sdk/1.2/docs/man/attributes-types.xml

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<refentry>
    <refentryinfo>
        <keywordset>
            <keyword>Attributes of Types</keyword>
        </keywordset>
    </refentryinfo>

    <refmeta>
        <refentrytitle>Attributes of Types</refentrytitle>

        <refmiscinfo>
            <copyright>
                <year>2007-2011</year>
                <holder>The Khronos Group Inc.
 Permission is hereby granted, free of charge, to any person obtaining a
copy of this software and/or associated documentation files (the
"Materials"), to deal in the Materials without restriction, including
without limitation the rights to use, copy, modify, merge, publish,
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permit persons to whom the Materials are furnished to do so, subject to
the condition that this copyright notice and permission notice shall be included
in all copies or substantial portions of the Materials.</holder>
            </copyright>
        </refmiscinfo>
        <manvolnum>3</manvolnum>
    </refmeta>

<!-- ================================ SYNOPSIS -->

    <refnamediv id="AttributesofTypes">
        <refname>Attributes of Types</refname>

        <refpurpose>
            Attributes of Types.
        </refpurpose>
    </refnamediv>

    <!-- ALTERNATIVE SYNTAX SYNOPSIS (NON-FUNCTION) -->

    <refsect2 id="synopsis">
        <title>
        </title>

        <informaltable frame="none">
            <tgroup cols="1" align="left" colsep="0" rowsep="0">
                <colspec colname="col1" colnum="1" />
                <tbody>
                    <row>
                        <entry>
__attribute__ ((aligned))
__attribute__ ((aligned (<emphasis>n</emphasis>)))
__attribute__ ((packed))
                        </entry>
                    </row>
                </tbody>
            </tgroup>
        </informaltable>
    </refsect2>

<!-- ================================ DESCRIPTION  -->

    <refsect1 id="description"><title>Description</title>
        <para>
          The keyword <constant>__attribute__</constant> allows you to specify special attributes
          of <code>enum</code>, <code>struct</code> and <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.
        </para>

        <para>
          You may specify type attributes in an <code>enum</code>, <code>struct</code>,
          or <code>union</code> type declaration or definition, or for other types in a
          <code>typedef</code> declaration.
        </para>

        <para>
          For an <code>enum</code>, <code>struct</code>, or <code>union</code> type, you
          may specify attributes either between the <code>enum</code>, <code>struct</code>
          or <code>union</code> tag and the name of the type, or just past the closing curly
          brace of the <emphasis>definition</emphasis>. The former syntax is preferred.
        </para>
    </refsect1>

    <refsect2 id="aligned"><title>aligned (<varname>alignment</varname>)</title>
        <!-- Using refsect2 so we can have shaded backgrounds behind examples  -->
        <para>
          This attribute specifies a minimum alignment (in bytes) for variables of the specified
          type. For example, the declarations:
        </para>

    <para>
        <informaltable frame="none">
            <tgroup cols="1" align="left" colsep="0" rowsep="0">
                <colspec colname="col1" colnum="1" /> 
                <tbody>
                    <row>
                        <entry>    <code>struct S { short f[3]; } __attribute__ ((aligned (8)));
    typedef int more_aligned_int __attribute__ ((aligned (8)));</code></entry>
                    </row>
                </tbody>
            </tgroup>
        </informaltable>
    </para>

    <para>
      force the compiler to ensure (as far as it can) that each variable whose type is
      <code>struct S</code> or <code>more_aligned_int</code> will be allocated and aligned
      <emphasis>at least</emphasis> on a 8-byte boundary.
    </para>

    <para>
      Note that the alignment of any given <code>struct</code> or <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>struct</code>
      or <code>union</code> in question and must also be a power of two. This means that
      you <emphasis>can</emphasis> effectively adjust the alignment of a <code>struct</code>
      or <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>struct</code> or <code>union</code> type.
    </para>

    <para>
      As in the preceding example, you can explicitly specify the alignment (in bytes) that
      you wish the compiler to use for a given <code>struct</code> or <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:
    </para>

    <para>
         <informaltable frame="none">
            <tgroup cols="1" align="left" colsep="0" rowsep="0">
                <colspec colname="col1" colnum="1" />
                <tbody>
                    <row>
                        <entry>    <code>struct S { short f[3]; } __attribute__ ((aligned));</code></entry>
                    </row>
                </tbody>
            </tgroup>
        </informaltable>
    </para>

    <para>
      Whenever you leave out the alignment factor in an <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>short</code> is 2 bytes, and therefore
      the size of the entire <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>struct S</code> type to 8 bytes.
    </para>

    <para>
      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>aligned(16)</code> in an <code>__attribute__</code> will still only provide
      you with 8 byte alignment. See your platform-specific documentation for further information.
    </para>

    <para>
      The <code>aligned</code> attribute can only increase the alignment; but you can decrease
      it by specifying <code>packed</code> as well. See below.
    </para>

</refsect2>

<refsect2 id="packed"><title>packed</title>
    <!-- Using refsect2 so we can have shaded backgrounds behind examples  -->

    <para>
      The <constant>packed</constant> attribute, attached to the <code>struct</code>
      or <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>enum</code>
      definition, it indicates that the smallest integral type should be used.
    </para>

    <para>
      Specifying this attribute for <code>struct</code> and <code>union</code> types is equivalent
      to specifying the <code>packed</code> attribute on each of the structure or union members.
    </para>

    <para>
      In the following example <code>struct my_packed_struct</code>'s members are packed closely
      together, but the internal layout of its <code>s</code> member is not packed. To do that,
      <code>struct my_unpacked_struct</code> would need to be packed, too.
    </para>

    <para>
         <informaltable frame="none">
            <tgroup cols="1" align="left" colsep="0" rowsep="0">
                <colspec colname="col1" colnum="1" /> <tbody>
                    <row>
                        <entry>    <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></entry>
                    </row>
                </tbody>
            </tgroup>
        </informaltable>
    </para>

    <para>
      You may only specify this attribute on the definition of a <code>enum</code>,
      <code>struct</code>, or <code>union</code>, not on a <code>typedef</code> which does not
      also define the enumerated type, structure or union.
    </para>
</refsect2>

<!-- ================================ EXAMPLE  -->
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    <refsect2 id="example1">
        <title>
            Example
        </title>

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                <colspec colname="col1" colnum="1" />
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                        <entry>
                            Example goes here - it will be set in "code" type with white space preserved.
                        </entry>
                    </row>
                </tbody>
            </tgroup>
        </informaltable>
    </refsect2>
-->

<!-- ================================ SPECIFICATION  -->
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    <refsect1 id="specification"><title>Specification</title>
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            <olink uri="attributes-types">OpenCL Specification</olink>
        </para>
    </refsect1>

<!-- ================================ ALSO SEE  -->

    <refsect1 id="seealso"><title>Also see</title>
        <para>
            <citerefentry href="attribute"><refentrytitle>__attribute__</refentrytitle></citerefentry>,
            <citerefentry href="attributes-blocksAndControlFlow"><refentrytitle>Blocks and Control-Flow Statement Attributes Attributes</refentrytitle></citerefentry>,
            <citerefentry href="attributes-variables"><refentrytitle>Variable Attributes</refentrytitle></citerefentry>
        </para>
    </refsect1>

<!-- ============================== COPYRIGHT -->
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    <refsect3 id="Copyright"><title></title>
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                <imagedata fileref="KhronosLogo.jpg" format="jpg" />
        </imageobject>
        <para />
    </refsect3>

<!-- 23-Oct-2011 -->
</refentry>