or variable name.
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</p>
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<p>You can use <code>__declspec(dllimport)</code> as a synonym for
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<code>__attribute__ ((dllimport))</code> for compatibility with other
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compilers.
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</p>
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<p>On systems that support the <code>visibility</code> attribute, this
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attribute also implies “default” visibility. It is an error to
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explicitly specify any other visibility.
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</p>
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<p>Currently, the attribute is ignored for inlined functions. If the
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attribute is applied to a symbol <em>definition</em>, an error is reported.
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If a symbol previously declared <code>dllimport</code> is later defined, the
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attribute is ignored in subsequent references, and a warning is emitted.
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The attribute is also overridden by a subsequent declaration as
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<code>dllexport</code>.
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</p>
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<p>When applied to C++ classes, the attribute marks non-inlined
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member functions and static data members as imports. However, the
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attribute is ignored for virtual methods to allow creation of vtables
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using thunks.
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</p>
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<p>On the SH Symbian OS target the <code>dllimport</code> attribute also has
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another affect—it can cause the vtable and run-time type information
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for a class to be exported. This happens when the class has a
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dllimported constructor or a non-inline, non-pure virtual function
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and, for either of those two conditions, the class also has an inline
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constructor or destructor and has a key function that is defined in
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the current translation unit.
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</p>
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<p>For Microsoft Windows targets the use of the <code>dllimport</code>
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attribute on functions is not necessary, but provides a small
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performance benefit by eliminating a thunk in the DLL. The use of the
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<code>dllimport</code> attribute on imported variables can be avoided by passing the
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<samp>--enable-auto-import</samp> switch to the GNU linker. As with
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functions, using the attribute for a variable eliminates a thunk in
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the DLL.
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</p>
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<p>One drawback to using this attribute is that a pointer to a
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<em>variable</em> marked as <code>dllimport</code> cannot be used as a constant
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address. However, a pointer to a <em>function</em> with the
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<code>dllimport</code> attribute can be used as a constant initializer; in
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this case, the address of a stub function in the import lib is
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referenced. On Microsoft Windows targets, the attribute can be disabled
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for functions by setting the <samp>-mnop-fun-dllimport</samp> flag.
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</p></dd>
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</dl>
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<hr>
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<div class="header">
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<p>
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Next: <a href="MIPS-Function-Attributes.html#MIPS-Function-Attributes" accesskey="n" rel="next">MIPS Function Attributes</a>, Previous: <a href="MicroBlaze-Function-Attributes.html#MicroBlaze-Function-Attributes" accesskey="p" rel="prev">MicroBlaze Function Attributes</a>, Up: <a href="Function-Attributes.html#Function-Attributes" accesskey="u" rel="up">Function Attributes</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Option-Index.html#Option-Index" title="Index" rel="index">Index</a>]</p>
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