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    <title>restrictions</title>
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    <div class="refentry">
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        <a xmlns="http://www.w3.org/1999/xhtml" id="restrictionsref"></a>
        <h1>Restrictions</h1>
        <p> </p>
      </div>
      <div class="refsect1">
        <a id="restrictions"></a>
        <h2></h2>
        <div class="itemizedlist">
          <ul class="itemizedlist" style="list-style-type: disc; ">
            <li class="listitem" style="list-style-type: disc">
              <p>The use of pointers is somewhat restricted. The following rules apply:</p>
              <div class="itemizedlist">
                <ul class="itemizedlist" style="list-style-type: opencircle; ">
                  <li class="listitem" style="list-style-type: circle">
                    <p>
                  Arguments to kernel
                  functions declared in a program that are pointers must be declared with the
                  <a class="citerefentry" href="global.html"><span class="citerefentry"><span class="refentrytitle">__global</span></span></a>,
                  <a class="citerefentry" href="constant.html"><span class="citerefentry"><span class="refentrytitle">__constant</span></span></a>, or
                  <a class="citerefentry" href="local.html"><span class="citerefentry"><span class="refentrytitle">__local</span></span></a>
                  qualifier.
                </p>
                  </li>
                  <li class="listitem" style="list-style-type: circle">
                    <p>
                    A pointer declared with the
                    <a class="citerefentry" href="constant.html"><span class="citerefentry"><span class="refentrytitle">__constant</span></span></a>
                    qualifier can only be assigned to a pointer declared with the __constant qualifier.
                </p>
                  </li>
                  <li class="listitem" style="list-style-type: circle">
                    <p>Pointers to functions are not allowed.</p>
                  </li>
                </ul>
              </div>
            </li>
            <li class="listitem" style="list-style-type: disc">
              <p>
            An image type (
            <a class="citerefentry" href="otherDataTypes.html"><span class="citerefentry"><span class="refentrytitle">image2d_t</span></span></a>,
            <a class="citerefentry" href="otherDataTypes.html"><span class="citerefentry"><span class="refentrytitle">image3d_t</span></span></a>,
            <a class="citerefentry" href="otherDataTypes.html"><span class="citerefentry"><span class="refentrytitle">image2d_array_t</span></span></a>,
            <a class="citerefentry" href="otherDataTypes.html"><span class="citerefentry"><span class="refentrytitle">image1d_t</span></span></a>,
            <a class="citerefentry" href="otherDataTypes.html"><span class="citerefentry"><span class="refentrytitle">image1d_buffer_t</span></span></a>, or
            <a class="citerefentry" href="otherDataTypes.html"><span class="citerefentry"><span class="refentrytitle">image1d_array_t</span></span></a>)
            can only be used as the type of a function argument. An image function
            argument cannot be modified.  Elements of an image can only be accessed
            using the <a class="citerefentry" href="imageFunctions.html"><span class="citerefentry"><span class="refentrytitle">Built-in Image
            Functions</span></span></a>.
          </p>
              <p>
            An image type cannot be used to declare a variable, a structure or
            union field, an array of images, a pointer to an image, or the return
            type of a function. An image type cannot be used with the
            <a class="citerefentry" href="global.html"><span class="citerefentry"><span class="refentrytitle">__global</span></span></a>,
            <a class="citerefentry" href="private.html"><span class="citerefentry"><span class="refentrytitle">__private</span></span></a>,
            <a class="citerefentry" href="local.html"><span class="citerefentry"><span class="refentrytitle">__local</span></span></a> and
            <a class="citerefentry" href="constant.html"><span class="citerefentry"><span class="refentrytitle">__constant</span></span></a>
            address space qualifiers.
          </p>
              <p>
            The sampler type
            (<a class="citerefentry" href="sampler_t.html"><span class="citerefentry"><span class="refentrytitle">sampler_t</span></span></a>) can only be
            used as the type of a function argument or a variable declared in the program scope
            or the outermost scope of a kernel function. The behavior of a sampler variable
            declared in a non-outermost scope of a kernel function is implementation-defined. A
            sampler argument or variable cannot be modified.
