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<?xml version="1.0" encoding="UTF-8"?>
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<!DOCTYPE book PUBLIC "-//OASIS//DTD DocBook MathML Module V1.1b1//EN"
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"http://www.oasis-open.org/docbook/xml/mathml/1.1CR1/dbmathml.dtd">
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<keyword>Attributes of Types</keyword>
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<refentrytitle>Attributes of Types</refentrytitle>
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<year>2007-2011</year>
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<holder>The Khronos Group Inc.
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Permission is hereby granted, free of charge, to any person obtaining a
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copy of this software and/or associated documentation files (the
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"Materials"), to deal in the Materials without restriction, including
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without limitation the rights to use, copy, modify, merge, publish,
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distribute, sublicense, and/or sell copies of the Materials, and to
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permit persons to whom the Materials are furnished to do so, subject to
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the condition that this copyright notice and permission notice shall be included
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in all copies or substantial portions of the Materials.</holder>
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<manvolnum>3</manvolnum>
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<!-- ================================ SYNOPSIS -->
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<refnamediv id="AttributesofTypes">
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<refname>Attributes of Types</refname>
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<!-- ALTERNATIVE SYNTAX SYNOPSIS (NON-FUNCTION) -->
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<refsect2 id="synopsis">
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<informaltable frame="none">
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<tgroup cols="1" align="left" colsep="0" rowsep="0">
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<colspec colname="col1" colnum="1" />
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__attribute__ ((aligned))
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__attribute__ ((aligned (<emphasis>n</emphasis>)))
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__attribute__ ((packed))
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<!-- ================================ DESCRIPTION -->
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<refsect1 id="description"><title>Description</title>
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The keyword <constant>__attribute__</constant> allows you to specify special attributes
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of <code>enum</code>, <code>struct</code> and <code>union</code> types when you
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define such types. This keyword is followed by an attribute specification inside
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double parentheses. Two attributes are currently defined for types: aligned, and packed.
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You may specify type attributes in an <code>enum</code>, <code>struct</code>,
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or <code>union</code> type declaration or definition, or for other types in a
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<code>typedef</code> declaration.
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For an <code>enum</code>, <code>struct</code>, or <code>union</code> type, you
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may specify attributes either between the <code>enum</code>, <code>struct</code>
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or <code>union</code> tag and the name of the type, or just past the closing curly
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brace of the <emphasis>definition</emphasis>. The former syntax is preferred.
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<refsect2 id="aligned"><title>aligned (<varname>alignment</varname>)</title>
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<!-- Using refsect2 so we can have shaded backgrounds behind examples -->
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This attribute specifies a minimum alignment (in bytes) for variables of the specified
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type. For example, the declarations:
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<informaltable frame="none">
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<tgroup cols="1" align="left" colsep="0" rowsep="0">
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<colspec colname="col1" colnum="1" />
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<entry> <code>struct S { short f[3]; } __attribute__ ((aligned (8)));
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typedef int more_aligned_int __attribute__ ((aligned (8)));</code></entry>
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force the compiler to ensure (as far as it can) that each variable whose type is
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<code>struct S</code> or <code>more_aligned_int</code> will be allocated and aligned
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<emphasis>at least</emphasis> on a 8-byte boundary.
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Note that the alignment of any given <code>struct</code> or <code>union</code> type
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is required by the ISO C standard to be at least a perfect multiple of the lowest
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common multiple of the alignments of all of the members of the <code>struct</code>
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or <code>union</code> in question and must also be a power of two. This means that
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you <emphasis>can</emphasis> effectively adjust the alignment of a <code>struct</code>
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or <code>union</code> type by attaching an aligned attribute to any one of the members
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of such a type, but the notation illustrated in the example above is a more obvious,
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intuitive, and readable way to request the compiler to adjust the alignment of an entire
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<code>struct</code> or <code>union</code> type.
