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101 lines
4.7 KiB
HTML
101 lines
4.7 KiB
HTML
4 years ago
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<html lang="en">
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<title>Label Attributes - Using the GNU Compiler Collection (GCC)</title>
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<meta name="description" content="Using the GNU Compiler Collection (GCC)">
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<link title="Top" rel="start" href="index.html#Top">
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<link rel="up" href="C-Extensions.html#C-Extensions" title="C Extensions">
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<link rel="prev" href="Function-Attributes.html#Function-Attributes" title="Function Attributes">
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<link rel="next" href="Attribute-Syntax.html#Attribute-Syntax" title="Attribute Syntax">
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Copyright (C) 1988-2015 Free Software Foundation, Inc.
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Permission is granted to copy, distribute and/or modify this document
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under the terms of the GNU Free Documentation License, Version 1.3 or
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any later version published by the Free Software Foundation; with the
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(a) The FSF's Front-Cover Text is:
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A GNU Manual
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(b) The FSF's Back-Cover Text is:
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</head>
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<body>
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<div class="node">
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<a name="Label-Attributes"></a>
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<p>
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Next: <a rel="next" accesskey="n" href="Attribute-Syntax.html#Attribute-Syntax">Attribute Syntax</a>,
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Previous: <a rel="previous" accesskey="p" href="Function-Attributes.html#Function-Attributes">Function Attributes</a>,
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Up: <a rel="up" accesskey="u" href="C-Extensions.html#C-Extensions">C Extensions</a>
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<hr>
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</div>
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<h3 class="section">6.32 Label Attributes</h3>
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<p><a name="index-Label-Attributes-3308"></a>
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GCC allows attributes to be set on C labels. See <a href="Attribute-Syntax.html#Attribute-Syntax">Attribute Syntax</a>, for
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details of the exact syntax for using attributes. Other attributes are
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available for functions (see <a href="Function-Attributes.html#Function-Attributes">Function Attributes</a>), variables
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(see <a href="Variable-Attributes.html#Variable-Attributes">Variable Attributes</a>) and for types (see <a href="Type-Attributes.html#Type-Attributes">Type Attributes</a>).
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<p>This example uses the <code>cold</code> label attribute to indicate the
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<code>ErrorHandling</code> branch is unlikely to be taken and that the
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<code>ErrorHandling</code> label is unused:
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<pre class="smallexample">
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asm goto ("some asm" : : : : NoError);
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/* This branch (the fall-through from the asm) is less commonly used */
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ErrorHandling:
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__attribute__((cold, unused)); /* Semi-colon is required here */
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printf("error\n");
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return 0;
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NoError:
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printf("no error\n");
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return 1;
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</pre>
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<dl>
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<dt><code>unused</code><dd><a name="index-g_t_0040code_007bunused_007d-label-attribute-3309"></a>This feature is intended for program-generated code that may contain
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unused labels, but which is compiled with <samp><span class="option">-Wall</span></samp>. It is
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not normally appropriate to use in it human-written code, though it
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could be useful in cases where the code that jumps to the label is
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contained within an <code>#ifdef</code> conditional.
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<br><dt><code>hot</code><dd><a name="index-g_t_0040code_007bhot_007d-label-attribute-3310"></a>The <code>hot</code> attribute on a label is used to inform the compiler that
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the path following the label is more likely than paths that are not so
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annotated. This attribute is used in cases where <code>__builtin_expect</code>
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cannot be used, for instance with computed goto or <code>asm goto</code>.
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<br><dt><code>cold</code><dd><a name="index-g_t_0040code_007bcold_007d-label-attribute-3311"></a>The <code>cold</code> attribute on labels is used to inform the compiler that
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the path following the label is unlikely to be executed. This attribute
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is used in cases where <code>__builtin_expect</code> cannot be used, for instance
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with computed goto or <code>asm goto</code>.
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</dl>
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</body></html>
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