          </p>
              <p>
            The sampler type cannot be used to declare a structure or union field, an array of
            samplers, a pointer to a sampler, or the return type of a function. The sampler type
            cannot be used with the 
            <a class="citerefentry" href="local.html"><span class="citerefentry"><span class="refentrytitle">__local</span></span></a> and 
            <a class="citerefentry" href="global.html"><span class="citerefentry"><span class="refentrytitle">__global</span></span></a> 
            address space qualifiers.
          </p>
            </li>
            <li class="listitem" style="list-style-type: disc">
              <p>Bit-fields struct members are currently not supported.</p>
            </li>
            <li class="listitem" style="list-style-type: disc">
              <p>Variable length arrays and structures with 
                flexible (or unsized) arrays are not supported.</p>
            </li>
            <li class="listitem" style="list-style-type: disc">
              <p>Variadic macros and functions with the exception of 
          <a class="citerefentry" href="printfFunction.html"><span class="citerefentry"><span class="refentrytitle">printf</span></span></a> and 
          <a class="citerefentry" href="enqueue_kernel.html"><span class="citerefentry"><span class="refentrytitle">enqueue_kernel</span></span></a> 
          are not supported.</p>
            </li>
            <li class="listitem" style="list-style-type: disc">
              <p>
            The library functions defined in the C99 standard headers <code class="code">assert.h</code>,
            <code class="code">ctype.h</code>, <code class="code">complex.h</code>, <code class="code">errno.h</code>,
            <code class="code">fenv.h</code>, <code class="code">float.h</code>, <code class="code">inttypes.h</code>,
            <code class="code">limits.h</code>, <code class="code">locale.h</code>, <code class="code">setjmp.h</code>,
            <code class="code">signal.h</code>, <code class="code">stdarg.h</code>, <code class="code">stdio.h</code>,
            <code class="code">stdlib.h</code>, <code class="code">string.h</code>, <code class="code">tgmath.h</code>,
            <code class="code">time.h</code>, <code class="code">wchar.h</code>, and <code class="code">wctype.h</code> are not
            available and cannot be included by a program.
          </p>
            </li>
            <li class="listitem" style="list-style-type: disc">
              <p>
             The <code class="code">auto</code> and <code class="code">register</code> 
             storage-class specifiers are not supported.
          </p>
            </li>
            <li class="listitem" style="list-style-type: disc">
              <p>Recursion is not supported.</p>
            </li>
            <li class="listitem" style="list-style-type: disc">
              <p>
            The return type of a kernel function must be <span class="type">void</span>.
          </p>
            </li>
            <li class="listitem" style="list-style-type: disc">
              <p>
            Arguments to kernel functions in a program cannot be declared with the built-in
            scalar types <span class="type">bool</span>, <span class="type">half</span>, <span class="type">size_t</span>,
            <span class="type">ptrdiff_t</span>, <span class="type">intptr_t</span>, and <span class="type">uintptr_t</span>, or
            a struct and/or union that contain fields declared to be one of these built-in
            scalar types. The size in bytes of these types except <span class="type">half</span> are
            implementation-defined and in addition can also be different for the OpenCL device
            and the host processor making it difficult to allocate buffer objects to be passed
            as arguments to a kernel declared as pointer to these types. <span class="type">half</span> is
            not supported as <span class="type">half</span> can be used as a storage format only (unless the
            <a class="citerefentry" href="cl_khr_fp16.html"><span class="citerefentry"><span class="refentrytitle">cl_khr_fp16</span></span></a> extension is
            supported) and is not a data type on which floating-point arithmetic can be performed.
