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    <title>clEnqueueReadBuffer</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="clEnqueueReadBuffer"></a>
        <h1>
            clEnqueueReadBuffer
        </h1>
        <p>
            Enqueue commands to read from a buffer object to host memory.
        </p>
      </div>
      <div class="refsynopsisdiv">
        <h2></h2>
        <div class="funcsynopsis">
          <table xmlns="" border="0" summary="Function synopsis" cellspacing="0" cellpadding="0">
            <tr valign="bottom">
              <td>
                <code xmlns="http://www.w3.org/1999/xhtml" class="funcdef">
                    <a class="link" href="scalarDataTypes.html" target="pagedisplay">cl_int</a>
 <strong class="fsfunc">clEnqueueReadBuffer</strong>
                (</code>
                <td><a xmlns="http://www.w3.org/1999/xhtml" class="link" href="abstractDataTypes.html" target="pagedisplay">cl_command_queue</a> <var xmlns="http://www.w3.org/1999/xhtml" class="pdparam">command_queue</var>, </td>
              </td>
            </tr>
            <tr valign="top">
              <td> </td>
              <td><a xmlns="http://www.w3.org/1999/xhtml" class="link" href="abstractDataTypes.html" target="pagedisplay">cl_mem</a> <var xmlns="http://www.w3.org/1999/xhtml" class="pdparam">buffer</var>, </td>
            </tr>
            <tr valign="top">
              <td> </td>
              <td><a xmlns="http://www.w3.org/1999/xhtml" class="link" href="enums.html#cl_bool" target="pagedisplay">cl_bool</a> <var xmlns="http://www.w3.org/1999/xhtml" class="pdparam">blocking_read</var>, </td>
            </tr>
            <tr valign="top">
              <td> </td>
              <td><a xmlns="http://www.w3.org/1999/xhtml" class="link" href="scalarDataTypes.html" target="pagedisplay">size_t</a> <var xmlns="http://www.w3.org/1999/xhtml" class="pdparam">offset</var>, </td>
            </tr>
            <tr valign="top">
              <td> </td>
              <td><a xmlns="http://www.w3.org/1999/xhtml" class="link" href="scalarDataTypes.html" target="pagedisplay">size_t</a> <var xmlns="http://www.w3.org/1999/xhtml" class="pdparam">size</var>, </td>
            </tr>
            <tr valign="top">
              <td> </td>
              <td><a xmlns="http://www.w3.org/1999/xhtml" class="link" href="scalarDataTypes.html" target="pagedisplay">void</a> <var xmlns="http://www.w3.org/1999/xhtml" class="pdparam">*ptr</var>, </td>
            </tr>
            <tr valign="top">
              <td> </td>
              <td><a xmlns="http://www.w3.org/1999/xhtml" class="link" href="scalarDataTypes.html" target="pagedisplay">cl_uint</a> <var xmlns="http://www.w3.org/1999/xhtml" class="pdparam">num_events_in_wait_list</var>, </td>
            </tr>
            <tr valign="top">
              <td> </td>
              <td>const <a xmlns="http://www.w3.org/1999/xhtml" class="link" href="abstractDataTypes.html" target="pagedisplay">cl_event</a> <var xmlns="http://www.w3.org/1999/xhtml" class="pdparam">*event_wait_list</var>, </td>
            </tr>
            <tr valign="top">
              <td> </td>
              <td><a xmlns="http://www.w3.org/1999/xhtml" class="link" href="abstractDataTypes.html" target="pagedisplay">cl_event</a> <var xmlns="http://www.w3.org/1999/xhtml" class="pdparam">*event</var><code>)</code></td>
            </tr>
          </table>
        </div>
      </div>
      <div class="refsect1">
        <a id="parameters"></a>
        <h2>Parameters</h2>
        <div class="variablelist">
          <dl class="variablelist">
            <dt>
              <span class="term">
                <code class="varname">command_queue</code>
              </span>
            </dt>
            <dd>
              <p>
                        Is a valid host command-queue in which the read command will be
                        queued. <code class="varname">command_queue</code> and <code class="varname">buffer</code>
                        must be created with the same OpenCL context.
                    </p>
            </dd>
            <dt>
              <span class="term">
                <code class="varname">buffer</code>
              </span>
            </dt>
            <dd>
              <p>Refers to a valid buffer object.</p>
            </dd>
            <dt>
              <span class="term">
                <code class="varname">blocking_read</code>
              </span>
            </dt>
            <dd>
              <p>
                      Indicates if the read operations are
                      <code class="varname">blocking</code> or <code class="varname">non-blocking</code>.
