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| 22 | <title>Type Alignment (GNU C Language Manual)</title>
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| 57 | <body lang="en">
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| 58 | <span id="Type-Alignment"></span><div class="header">
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| 59 | <p>
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| 60 | Next: <a href="Aliasing.html" accesskey="n" rel="next">Aliasing</a>, Previous: <a href="Directing-Compilation.html" accesskey="p" rel="prev">Directing Compilation</a>, Up: <a href="index.html" accesskey="u" rel="up">Top</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="Type-Alignment-1"></span><h2 class="appendix">Appendix A Type Alignment</h2>
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| 64 | <span id="index-type-alignment"></span>
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| 65 | <span id="index-alignment-of-type"></span>
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| 66 | <span id="index-_005fAlignof"></span>
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| 67 | <span id="index-_005f_005falignof_005f_005f"></span>
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| 68 |
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| 69 | <p>Code for device drivers and other communication with low-level
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| 70 | hardware sometimes needs to be concerned with the alignment of
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| 71 | data objects in memory.
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| 72 | </p>
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| 73 | <p>Each data type has a required <em>alignment</em>, always a power of 2,
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| 74 | that says at which memory addresses an object of that type can validly
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| 75 | start. A valid address for the type must be a multiple of its
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| 76 | alignment. If a type’s alignment is 1, that means it can validly
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| 77 | start at any address. If a type’s alignment is 2, that means it can
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| 78 | only start at an even address. If a type’s alignment is 4, that means
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| 79 | it can only start at an address that is a multiple of 4.
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| 80 | </p>
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| 81 | <p>The alignment of a type (except <code>char</code>) can vary depending on the
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| 82 | kind of computer in use. To refer to the alignment of a type in a C
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| 83 | program, use <code>_Alignof</code>, whose syntax parallels that of
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| 84 | <code>sizeof</code>. Like <code>sizeof</code>, <code>_Alignof</code> is a compile-time
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| 85 | operation, and it doesn’t compute the value of the expression used
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| 86 | as its argument.
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| 87 | </p>
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| 88 | <p>Nominally, each integer and floating-point type has an alignment equal to
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| 89 | the largest power of 2 that divides its size. Thus, <code>int</code> with
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| 90 | size 4 has a nominal alignment of 4, and <code>long long int</code> with
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| 91 | size 8 has a nominal alignment of 8.
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| 92 | </p>
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| 93 | <p>However, each kind of computer generally has a maximum alignment, and
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| 94 | no type needs more alignment than that. If the computer’s maximum
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| 95 | alignment is 4 (which is common), then no type’s alignment is more
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| 96 | than 4.
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| 97 | </p>
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| 98 | <p>The size of any type is always a multiple of its alignment; that way,
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| 99 | in an array whose elements have that type, all the elements are
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| 100 | properly aligned if the first one is.
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| 101 | </p>
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| 102 | <p>These rules apply to all real computers today, but some embedded
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| 103 | controllers have odd exceptions. We don’t have references to cite for
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| 104 | them.
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| 105 | </p>
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| 106 | <p>Ordinary C code guarantees that every object of a given type is in
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| 107 | fact aligned as that type requires.
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| 108 | </p>
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| 109 | <p>If the operand of <code>_Alignof</code> is a structure field, the value
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| 110 | is the alignment it requires. It may have a greater alignment by
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| 111 | coincidence, due to the other fields, but <code>_Alignof</code> is not
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| 112 | concerned about that. See <a href="Structures.html">Structures</a>.
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| 113 | </p>
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| 114 | <p>Older versions of GNU C used the keyword <code>__alignof__</code> for this,
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| 115 | but now that the feature has been standardized, it is better
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| 116 | to use the standard keyword <code>_Alignof</code>.
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| 117 | </p>
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| 118 | <span id="index-_005fAlignas"></span>
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| 119 | <span id="index-_005f_005faligned_005f_005f"></span>
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| 120 | <p>You can explicitly specify an alignment requirement for a particular
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| 121 | variable or structure field by adding <code>_Alignas
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| 122 | (<var>alignment</var>)</code> to the declaration, where <var>alignment</var> is a
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| 123 | power of 2 or a type name. For instance:
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| 124 | </p>
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| 125 | <div class="example">
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| 126 | <pre class="example">char _Alignas (8) x;
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| 127 | </pre></div>
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| 128 |
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| 129 | <p>or
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| 130 | </p>
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| 131 | <div class="example">
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| 132 | <pre class="example">char _Alignas (double) x;
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| 133 | </pre></div>
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| 134 |
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| 135 | <p>specifies that <code>x</code> must start on an address that is a multiple of
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| 136 | 8. However, if <var>alignment</var> exceeds the maximum alignment for the
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| 137 | machine, that maximum is how much alignment <code>x</code> will get.
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| 138 | </p>
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| 139 | <p>The older GNU C syntax for this feature looked like
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| 140 | <code>__attribute__ ((__aligned__ (<var>alignment</var>)))</code> to the
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| 141 | declaration, and was added after the variable. For instance:
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| 142 | </p>
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| 143 | <div class="example">
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| 144 | <pre class="example">char x __attribute__ ((__aligned__ 8));
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| 145 | </pre></div>
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| 146 |
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| 147 | <p>See <a href="Attributes.html">Attributes</a>.
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| 148 | </p>
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| 149 | <hr>
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| 150 | <div class="header">
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| 152 | Next: <a href="Aliasing.html" accesskey="n" rel="next">Aliasing</a>, Previous: <a href="Directing-Compilation.html" accesskey="p" rel="prev">Directing Compilation</a>, Up: <a href="index.html" accesskey="u" rel="up">Top</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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