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    <title>Attributes of Variables</title>
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    <div class="refentry">
      <a id="id-1"></a>
      <div class="titlepage"></div>
      <div xmlns="" class="refnamediv">
        <a xmlns="http://www.w3.org/1999/xhtml" id="AttributesofVariables"></a>
        <h1>Attributes of Variables</h1>
        <p>
            Attributes of Variables.
        </p>
      </div>
      <div class="refsect2">
        <a id="synopsis"></a>
        <h3>
        </h3>
        <div class="informaltable">
          <table border="0">
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            </colgroup>
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                <td align="left">
    __attribute__ ((aligned))
    __attribute__ ((aligned (<span class="emphasis"><em>n</em></span>)))
    __attribute__ ((packed))
    __attribute__ ((endian(host)))
    __attribute__ ((endian(device)))
                        </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 variables or structure fields. This keyword is followed by an attribute
          specification inside double parentheses. The <code class="code">aligned</code>, <code class="code">packed</code>,
          and <code class="code">endian</code> attribute qualifiers are defined below.
        </p>
      </div>
      <div class="refsect2">
        <a id="aligned"></a>
        <h3>aligned (<code class="varname">alignment</code>)</h3>
        <p>
      This attribute specifies a minimum alignment for the variable or structure field, measured
      in bytes. For example, the declaration:
    </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">int x __attribute__ ((aligned (16))) = 0;</code></td>
              </tr>
            </tbody>
          </table>
        </div>
        <p>
    </p>
        <p>
      causes the compiler to allocate the global variable <code class="code">x</code> on a 16-byte boundary. The
      alignment value specified must be a power of two.
    </p>
        <p>
      You can also specify the alignment of structure fields. For example, to create double-word
      aligned <code class="code">int</code> pair, 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 foo { int x[2] __attribute__ ((aligned (8)));
                        };</code></td>
              </tr>
            </tbody>
          </table>
        </div>
        <p>
    </p>
        <p>
      This is an alternative to creating a union with a <code class="code">double</code> member that forces
      the union to be double-word aligned.
    </p>
        <p>
      As in the preceding examples, you can explicitly specify the alignment (in bytes) that you
      wish the compiler to use for a given variable or structure field. Alternatively, you can
      leave out the alignment factor and just ask the compiler to align a variable or field 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">
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            </colgroup>
            <tbody>
              <tr>
                <td align="left">    <code class="code">short array[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 OpenCL compiler automatically sets the alignment for the declared
      variable or field to the largest alignment which is ever used for any data type on the
      target device you are compiling for.
    </p>
        <p>
      When used on a <code class="code">struct</code>, or <code class="code">struct</code> member, the aligned
      <code class="code">attribute</code> can only increase the alignment; in order to decrease it,
      the <code class="code">packed</code> attribute must be specified as well.  When used as part of a
      <code class="code">typedef</code>, the <code class="code">aligned</code> attribute can both increase and decrease
      alignment, and specifying the <code class="code">packed</code> attribute will generate a warning.
    </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>
      </div>
      <div class="refsect2">
        <a id="packed"></a>
        <h3>packed</h3>
        <p>
      The <code class="code">packed</code> attribute specifies that a variable or structure field should
      have the smallest possible alignment -- one byte for a variable, unless you specify a
      larger value with the <code class="code">aligned</code> attribute.
    </p>
        <p>
      Here is a structure in which the field <code class="code">x</code> is packed, so that it immediately
      follows <code class="code">a</code>:
    </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 foo
    {
        char a;
        int x[2] __attribute__ ((packed));
    };</code></td>
              </tr>
            </tbody>
          </table>
        </div>
        <p>
    </p>
        <p>
      An attribute list placed at the beginning of a user-defined type applies to the variable
      of that type and not the type, while attributes following the type body apply to the type.
    </p>
        <p>
      For example:
    </p>
        <p>
        </p>
        <div class="informaltable">
          <table border="0">
            <colgroup>
              <col align="left" class="col1" />
            </colgroup>
            <tbody>
              <tr>
                <td align="left">    <code class="code">/* a has alignment of 128 */
    __attribute__((aligned(128))) struct A {int i;} a;

    /* b has alignment of 16 */
    __attribute__((aligned(16))) struct B {double d;}
                      __attribute__((aligned(32))) b ;

    struct A a1; /* a1 has alignment of 4 */

    struct B b1; /* b1 has alignment of 32 */</code></td>
              </tr>
            </tbody>
          </table>
        </div>
        <p>
    </p>
      </div>
      <div class="refsect2">
        <a id="endian"></a>
        <h3>endian (<code class="varname">endiantype</code>)</h3>
        <p>
      The <code class="code">endian</code> attribute determines the byte ordering of a variable.
      <code class="code">endiantype</code> can be set to <code class="code">host</code> indicating the variable uses
      the endianness of the host processor or can be set to <code class="code">device</code> indicating
      the variable uses the endianness of the device on which the kernel will be executed. The
      default is <code class="code">device</code>. For example:
    </p>
        <p>
        </p>
        <div class="informaltable">
          <table border="0">
            <colgroup>
              <col align="left" class="col1" />
            </colgroup>
            <tbody>
              <tr>
                <td align="left">    <code class="code">float4 *p   __attribute__ ((endian(host)));</code></td>
              </tr>
            </tbody>
          </table>
        </div>
        <p>
    </p>
        <p>
      specifies that data stored in memory pointed to by <code class="code">p</code> will be in the host
      endian format.
    </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=239" 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-types.html"><span class="citerefentry"><span class="refentrytitle">Types 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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