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| 22 | <title>Bit Fields (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="Bit-Fields"></span><div class="header">
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| 59 | <p>
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| 60 | Next: <a href="Bit-Field-Packing.html" accesskey="n" rel="next">Bit Field Packing</a>, Previous: <a href="Packed-Structures.html" accesskey="p" rel="prev">Packed Structures</a>, Up: <a href="Structures.html" accesskey="u" rel="up">Structures</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="Bit-Fields-1"></span><h3 class="section">15.6 Bit Fields</h3>
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| 64 | <span id="index-bit-fields"></span>
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| 65 |
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| 66 | <p>A structure field declaration with an integer type can specify the
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| 67 | number of bits the field should occupy. We call that a <em>bit
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| 68 | field</em>. These are useful because consecutive bit fields are packed
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| 69 | into a larger storage unit. For instance,
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| 70 | </p>
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| 71 | <div class="example">
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| 72 | <pre class="example">unsigned char opcode: 4;
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| 73 | </pre></div>
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| 74 |
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| 75 | <p>specifies that this field takes just 4 bits.
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| 76 | Since it is unsigned, its possible values range
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| 77 | from 0 to 15. A signed field with 4 bits, such as this,
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| 78 | </p>
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| 79 | <div class="example">
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| 80 | <pre class="example">signed char small: 4;
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| 81 | </pre></div>
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| 82 |
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| 83 | <p>can hold values from -8 to 7.
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| 84 | </p>
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| 85 | <p>You can subdivide a single byte into those two parts by writing
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| 86 | </p>
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| 87 | <div class="example">
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| 88 | <pre class="example">unsigned char opcode: 4;
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| 89 | signed char small: 4;
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| 90 | </pre></div>
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| 91 |
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| 92 | <p>in the structure. With bit fields, these two numbers fit into
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| 93 | a single <code>char</code>.
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| 94 | </p>
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| 95 | <p>Here’s how to declare a one-bit field that can hold either 0 or 1:
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| 96 | </p>
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| 97 | <div class="example">
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| 98 | <pre class="example">unsigned char special_flag: 1;
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| 99 | </pre></div>
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| 100 |
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| 101 | <p>You can also use the <code>bool</code> type for bit fields:
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| 102 | </p>
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| 103 | <div class="example">
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| 104 | <pre class="example">bool special_flag: 1;
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| 105 | </pre></div>
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| 106 |
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| 107 | <p>Except when using <code>bool</code> (which is always unsigned,
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| 108 | see <a href="Boolean-Type.html">Boolean Type</a>), always specify <code>signed</code> or <code>unsigned</code>
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| 109 | for a bit field. There is a default, if that’s not specified: the bit
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| 110 | field is signed if plain <code>char</code> is signed, except that the option
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| 111 | <samp>-funsigned-bitfields</samp> forces unsigned as the default. But it
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| 112 | is cleaner not to depend on this default.
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| 113 | </p>
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| 114 | <p>Bit fields are special in that you cannot take their address with
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| 115 | ‘<samp>&</samp>’. They are not stored with the size and alignment appropriate
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| 116 | for the specified type, so they cannot be addressed through pointers
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| 117 | to that type.
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| 118 | </p>
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| 119 | <hr>
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| 120 | <div class="header">
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| 121 | <p>
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| 122 | Next: <a href="Bit-Field-Packing.html" accesskey="n" rel="next">Bit Field Packing</a>, Previous: <a href="Packed-Structures.html" accesskey="p" rel="prev">Packed Structures</a>, Up: <a href="Structures.html" accesskey="u" rel="up">Structures</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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