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<title>Using the GNU Compiler Collection (GCC): Structures unions enumerations and bit-fields implementation</title>
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<a name="Structures-unions-enumerations-and-bit_002dfields-implementation"></a>
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<div class="header">
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<p>
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Next: <a href="Qualifiers-implementation.html#Qualifiers-implementation" accesskey="n" rel="next">Qualifiers implementation</a>, Previous: <a href="Hints-implementation.html#Hints-implementation" accesskey="p" rel="prev">Hints implementation</a>, Up: <a href="C-Implementation.html#C-Implementation" accesskey="u" rel="up">C Implementation</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="Structures_002c-Unions_002c-Enumerations_002c-and-Bit_002dFields"></a>
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<h3 class="section">4.9 Structures, Unions, Enumerations, and Bit-Fields</h3>
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<ul>
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<li> <cite>A member of a union object is accessed using a member of a
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different type (C90 6.3.2.3).</cite>
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<p>The relevant bytes of the representation of the object are treated as
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an object of the type used for the access. See <a href="Optimize-Options.html#Type_002dpunning">Type-punning</a>. This
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may be a trap representation.
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</p>
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</li><li> <cite>Whether a “plain” <code>int</code> bit-field is treated as a
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<code>signed int</code> bit-field or as an <code>unsigned int</code> bit-field
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(C90 6.5.2, C90 6.5.2.1, C99 and C11 6.7.2, C99 and C11 6.7.2.1).</cite>
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<a name="index-funsigned_002dbitfields-1"></a>
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<p>By default it is treated as <code>signed int</code> but this may be changed
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by the <samp>-funsigned-bitfields</samp> option.
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</p>
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</li><li> <cite>Allowable bit-field types other than <code>_Bool</code>, <code>signed int</code>,
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and <code>unsigned int</code> (C99 and C11 6.7.2.1).</cite>
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<p>Other integer types, such as <code>long int</code>, and enumerated types are
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permitted even in strictly conforming mode.
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</p>
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</li><li> <cite>Whether atomic types are permitted for bit-fields (C11 6.7.2.1).</cite>
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<p>Atomic types are not permitted for bit-fields.
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</p>
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</li><li> <cite>Whether a bit-field can straddle a storage-unit boundary (C90
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6.5.2.1, C99 and C11 6.7.2.1).</cite>
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<p>Determined by ABI.
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</p>
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</li><li> <cite>The order of allocation of bit-fields within a unit (C90
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6.5.2.1, C99 and C11 6.7.2.1).</cite>
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<p>Determined by ABI.
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</p>
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</li><li> <cite>The alignment of non-bit-field members of structures (C90
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6.5.2.1, C99 and C11 6.7.2.1).</cite>
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<p>Determined by ABI.
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</p>
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</li><li> <cite>The integer type compatible with each enumerated type (C90
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6.5.2.2, C99 and C11 6.7.2.2).</cite>
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<a name="index-fshort_002denums-1"></a>
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<p>Normally, the type is <code>unsigned int</code> if there are no negative
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values in the enumeration, otherwise <code>int</code>. If
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<samp>-fshort-enums</samp> is specified, then if there are negative values
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it is the first of <code>signed char</code>, <code>short</code> and <code>int</code>
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that can represent all the values, otherwise it is the first of
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<code>unsigned char</code>, <code>unsigned short</code> and <code>unsigned int</code>
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that can represent all the values.
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</p>
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<p>On some targets, <samp>-fshort-enums</samp> is the default; this is
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determined by the ABI.
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</p>
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</li></ul>
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<hr>
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<div class="header">
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<p>
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Next: <a href="Qualifiers-implementation.html#Qualifiers-implementation" accesskey="n" rel="next">Qualifiers implementation</a>, Previous: <a href="Hints-implementation.html#Hints-implementation" accesskey="p" rel="prev">Hints implementation</a>, Up: <a href="C-Implementation.html#C-Implementation" accesskey="u" rel="up">C Implementation</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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