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    <title>Attributes of Types</title>
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      <div class="titlepage"></div>
      <div xmlns="" class="refnamediv">
        <a xmlns="http://www.w3.org/1999/xhtml" id="AttributesofTypes"></a>
        <h1>Attributes of Types</h1>
        <p>
            Attributes of Types.
        </p>
      </div>
      <div class="refsect2">
        <a id="synopsis"></a>
        <h3>
        </h3>
        <div class="informaltable">
          <table border="0">
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                <td align="left">
__attribute__ ((aligned))
__attribute__ ((aligned (<span class="emphasis"><em>n</em></span>)))
__attribute__ ((packed))
                        </td>
              </tr>
            </tbody>
          </table>
        </div>
      </div>
      <div class="refsect1">
        <a id="description"></a>
        <h2>Description</h2>
        <p>
          The keyword <code class="constant">__attribute__</code> allows you to specify special attributes
          of <code class="code">enum</code>, <code class="code">struct</code> and <code class="code">union</code> types when you
          define such types.  This keyword is followed by an attribute specification inside
          double parentheses. Two attributes are currently defined for types: aligned, and packed.
        </p>
        <p>
          You may specify type attributes in an <code class="code">enum</code>, <code class="code">struct</code>,
          or <code class="code">union</code> type declaration or definition, or for other types in a
          <code class="code">typedef</code> declaration.
        </p>
        <p>
          For an <code class="code">enum</code>, <code class="code">struct</code>, or <code class="code">union</code> type, you
          may specify attributes either between the <code class="code">enum</code>, <code class="code">struct</code>
          or <code class="code">union</code> tag and the name of the type, or just past the closing curly
          brace of the <span class="emphasis"><em>definition</em></span>. The former syntax is preferred.
        </p>
      </div>
      <div class="refsect2">
        <a id="aligned"></a>
        <h3>aligned (<code class="varname">alignment</code>)</h3>
        <p>
          This attribute specifies a minimum alignment (in bytes) for variables of the specified
          type. For example, the declarations:
        </p>
        <p>
        </p>
        <div class="informaltable">
          <table border="0">
            <colgroup>
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            </colgroup>
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              <tr>
                <td align="left">    <code class="code">struct S { short f[3]; } __attribute__ ((aligned (8)));
    typedef int more_aligned_int __attribute__ ((aligned (8)));</code></td>
              </tr>
            </tbody>
          </table>
        </div>
        <p>
    </p>
        <p>
      force the compiler to ensure (as far as it can) that each variable whose type is
      <code class="code">struct S</code> or <code class="code">more_aligned_int</code> will be allocated and aligned
      <span class="emphasis"><em>at least</em></span> on a 8-byte boundary.
    </p>
        <p>
      Note that the alignment of any given <code class="code">struct</code> or <code class="code">union</code> type
      is required by the ISO C standard to be at least a perfect multiple of the lowest
      common multiple of the alignments of all of the members of the <code class="code">struct</code>
      or <code class="code">union</code> in question and must also be a power of two. This means that
      you <span class="emphasis"><em>can</em></span> effectively adjust the alignment of a <code class="code">struct</code>
      or <code class="code">union</code> type by attaching an aligned attribute to any one of the members
      of such a type, but the notation illustrated in the example above is a more obvious,
      intuitive, and readable way to request the compiler to adjust the alignment of an entire
      <code class="code">struct</code> or <code class="code">union</code> type.
    </p>
        <p>
      As in the preceding example, you can explicitly specify the alignment (in bytes) that
      you wish the compiler to use for a given <code class="code">struct</code> or <code class="code">union</code>
      type. Alternatively, you can leave out the alignment factor and just ask the compiler
      to align a type to the maximum useful alignment for the target machine you are compiling
      for. For example, you could write:
    </p>
        <p>
         </p>
        <div class="informaltable">
          <table border="0">
            <colgroup>
              <col align="left" class="col1" />
            </colgroup>
            <tbody>
              <tr>
                <td align="left">    <code class="code">struct S { short f[3]; } __attribute__ ((aligned));</code></td>
              </tr>
            </tbody>
          </table>
        </div>
        <p>
    </p>
        <p>
      Whenever you leave out the alignment factor in an <code class="code">aligned</code> attribute
      specification, the compiler automatically sets the alignment for the type to the largest
      alignment which is ever used for any data type on the target machine you are compiling
      for. In the example above, the size of each <code class="code">short</code> is 2 bytes, and therefore
      the size of the entire <code class="code">struct S</code> type is 6 bytes. The smallest power of two
      which is greater than or equal to that is 8, so the compiler sets the alignment for the
      entire <code class="code">struct S</code> type to 8 bytes.