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As in the preceding example, you can explicitly specify the alignment (in bytes) that
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you wish the compiler to use for a given <code>struct</code> or <code>union</code>
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type. Alternatively, you can leave out the alignment factor and just ask the compiler
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to align a type to the maximum useful alignment for the target machine you are compiling
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for. For example, you could write:
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<informaltable frame="none">
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<tgroup cols="1" align="left" colsep="0" rowsep="0">
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<colspec colname="col1" colnum="1" />
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<entry> <code>struct S { short f[3]; } __attribute__ ((aligned));</code></entry>
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Whenever you leave out the alignment factor in an <code>aligned</code> attribute
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specification, the compiler automatically sets the alignment for the type to the largest
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alignment which is ever used for any data type on the target machine you are compiling
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for. In the example above, the size of each <code>short</code> is 2 bytes, and therefore
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the size of the entire <code>struct S</code> type is 6 bytes. The smallest power of two
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which is greater than or equal to that is 8, so the compiler sets the alignment for the
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entire <code>struct S</code> type to 8 bytes.
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Note that the effectiveness of aligned attributes may be limited by inherent limitations
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of the OpenCL device and compiler. For some devices, the OpenCL compiler may only be
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able to arrange for variables to be aligned up to a certain maximum alignment. If the
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OpenCL compiler is only able to align variables up to a maximum of 8 byte alignment, then
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specifying <code>aligned(16)</code> in an <code>__attribute__</code> will still only provide
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you with 8 byte alignment. See your platform-specific documentation for further information.
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The <code>aligned</code> attribute can only increase the alignment; but you can decrease
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it by specifying <code>packed</code> as well. See below.
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<refsect2 id="packed"><title>packed</title>
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<!-- Using refsect2 so we can have shaded backgrounds behind examples -->
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The <constant>packed</constant> attribute, attached to the <code>struct</code>
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or <code>union</code> type definition, specifies that each member of the structure or
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union is placed to minimize the memory required. When attached to an <code>enum</code>
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definition, it indicates that the smallest integral type should be used.
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Specifying this attribute for <code>struct</code> and <code>union</code> types is equivalent
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to specifying the <code>packed</code> attribute on each of the structure or union members.
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In the following example <code>struct my_packed_struct</code>'s members are packed closely
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together, but the internal layout of its <code>s</code> member is not packed. To do that,
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<code>struct my_unpacked_struct</code> would need to be packed, too.
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<informaltable frame="none">
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<tgroup cols="1" align="left" colsep="0" rowsep="0">
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<colspec colname="col1" colnum="1" /> <tbody>
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<entry> <code>struct my_unpacked_struct
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struct __attribute__ ((packed)) my_packed_struct
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struct my_unpacked_struct s;
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You may only specify this attribute on the definition of a <code>enum</code>,
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<code>struct</code>, or <code>union</code>, not on a <code>typedef</code> which does not
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also define the enumerated type, structure or union.
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<!-- ================================ EXAMPLE -->
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<!-- DO NOT DELETE IN CASE AN EXAMPLE IS ADDED IN THE FUTURE -->
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<refsect2 id="example1">
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<informaltable frame="none">
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<tgroup cols="1" align="left" colsep="0" rowsep="0">
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<colspec colname="col1" colnum="1" />
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Example goes here - it will be set in "code" type with white space preserved.
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<!-- ================================ SPECIFICATION -->
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<!-- Set the "uri" attribute in the <olink /> element to the "named destination" for the PDF page
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<refsect1 id="specification"><title>Specification</title>
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<olink uri="attributes-types">OpenCL Specification</olink>
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<!-- ================================ ALSO SEE -->
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<refsect1 id="seealso"><title>Also see</title>
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<citerefentry href="attribute"><refentrytitle>__attribute__</refentrytitle></citerefentry>,
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<citerefentry href="attributes-blocksAndControlFlow"><refentrytitle>Blocks and Control-Flow Statement Attributes Attributes</refentrytitle></citerefentry>,
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<citerefentry href="attributes-variables"><refentrytitle>Variable Attributes</refentrytitle></citerefentry>
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<!-- ============================== COPYRIGHT -->
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<!-- Content included from copyright.inc.xsl -->
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<refsect3 id="Copyright"><title></title>
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