sdk/2.0/docs/man/xhtml/attributes-variables.html - external/github.com/KhronosGroup/OpenCL-Registry - Git at Google

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     <title>Attributes of Variables</title>
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       <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">
             <colgroup>
               <col align="left" class="col1" />
             </colgroup>
             <tbody>
               <tr>
                 <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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                 <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">
             <colgroup>
               <col align="left" class="col1" />
             </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">global 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>
         <p>
         The endian attribute can only be applied to pointer
         types that are in the <code class="code">global</code> or
         <code class="code">constant</code> address space. The endian
         attribute cannot be used for variables that are
         not a pointer type. The <code class="code">endian</code> attribute value
         for both pointers must be the same when
         one pointer is assigned to another.
     </p>
       </div>
       <div class="refsect1">
         <a id="specification"></a>
         <h2>Specification</h2>
         <p>
             <img src="pdficon_small1.gif" />

             <a href="https://www.khronos.org/registry/cl/specs/opencl-2.0-openclc.pdf#page=59" 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</span></span></a>,
             <a class="citerefentry" href="attributes-types.html"><span class="citerefentry"><span class="refentrytitle">Types Attributes</span></span></a>,
             <a class="citerefentry" href="attributes-loopUnroll.html"><span class="citerefentry"><span class="refentrytitle">Loop Unroll Attributes</span></span></a>,
             <a class="citerefentry" href="qualifiers.html"><span class="citerefentry"><span class="refentrytitle">Qualifiers</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 © 2007-2013 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,
 distribute, sublicense, and/or sell copies of the Materials, and to
 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.
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	<title>Attributes of Variables</title>
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	<body>
	<div class="refentry">
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	<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">
	<colgroup>
	<col align="left" class="col1" />
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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>
	<col align="left" class="col1" />
	</colgroup>
	<tbody>
	<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">
	<colgroup>
	<col align="left" class="col1" />
	</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">global 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>
	<p>
	The endian attribute can only be applied to pointer
	types that are in the <code class="code">global</code> or
	<code class="code">constant</code> address space. The endian
	attribute cannot be used for variables that are
	not a pointer type. The <code class="code">endian</code> attribute value
	for both pointers must be the same when
	one pointer is assigned to another.
	</p>
	</div>
	<div class="refsect1">
	<a id="specification"></a>
	<h2>Specification</h2>
	<p>
	<img src="pdficon_small1.gif" />

	<a href="https://www.khronos.org/registry/cl/specs/opencl-2.0-openclc.pdf#page=59" 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</span></span></a>,
	<a class="citerefentry" href="attributes-types.html"><span class="citerefentry"><span class="refentrytitle">Types Attributes</span></span></a>,
	<a class="citerefentry" href="attributes-loopUnroll.html"><span class="citerefentry"><span class="refentrytitle">Loop Unroll Attributes</span></span></a>,
	<a class="citerefentry" href="qualifiers.html"><span class="citerefentry"><span class="refentrytitle">Qualifiers</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 © 2007-2013 The Khronos Group Inc.
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	copy of this software and/or associated documentation files (the
	"Materials"), to deal in the Materials without restriction, including
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