    </p>
        <p>
      Note that the effectiveness of aligned attributes may be limited by inherent limitations
      of the OpenCL device and compiler. For some devices, the OpenCL compiler may only be
      able to arrange for variables to be aligned up to a certain maximum alignment. If the
      OpenCL compiler is only able to align variables up to a maximum of 8 byte alignment, then
      specifying <code class="code">aligned(16)</code> in an <code class="code">__attribute__</code> will still only provide
      you with 8 byte alignment. See your platform-specific documentation for further information.
    </p>
        <p>
      The <code class="code">aligned</code> attribute can only increase the alignment; but you can decrease
      it by specifying <code class="code">packed</code> as well. See below.
    </p>
      </div>
      <div class="refsect2">
        <a id="packed"></a>
        <h3>packed</h3>
        <p>
      The <code class="constant">packed</code> attribute, attached to the <code class="code">struct</code>
      or <code class="code">union</code> type definition, specifies that each member of the structure or
      union is placed to minimize the memory required. When attached to an <code class="code">enum</code>
      definition, it indicates that the smallest integral type should be used.
    </p>
        <p>
      Specifying this attribute for <code class="code">struct</code> and <code class="code">union</code> types is equivalent
      to specifying the <code class="code">packed</code> attribute on each of the structure or union members.
    </p>
        <p>
      In the following example <code class="code">struct my_packed_struct</code>'s members are packed closely
      together, but the internal layout of its <code class="code">s</code> member is not packed. To do that,
      <code class="code">struct my_unpacked_struct</code> would need to be packed, too.
    </p>
        <p>
         </p>
        <div class="informaltable">
          <table border="0">
            <colgroup>
              <col align="left" class="col1" />
            </colgroup>
            <tbody>
              <tr>
                <td align="left">    <code class="code">struct my_unpacked_struct
    {
        char c; int i;
    };

    struct __attribute__ ((packed)) my_packed_struct
    {
        char c;
        int i;
        struct my_unpacked_struct s;
    };</code></td>
              </tr>
            </tbody>
          </table>
        </div>
        <p>
    </p>
        <p>
      You may only specify this attribute on the definition of a <code class="code">enum</code>,
      <code class="code">struct</code>, or <code class="code">union</code>, not on a <code class="code">typedef</code> which does not
      also define the enumerated type, structure or union.
    </p>
      </div>
      <div class="refsect1">
        <a id="specification"></a>
        <h2>Specification</h2>
        <p>
            <img src="pdficon_small1.gif" />

            <a href="http://www.khronos.org/registry/cl/specs/opencl-1.2.pdf#page=237" target="OpenCL Spec">OpenCL Specification</a>
        </p>
      </div>
      <div class="refsect1">
        <a id="seealso"></a>
        <h2>Also see</h2>
        <p>
            <a class="citerefentry" href="attribute.html"><span class="citerefentry"><span class="refentrytitle">__attribute__</span></span></a>,
            <a class="citerefentry" href="attributes-blocksAndControlFlow.html"><span class="citerefentry"><span class="refentrytitle">Blocks and Control-Flow Statement Attributes Attributes</span></span></a>,
            <a class="citerefentry" href="attributes-variables.html"><span class="citerefentry"><span class="refentrytitle">Variable Attributes</span></span></a>
        </p>
      </div>
      <div xmlns="" class="refsect3" lang="en" xml:lang="en"><a xmlns="http://www.w3.org/1999/xhtml" id="Copyright"></a><h4 xmlns="http://www.w3.org/1999/xhtml"></h4><img xmlns="http://www.w3.org/1999/xhtml" src="KhronosLogo.jpg" /><p xmlns="http://www.w3.org/1999/xhtml"></p>Copyright © 2007-2011 The Khronos Group Inc.
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