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<title>__constant</title>
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<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">
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__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">
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// 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>
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__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.
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