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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">
          <table border="0">
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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>
      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>
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          <table border="0">
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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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