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<h3 class="section">4.9 Structures, Unions, Enumerations, and Bit-Fields</h3>
<ul>
<li><cite>A member of a union object is accessed using a member of a
different type (C90 6.3.2.3).</cite>
<p>The relevant bytes of the representation of the object are treated as
an object of the type used for the access. See <a href="Type_002dpunning.html#Type_002dpunning">Type-punning</a>. This
may be a trap representation.
<li><cite>Whether a “plain” </cite><code>int</code><cite> bit-field is treated as a
</cite><code>signed int</code><cite> bit-field or as an </cite><code>unsigned int</code><cite> bit-field
(C90 6.5.2, C90 6.5.2.1, C99 and C11 6.7.2, C99 and C11 6.7.2.1).</cite>
<p><a name="index-funsigned_002dbitfields-2862"></a>By default it is treated as <code>signed int</code> but this may be changed
by the <samp><span class="option">-funsigned-bitfields</span></samp> option.
<li><cite>Allowable bit-field types other than </cite><code>_Bool</code><cite>, </cite><code>signed int</code><cite>,
and </cite><code>unsigned int</code><cite> (C99 and C11 6.7.2.1).</cite>
<p>Other integer types, such as <code>long int</code>, and enumerated types are
permitted even in strictly conforming mode.
<li><cite>Whether atomic types are permitted for bit-fields (C11 6.7.2.1).</cite>
<p>Atomic types are not permitted for bit-fields.
<li><cite>Whether a bit-field can straddle a storage-unit boundary (C90
6.5.2.1, C99 and C11 6.7.2.1).</cite>
<p>Determined by ABI.
<li><cite>The order of allocation of bit-fields within a unit (C90
6.5.2.1, C99 and C11 6.7.2.1).</cite>
<p>Determined by ABI.
<li><cite>The alignment of non-bit-field members of structures (C90
6.5.2.1, C99 and C11 6.7.2.1).</cite>
<p>Determined by ABI.
<li><cite>The integer type compatible with each enumerated type (C90
6.5.2.2, C99 and C11 6.7.2.2).</cite>
<p><a name="index-fshort_002denums-2863"></a>Normally, the type is <code>unsigned int</code> if there are no negative
values in the enumeration, otherwise <code>int</code>. If
<samp><span class="option">-fshort-enums</span></samp> is specified, then if there are negative values
it is the first of <code>signed char</code>, <code>short</code> and <code>int</code>
that can represent all the values, otherwise it is the first of
<code>unsigned char</code>, <code>unsigned short</code> and <code>unsigned int</code>
that can represent all the values.
<!-- On a few unusual targets with 64-bit int, this doesn't agree with -->
<!-- the code and one of the types accessed via mode attributes (which -->
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<p>On some targets, <samp><span class="option">-fshort-enums</span></samp> is the default; this is
determined by the ABI.
</ul>
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