                    </p>
              <p>
                      If <code class="varname">blocking_read</code> is <code class="constant">CL_TRUE</code>
                      i.e. the read command is blocking, <code class="function">clEnqueueReadBuffer</code>
                      does not return until the buffer data has been read and copied into memory
                      pointed to by <code class="varname">ptr</code>.
                    </p>
              <p>
                      If <code class="varname">blocking_read</code> is <code class="constant">CL_FALSE</code>
                      i.e. the read command is <code class="varname">non-blocking</code>,
                      <code class="function">clEnqueueReadBuffer</code> queues a
                      <code class="varname">non-blocking</code> read command and returns. The contents of
                      the buffer that <code class="varname">ptr</code> points to cannot be used until the
                      read command has completed. The <code class="varname">event</code> argument returns
                      an event object which can be used to query the execution status of the
                      read command. When the read command has completed, the contents of the
                      buffer that <code class="varname">ptr</code> points to can be used by the application.
                    </p>
            </dd>
            <dt>
              <span class="term">
                <code class="varname">offset</code>
              </span>
            </dt>
            <dd>
              <p>The offset in bytes in the buffer object to read from.</p>
            </dd>
            <dt>
              <span class="term">
                <code class="varname">size</code>
              </span>
            </dt>
            <dd>
              <p>The size in bytes of data being read.</p>
            </dd>
            <dt>
              <span class="term"> <code class="varname">ptr </code> </span>
            </dt>
            <dd>
              <p>
                        The pointer to buffer in host memory where data is to be read into.
                    </p>
            </dd>
            <dt>
              <span class="term">
                    <code class="varname">event_wait_list</code>
                , </span>
              <span class="term">
                    <code class="varname">num_events_in_wait_list</code>
                </span>
            </dt>
            <dd>
              <p>
                      <code class="varname">event_wait_list</code>
                      and <code class="varname">num_events_in_wait_list</code> specify events that
                      need to complete before this particular command can be executed.
                      If <code class="varname">event_wait_list</code> is NULL, then this particular command
                      does not wait on any event to complete. If <code class="varname">event_wait_list</code>
                      is NULL, <code class="varname">num_events_in_wait_list</code> must be 0. If
                      <code class="varname">event_wait_list</code> is not NULL, the list of events
                      pointed to by <code class="varname">event_wait_list</code> must be valid
                      and <code class="varname">num_events_in_wait_list</code> must be greater
                      than 0. The events specified in <code class="varname">event_wait_list</code>
                      act as synchronization points. The context associated with events in
                      <code class="varname">event_wait_list</code> and <code class="varname">command_queue</code> must
                      be the same. The memory associated with <code class="varname">event_wait_list</code>
                      can be reused or freed after the function returns.
                    </p>
            </dd>
            <dt>
              <span class="term"> <code class="varname"> event </code> </span>
            </dt>
            <dd>
              <p>
                      Returns an event object that identifies
                      this particular read command and can be used to query or queue a wait
                      for this particular command to complete.  <code class="varname">event</code> can be
                      NULL in which case it will not be possible for the application to query
                      the status of this command or queue a wait for this command to complete.
                      If the <code class="varname">event_wait_list</code> and the <code class="varname">event</code> arguments are
                      not NULL, the <code class="varname">event</code> argument should not refer to an
                      element of the <code class="varname">event_wait_list</code> array.
                    </p>
            </dd>
          </dl>
        </div>
      </div>
      <div class="refsect1">
        <a id="notes"></a>
        <h2>Notes</h2>
        <p>
          Calling <code class="function">clEnqueueReadBuffer</code> to read a region of the buffer object
          with the <code class="varname">ptr</code> argument value set to <code class="varname">host_ptr</code>
          + <code class="varname">offset</code>, where <code class="varname">host_ptr</code> is a pointer to
          the memory region specified when the buffer object being read is created with
          <code class="constant">CL_MEM_USE_HOST_PTR</code>, must meet the following requirements in
          order to avoid undefined behavior:
        </p>
        <div class="itemizedlist">
          <ul class="itemizedlist" style="list-style-type: disc; ">
            <li class="listitem" style="list-style-type: disc">
              All commands that use this buffer object or a memory object (buffer or image)
              created from this buffer object have finished execution before the read command
              begins execution.