          </p>
            </li>
            <li class="listitem" style="list-style-type: disc">
              <p>Whether or not irreducible control flow is illegal is implementation defined.</p>
            </li>
            <li class="listitem" style="list-style-type: disc">
              <p>
            The type qualifiers <code class="code">const</code>, <code class="code">restrict</code> 
            and <code class="code">volatile</code> as defined by the C99 
            specification are supported. These qualifiers cannot be used with 
            <a class="citerefentry" href="otherDataTypes.html"><span class="citerefentry"><span class="refentrytitle">image2d_t</span></span></a>, 
            <a class="citerefentry" href="otherDataTypes.html"><span class="citerefentry"><span class="refentrytitle">image3d_t</span></span></a>, 
            <a class="citerefentry" href="otherDataTypes.html"><span class="citerefentry"><span class="refentrytitle">image2d_array_t</span></span></a>, 
            <a class="citerefentry" href="otherDataTypes.html"><span class="citerefentry"><span class="refentrytitle">image2d_depth_t</span></span></a>,
            <a class="citerefentry" href="otherDataTypes.html"><span class="citerefentry"><span class="refentrytitle">image2d_array_depth_t</span></span></a>, 
            <a class="citerefentry" href="otherDataTypes.html"><span class="citerefentry"><span class="refentrytitle">image1d_t</span></span></a>, 
            <a class="citerefentry" href="otherDataTypes.html"><span class="citerefentry"><span class="refentrytitle">image1d_buffer_t</span></span></a> and 
            <a class="citerefentry" href="otherDataTypes.html"><span class="citerefentry"><span class="refentrytitle">image1d_array_t</span></span></a> 
            types. Types other than pointer types shall not use the 
            <code class="code">restrict</code> qualifier.
          </p>
            </li>
            <li class="listitem" style="list-style-type: disc">
              <p>
            The event type 
            (<a class="citerefentry" href="otherDataTypes.html"><span class="citerefentry"><span class="refentrytitle">event_t</span></span></a>) 
            cannot be used as the type of a kernel function argument. 
            The event type cannot be used to declare a program scope variable. The event type 
            cannot be used to declare a structure or union field. The event type cannot be used with the 
            <a class="citerefentry" href="local.html"><span class="citerefentry"><span class="refentrytitle">__local</span></span></a>, 
            <a class="citerefentry" href="constant.html"><span class="citerefentry"><span class="refentrytitle">__constant</span></span></a> and 
            <a class="citerefentry" href="global.html"><span class="citerefentry"><span class="refentrytitle">__global</span></span></a> 
            address space qualifiers.
          </p>
            </li>
            <li class="listitem" style="list-style-type: disc">
              <p>
            The <span class="type">clk_event_t</span>, <span class="type">ndrange_t</span> and 
            <span class="type">reserve_id_t</span> types cannot be used as arguments to 
            kernel functions that get enqueued from the host. The 
            <span class="type">clk_event_t</span> and <span class="type">reserve_id_t</span> types 
            cannot be declared in program scope.
          </p>
            </li>
            <li class="listitem" style="list-style-type: disc">
              <p>
            The behavior of applying the <code class="code">sizeof</code> operator 
            to the <span class="type">queue_t</span>, <span class="type">clk_event_t</span>, 
            <span class="type">ndrange_t</span> and <span class="type">reserve_id_t</span> 
            types is implementation-defined.
          </p>
            </li>
            <li class="listitem" style="list-style-type: disc">
              <p>
            Kernels enqueued by the host must continue to have 
            their arguments that are a pointer to 
            a type declared to point to a named address space.
          </p>
            </li>
            <li class="listitem" style="list-style-type: disc">
              <p>
            A function in an OpenCL program cannot be called <code class="code">main</code>.
          </p>
            </li>
          </ul>
        </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-2.1-openclc.pdf#page=51" target="OpenCL Spec">OpenCL Specification</a>
        </p>
      </div>
      <div class="refsect1">
        <a id="seealso"></a>
        <h2>Also see</h2>
        <p>
            <a class="citerefentry" href="imageFunctions.html"><span class="citerefentry"><span class="refentrytitle">Image Functions</span></span></a>,
            <a class="citerefentry" href="functionQualifiers.html"><span class="citerefentry"><span class="refentrytitle">Function Qualifiers</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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in all copies or substantial portions of the Materials.
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