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| <title>__constant</title> |
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| <body> |
| <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="__constant"></a> |
| <h1>__constant</h1> |
| <p> |
| Address Space Qualifier. |
| </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"> |
| __constant |
| constant |
| </td> |
| </tr> |
| </tbody> |
| </table> |
| </div> |
| </div> |
| <div class="refsect1"> |
| <a id="description"></a> |
| <h2>Description</h2> |
| <p> |
| OpenCL implements the following disjoint address spaces: <code class="function">__global</code>, |
| <code class="function">__local</code>, <code class="function">__constant</code>, and |
| <code class="function">__private</code> The address space qualifier may be used in variable |
| declarations to specify the region of memory that is used to allocate the object. The |
| C syntax for type qualifiers is extended in OpenCL to include an address space name as |
| a valid type qualifier. If the type of an object is qualified by an address space name, |
| the object is allocated in the specified address name; otherwise, the object is allocated |
| in the generic address space. |
| </p> |
| <p> |
| The address space names without the __prefix i.e. <code class="function">global</code>, |
| <code class="function">local</code>, <code class="function">constant</code> and <code class="function">private</code> |
| may be substituted for the corresponding address space names with the __prefix. |
| </p> |
| <p> |
| The generic address space name for arguments to a function in a program, or local variables |
| of a function is <code class="function">__private</code>. All function arguments shall be in the |
| <code class="function">__private</code> address space. |
| </p> |
| <p> |
| <a class="citerefentry" href="functionQualifiers.html"><span class="citerefentry"><span class="refentrytitle">__kernel</span></span></a> |
| function arguments declared to be a pointer of a type can point to one of the following |
| address spaces only: <code class="function">__global</code>, <code class="function">__local</code> or |
| <code class="function">__constant</code>. A pointer to address space A can only be assigned to |
| a pointer to the same address space A. Casting a pointer to address space A to a pointer |
| to address space B is illegal. |
| </p> |
| <p> |
| Function arguments of type |
| <span class="type"><a class="citerefentry" href="otherDataTypes.html"><span class="citerefentry"><span class="refentrytitle">image2d_t</span></span></a></span>, |
| <span class="type"><a class="citerefentry" href="otherDataTypes.html"><span class="citerefentry"><span class="refentrytitle">image3d_t</span></span></a></span>, |
| <span class="type"><a class="citerefentry" href="otherDataTypes.html"><span class="citerefentry"><span class="refentrytitle">image2d_array_t</span></span></a></span>, |
| <span class="type"><a class="citerefentry" href="otherDataTypes.html"><span class="citerefentry"><span class="refentrytitle">image1d_t</span></span></a></span>, |
| <span class="type"><a class="citerefentry" href="otherDataTypes.html"><span class="citerefentry"><span class="refentrytitle">image1d_buffer_t</span></span></a></span>, and |
| <span class="type"><a class="citerefentry" href="otherDataTypes.html"><span class="citerefentry"><span class="refentrytitle">image1d_array_t</span></span></a></span> |
| refer to image memory objects allocated in the <code class="function">__global</code> address space. |
| </p> |
| </div> |
| <div class="refsect1"> |
| <a id="notes"></a> |
| <h2>Notes</h2> |
| <p> |
| The <code class="function">__constant</code> or <code class="function">constant</code> address space |
| name is used to describe variables allocated in global memory and which are accessed |
| inside a kernel(s) as read-only variables. These readonly variables can be accessed |
| by all (global) work-items of the kernel during its execution. Pointers to the |
| <code class="function">__constant</code> address space are allowed as arguments to functions |
| (including kernel functions) and for variables declared inside functions. |
| </p> |
| <p> |
| All string literal storage shall be in the <code class="function">__constant</code> address space. |
| </p> |
| <p> |
| NOTE: Each argument to a kernel that is a pointer to the <code class="function">__constant</code> |
| address space is counted separately towards the maximum number of such arguments, |
| defined as <code class="constant">CL_DEVICE_MAX_CONSTANT_ARGS</code> are described in the |
| table for <a class="citerefentry" href="clGetDeviceInfo.html"><span class="citerefentry"><span class="refentrytitle">clGetDeviceInfo</span></span></a>. |
| </p> |
| <p> |
| Variables in the program scope or the outermost scope of kernel functions can be |
| declared in the <code class="function">__constant</code> address space. These variables are |
| required to be initialized and the values used to initialize these variables must be |
| a compile time constant. Writing to such a variable results in a compile-time error. |
| </p> |
| <p> |
| Implementations are not required to aggregate these declarations into the fewest |
| number of constant arguments. This behavior is implementation defined. |
| </p> |
| <p> |
| Thus portable code must conservatively assume that each variable declared inside |
| a function or in program scope allocated in the <code class="function">__constant</code> |
| address space counts as a separate constant argument. |
| </p> |
| </div> |
| <div class="refsect2"> |
| <a id="example1"></a> |
| <h3> |
| Example |
| </h3> |
| <p> |
| There is no generic address space name for program scope variables. All program scope |
| variables must be declared in the <code class="function">__constant</code> address space. For example: |
| </p> |
| <div class="informaltable"> |
| <table border="0"> |
| <colgroup> |
| <col align="left" class="col1" /> |
| </colgroup> |
| <tbody> |
| <tr> |
| <td align="left"> |
| // declares a pointer p in the __private address space that |
| // points to an int object in address space __global |
| __global int *p; |
| |
| // declares an array of 4 floats in the __private address space. |
| float x[4]; |
| </td> |
| </tr> |
| </tbody> |
| </table> |
| </div> |
| <p> |
| There is no address space for function return values. Using an address space qualifier in |
| a function return type declaration will generate a compilation error, unless the return |
| type is declared as a pointer type and the qualifier is used on the points-to address space. |
| </p> |
| <p> |
| For example: |
| </p> |
| <div class="informaltable"> |
| <table border="0"> |
| <colgroup> |
| <col align="left" class="col1" /> |
| </colgroup> |
| <tbody> |
| <tr> |
| <td align="left"> |
| __private int f() { ... } // should generate an error |
| __local int *f() { ... } // allowed |
| __local int * __private f() { ... }; // should generate an error. |
| </td> |
| </tr> |
| </tbody> |
| </table> |
| </div> |
| </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=222" target="OpenCL Spec">OpenCL Specification</a> |
| </p> |
| </div> |
| <div class="refsect1"> |
| <a id="seealso"></a> |
| <h2>Also see</h2> |
| <p> |
| <a class="citerefentry" href="global.html"><span class="citerefentry"><span class="refentrytitle">__global</span></span></a>, |
| <a class="citerefentry" href="local.html"><span class="citerefentry"><span class="refentrytitle">__local</span></span></a>, |
| <a class="citerefentry" href="private.html"><span class="citerefentry"><span class="refentrytitle">__private</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. |
| 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 |
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| in all copies or substantial portions of the Materials. |
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