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| 22 | <title>Attributes (GNU C Language Manual)</title>
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| 55 | </head>
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| 57 | <body lang="en">
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| 58 | <span id="Attributes"></span><div class="header">
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| 59 | <p>
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| 60 | Next: <a href="Signals.html" accesskey="n" rel="next">Signals</a>, Previous: <a href="Digraphs.html" accesskey="p" rel="prev">Digraphs</a>, Up: <a href="index.html" accesskey="u" rel="up">Top</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Symbol-Index.html" title="Index" rel="index">Index</a>]</p>
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| 61 | </div>
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| 62 | <hr>
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| 63 | <span id="Attributes-in-Declarations"></span><h2 class="appendix">Appendix D Attributes in Declarations</h2>
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| 64 | <span id="index-attributes"></span>
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| 65 | <span id="index-_005f_005fattribute_005f_005f"></span>
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| 66 |
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| 67 | <p>You can specify certain additional requirements in a declaration, to
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| 68 | get fine-grained control over code generation, and helpful
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| 69 | informational messages during compilation. We use a few attributes in
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| 70 | code examples throughout this manual, including
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| 71 | </p>
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| 72 | <dl compact="compact">
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| 73 | <dt><code>aligned</code></dt>
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| 74 | <dd><p>The <code>aligned</code> attribute specifies a minimum alignment for a
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| 75 | variable or structure field, measured in bytes:
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| 76 | </p>
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| 77 | <div class="example">
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| 78 | <pre class="example">int foo __attribute__ ((aligned (8))) = 0;
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| 79 | </pre></div>
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| 80 |
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| 81 | <p>This directs GNU C to allocate <code>foo</code> at an address that is a
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| 82 | multiple of 8 bytes. However, you can’t force an alignment bigger
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| 83 | than the computer’s maximum meaningful alignment.
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| 84 | </p>
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| 85 | </dd>
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| 86 | <dt><code>packed</code></dt>
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| 87 | <dd><p>The <code>packed</code> attribute specifies to compact the fields of a
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| 88 | structure by not leaving gaps between fields. For example,
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| 89 | </p>
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| 90 | <div class="example">
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| 91 | <pre class="example">struct __attribute__ ((packed)) bar
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| 92 | {
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| 93 | char a;
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| 94 | int b;
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| 95 | };
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| 96 | </pre></div>
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| 97 |
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| 98 | <p>allocates the integer field <code>b</code> at byte 1 in the structure,
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| 99 | immediately after the character field <code>a</code>. The packed structure
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| 100 | is just 5 bytes long (assuming <code>int</code> is 4 bytes) and its
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| 101 | alignment is 1, that of <code>char</code>.
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| 102 | </p>
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| 103 | </dd>
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| 104 | <dt><code>deprecated</code></dt>
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| 105 | <dd><p>Applicable to both variables and functions, the <code>deprecated</code>
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| 106 | attribute tells the compiler to issue a warning if the variable or
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| 107 | function is ever used in the source file.
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| 108 | </p>
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| 109 | <div class="example">
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| 110 | <pre class="example">int old_foo __attribute__ ((deprecated));
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| 111 |
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| 112 | int old_quux () __attribute__ ((deprecated));
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| 113 | </pre></div>
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| 114 |
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| 115 | </dd>
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| 116 | <dt><code>__noinline__</code></dt>
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| 117 | <dd><p>The <code>__noinline__</code> attribute, in a function’s declaration or
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| 118 | definition, specifies never to inline calls to that function. All
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| 119 | calls to that function, in a compilation unit where it has this
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| 120 | attribute, will be compiled to invoke the separately compiled
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| 121 | function. See <a href="Inline-Function-Definitions.html">Inline Function Definitions</a>.
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| 122 | </p>
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| 123 | </dd>
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| 124 | <dt><code>__noclone__</code></dt>
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| 125 | <dd><p>The <code>__noclone__</code> attribute, in a function’s declaration or
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| 126 | definition, specifies never to clone that function. Thus, there will
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| 127 | be only one compiled version of the function. See <a href="Label-Value-Caveats.html">Label Value Caveats</a>, for more information about cloning.
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| 128 | </p>
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| 129 | </dd>
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| 130 | <dt><code>always_inline</code></dt>
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| 131 | <dd><p>The <code>always_inline</code> attribute, in a function’s declaration or
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| 132 | definition, specifies to inline all calls to that function (unless
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| 133 | something about the function makes inlining impossible). This applies
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| 134 | to all calls to that function in a compilation unit where it has this
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| 135 | attribute. See <a href="Inline-Function-Definitions.html">Inline Function Definitions</a>.
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| 136 | </p>
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| 137 | </dd>
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| 138 | <dt><code>gnu_inline</code></dt>
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| 139 | <dd><p>The <code>gnu_inline</code> attribute, in a function’s declaration or
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| 140 | definition, specifies to handle the <code>inline</code> keywprd the way GNU
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| 141 | C originally implemented it, many years before ISO C said anything
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| 142 | about inlining. See <a href="Inline-Function-Definitions.html">Inline Function Definitions</a>.
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| 143 | </p></dd>
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| 144 | </dl>
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| 145 |
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| 146 | <p>For full documentation of attributes, see the GCC manual.
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| 147 | See <a href="https://gcc.gnu.org/onlinedocs/gcc/Attribute-Syntax.html#Attribute-Syntax">System Headers</a> in <cite>Using
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| 148 | the GNU Compiler Collection</cite>.
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| 149 | </p>
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| 150 | <hr>
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| 151 | <div class="header">
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| 152 | <p>
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| 153 | Next: <a href="Signals.html" accesskey="n" rel="next">Signals</a>, Previous: <a href="Digraphs.html" accesskey="p" rel="prev">Digraphs</a>, Up: <a href="index.html" accesskey="u" rel="up">Top</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Symbol-Index.html" title="Index" rel="index">Index</a>]</p>
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