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Next: <a href="_005f_005fhas_005fcpp_005fattribute.html#g_t_005f_005fhas_005fcpp_005fattribute" accesskey="n" rel="next"><code>__has_cpp_attribute</code></a>, Previous: <a href="Elif.html#Elif" accesskey="p" rel="previous">Elif</a>, Up: <a href="Conditional-Syntax.html#Conditional-Syntax" accesskey="u" rel="up">Conditional Syntax</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Index-of-Directives.html#Index-of-Directives" title="Index" rel="index">Index</a>]</p>
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<h4 class="subsection">4.2.6 <code>__has_attribute</code></h4>
<a name="index-_005f_005fhas_005fattribute"></a>
<p>The special operator <code>__has_attribute (<var>operand</var>)</code> may be used
in ‘<samp>#if</samp>’ and ‘<samp>#elif</samp>’ expressions to test whether the attribute
referenced by its <var>operand</var> is recognized by GCC. Using the operator
in other contexts is not valid. In C code, if compiling for strict
conformance to standards before C2x, <var>operand</var> must be
a valid identifier. Otherwise, <var>operand</var> may be optionally
introduced by the <code><var>attribute-scope</var>::</code> prefix.
The <var>attribute-scope</var> prefix identifies the “namespace” within
which the attribute is recognized. The scope of GCC attributes is
‘<samp>gnu</samp>’ or ‘<samp>__gnu__</samp>’. The <code>__has_attribute</code> operator by
itself, without any <var>operand</var> or parentheses, acts as a predefined
macro so that support for it can be tested in portable code. Thus,
the recommended use of the operator is as follows:
</p>
<div class="smallexample">
<pre class="smallexample">#if defined __has_attribute
# if __has_attribute (nonnull)
# define ATTR_NONNULL __attribute__ ((nonnull))
# endif
#endif
</pre></div>
<p>The first ‘<samp>#if</samp>’ test succeeds only when the operator is supported
by the version of GCC (or another compiler) being used. Only when that
test succeeds is it valid to use <code>__has_attribute</code> as a preprocessor
operator. As a result, combining the two tests into a single expression as
shown below would only be valid with a compiler that supports the operator
but not with others that don’t.
</p>
<div class="smallexample">
<pre class="smallexample">#if defined __has_attribute && __has_attribute (nonnull) /* not portable */
…
#endif
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