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    <title>__constant</title>
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      <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" />
            </colgroup>
            <tbody>
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                <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>
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              <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">
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            <colgroup>
              <col align="left" class="col1" />
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                <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.
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