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<title>__constant</title>
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<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 class="informaltable" border="0">
<colgroup>
<col align="left" class="col1" />
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<tbody>
<tr>
<td align="left">
__constant
constant
</td>
</tr>
</tbody>
</table>
</div>
</div>
<div class="refsect1">
<a id="description"></a>
<h2>Description</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 read-only 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 must be declared in the
<code class="function">__constant</code> address space. Variables
in 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="refsect1">
<a id="notes"></a>
<h2>Notes</h2>
<h4><a id="id-1.6.2"></a>General information about address space qualifiers</h4>
<p>
OpenCL implements the following disjoint named 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 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.
The address space for a variable at program scope
or a static variable inside a function can either
be <code class="function">__global</code> or
<code class="function">__constant</code>, but defaults to
<code class="function">__global</code> if not specified.
</p>
<p>
OpenCL 2.0 adds support for an unnamed <code class="function">generic</code>
address space. Pointers that are declared
without pointing to a named address space point to the
<code class="function">generic</code> address space. Before
referring to the region pointed to, the pointer
must be associated with a named address space.
Functions may be written with arguments and return values that point to the
<code class="function">generic</code> address space.
</p>
<p>
kernel function arguments declared to be a pointer or
an array of a type must point to one of
the named address spaces <code class="function">__global</code>,
<code class="function">__local</code> or <code class="function">__constant</code>.
</p>
<p>
The named address spaces are a subset of the generic
address space except for the <code class="function">constant</code>
address space.
</p>
<p>
A pointer to address space A can only be assigned to
a pointer to the same address space A or a
pointer to the <code class="function">generic</code> address
space. Casting a pointer to address space A to a pointer to
address space B is illegal if A and B are named
address spaces and A is not the same as B.
</p>
<p>
The <code class="function">__global</code>, <code class="function">__constant</code>,
<code class="function">__local</code>,
<code class="function">__private</code>, <code class="function">__generic</code>,
<code class="function">global</code>,
<code class="function">constant</code>, <code class="function">local</code>, and
<code class="function">private</code> names are reserved for use as address space
qualifiers and shall not be used otherwise.
The <code class="function">__generic</code> and <code class="function">generic</code>
names are reserved for future use.
</p>
<p>
The size of pointers to different address spaces
may differ. It is not correct to assume
that, for example, <code class="code">sizeof(__global int *)</code>
always equals <code class="code">sizeof(__local int *)</code>.
</p>
</div>
<div class="refsect2">
<a id="example1"></a>
<h3>
Example
</h3>
<p>
General qualifier examples follow:
</p>
<div class="informaltable">
<table class="informaltable" 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 object in address space global
global int *p;
void foo (...)
{
// declares an array of 4 floats in the private address space
float x[4];
...
}
</td>
</tr>
</tbody>
</table>
</div>
<p>
Example:
</p>
<div class="informaltable">
<table class="informaltable" 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="https://www.khronos.org/registry/cl/specs/opencl-2.0-openclc.pdf#namedest=addressSpaceQualifiers" 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>,
<a class="citerefentry" href="genericAddressSpace.html"><span class="citerefentry"><span class="refentrytitle">genericAddressSpace</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 © 2007-2015 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
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