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<title>Structures unions enumerations and bit-fields implementation - Using the GNU Compiler Collection (GCC)</title>
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<a name="Structures-unions-enumerations-and-bit-fields-implementation"></a>
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<a name="Structures-unions-enumerations-and-bit_002dfields-implementation"></a>
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Next: <a rel="next" accesskey="n" href="Qualifiers-implementation.html#Qualifiers-implementation">Qualifiers implementation</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="Type_002dpunning.html#Type_002dpunning">Type-punning</a>. This
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may be a trap representation.
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<li><cite>Whether a “plain” </cite><code>int</code><cite> bit-field is treated as a
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</cite><code>signed int</code><cite> bit-field or as an </cite><code>unsigned int</code><cite> 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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<p><a name="index-funsigned_002dbitfields-2862"></a>By default it is treated as <code>signed int</code> but this may be changed
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by the <samp><span class="option">-funsigned-bitfields</span></samp> option.
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<li><cite>Allowable bit-field types other than </cite><code>_Bool</code><cite>, </cite><code>signed int</code><cite>,
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and </cite><code>unsigned int</code><cite> (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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<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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<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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<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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<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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<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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<p><a name="index-fshort_002denums-2863"></a>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><span class="option">-fshort-enums</span></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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<!-- On a few unusual targets with 64-bit int, this doesn't agree with -->
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<!-- the code and one of the types accessed via mode attributes (which -->
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<!-- are not currently considered extended integer types) may be used. -->
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<!-- If these types are made extended integer types, it would still be -->
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<!-- the case that -fshort-enums stops the implementation from -->
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<!-- conforming to C90 on those targets. -->
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<p>On some targets, <samp><span class="option">-fshort-enums</span></samp> is the default; this is
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determined by the ABI.
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</ul>
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