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<a name="Microsoft-Windows-Function-Attributes"></a>
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<a name="Microsoft-Windows-Function-Attributes-1"></a>
<h4 class="subsection">6.33.20 Microsoft Windows Function Attributes</h4>

<p>The following attributes are available on Microsoft Windows and Symbian OS
targets.
</p>
<dl compact="compact">
<dt><code>dllexport</code></dt>
<dd><a name="index-dllexport-function-attribute"></a>
<a name="index-_005f_005fdeclspec_0028dllexport_0029"></a>
<p>On Microsoft Windows targets and Symbian OS targets the
<code>dllexport</code> attribute causes the compiler to provide a global
pointer to a pointer in a DLL, so that it can be referenced with the
<code>dllimport</code> attribute.  On Microsoft Windows targets, the pointer
name is formed by combining <code>_imp__</code> and the function or variable
name.
</p>
<p>You can use <code>__declspec(dllexport)</code> as a synonym for
<code>__attribute__ ((dllexport))</code> for compatibility with other
compilers.
</p>
<p>On systems that support the <code>visibility</code> attribute, this
attribute also implies &ldquo;default&rdquo; visibility.  It is an error to
explicitly specify any other visibility.
</p>
<p>GCC&rsquo;s default behavior is to emit all inline functions with the
<code>dllexport</code> attribute.  Since this can cause object file-size bloat,
you can use <samp>-fno-keep-inline-dllexport</samp>, which tells GCC to
ignore the attribute for inlined functions unless the 
<samp>-fkeep-inline-functions</samp> flag is used instead.
</p>
<p>The attribute is ignored for undefined symbols.
</p>
<p>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.
</p>
<p>For Microsoft Windows targets there are alternative methods for
including the symbol in the DLL&rsquo;s export table such as using a
<samp>.def</samp> file with an <code>EXPORTS</code> section or, with GNU ld, using
the <samp>--export-all</samp> linker flag.
</p>
</dd>
<dt><code>dllimport</code></dt>
<dd><a name="index-dllimport-function-attribute"></a>
<a name="index-_005f_005fdeclspec_0028dllimport_0029"></a>
<p>On Microsoft Windows and Symbian OS targets, the <code>dllimport</code>
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 <code>extern</code>.  On Microsoft Windows
targets, the pointer name is formed by combining <code>_imp__</code> and the
function or variable name.
</p>
<p>You can use <code>__declspec(dllimport)</code> as a synonym for
<code>__attribute__ ((dllimport))</code> for compatibility with other
compilers.
</p>
<p>On systems that support the <code>visibility</code> attribute, this
attribute also implies &ldquo;default&rdquo; visibility.  It is an error to
explicitly specify any other visibility.
</p>
<p>Currently, the attribute is ignored for inlined functions.  If the
attribute is applied to a symbol <em>definition</em>, an error is reported.
If a symbol previously declared <code>dllimport</code> 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
<code>dllexport</code>.
</p>
<p>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.
</p>
<p>On the SH Symbian OS target the <code>dllimport</code> attribute also has
another affect&mdash;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.
</p>
<p>For Microsoft Windows targets the use of the <code>dllimport</code>
attribute on functions is not necessary, but provides a small
performance benefit by eliminating a thunk in the DLL.  The use of the
<code>dllimport</code> attribute on imported variables can be avoided by passing the
<samp>--enable-auto-import</samp> switch to the GNU linker.  As with
functions, using the attribute for a variable eliminates a thunk in
the DLL.
</p>
<p>One drawback to using this attribute is that a pointer to a
<em>variable</em> marked as <code>dllimport</code> cannot be used as a constant
address. However, a pointer to a <em>function</em> with the
<code>dllimport</code> 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 <samp>-mnop-fun-dllimport</samp> flag.
</p></dd>
</dl>

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