            </li>
            <li class="listitem" style="list-style-type: disc">
              The buffer object or memory objects created from this buffer object are not mapped.
            </li>
            <li class="listitem" style="list-style-type: disc">
              The buffer object or memory objects created from this buffer object are not used
              by any command-queue until the read command has finished execution.
            </li>
          </ul>
        </div>
      </div>
      <div class="refsect1">
        <a id="errors"></a>
        <h2>Errors</h2>
        <p>
          <code class="function">clEnqueueReadBuffer</code> returns <span class="errorname">CL_SUCCESS</span> if
          the function is executed successfully. Otherwise, it returns one of the following errors:
        </p>
        <div class="itemizedlist">
          <ul class="itemizedlist" style="list-style-type: disc; ">
            <li class="listitem" style="list-style-type: disc"><span class="errorname">CL_INVALID_COMMAND_QUEUE</span> if <code class="varname">command_queue</code>
              is not a valid host command-queue.
            </li>
            <li class="listitem" style="list-style-type: disc"><span class="errorname">CL_INVALID_CONTEXT</span> if the context associated with
              <code class="varname">command_queue</code> and <code class="varname">buffer</code> are not the same
              or if the context associated with <code class="varname">command_queue</code> and events in
              <code class="varname">event_wait_list</code> are not the same.
            </li>
            <li class="listitem" style="list-style-type: disc"><span class="errorname">CL_INVALID_MEM_OBJECT</span> if <code class="varname">buffer</code> is not
              a valid buffer object.
            </li>
            <li class="listitem" style="list-style-type: disc"><span class="errorname">CL_INVALID_VALUE</span> if the region being read specified by
              (<code class="varname">offset</code>, <code class="varname">size</code>) is out of bounds or if
              <code class="varname">ptr</code> is a NULL value.
            </li>
            <li class="listitem" style="list-style-type: disc"><span class="errorname">CL_INVALID_EVENT_WAIT_LIST</span>
              if <code class="varname">event_wait_list</code> is NULL and
              <code class="varname">num_events_in_wait_list</code> &gt;
              0, or <code class="varname">event_wait_list</code> is not NULL and
              <code class="varname">num_events_in_wait_list</code> is 0, or if event objects in
              <code class="varname">event_wait_list</code> are not valid events.
            </li>
            <li class="listitem" style="list-style-type: disc"><span class="errorname">CL_MISALIGNED_SUB_BUFFER_OFFSET</span> if
              <code class="varname">buffer</code> is a sub-buffer object and <code class="varname">offset</code>
              specified when the sub-buffer object is created is not aligned to
              <code class="constant">CL_DEVICE_MEM_BASE_ADDR_ALIGN</code> value for device associated
              with <code class="varname">queue</code>.
            </li>
            <li class="listitem" style="list-style-type: disc"><span class="errorname">CL_EXEC_STATUS_ERROR_FOR_EVENTS_IN_WAIT_LIST</span> if the read
              and write operations are blocking and the execution status of any of the events
              in <code class="varname">event_wait_list</code> is a negative integer value.
            </li>
            <li class="listitem" style="list-style-type: disc"><span class="errorname">CL_MEM_OBJECT_ALLOCATION_FAILURE</span> if there is a failure to
              allocate memory for data store associated with <code class="varname">buffer</code>.
            </li>
            <li class="listitem" style="list-style-type: disc"><span class="errorname">CL_INVALID_OPERATION</span> if
              <code class="function">clEnqueueReadBuffer</code> is called on <code class="varname">buffer</code>
              which has been created with <code class="constant">CL_MEM_HOST_WRITE_ONLY</code> or
              <code class="constant">CL_MEM_HOST_NO_ACCESS</code>.
            </li>
            <li class="listitem" style="list-style-type: disc"><span class="errorname">CL_OUT_OF_RESOURCES</span> if there is a failure to allocate
              resources required by the OpenCL implementation on the device.
            </li>
            <li class="listitem" style="list-style-type: disc"><span class="errorname">CL_OUT_OF_HOST_MEMORY</span> if there is a failure to allocate
              resources required by the OpenCL implementation on the host.
            </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.pdf#page=110" target="OpenCL Spec">OpenCL Specification</a>
        </p>
      </div>
      <div class="refsect1">
        <a id="seealso"></a>
        <h2>Also see</h2>
        <p>
            <a class="citerefentry" href="clEnqueueCopyBuffer.html"><span class="citerefentry"><span class="refentrytitle">clEnqueueCopyBuffer</span></span></a>,
            <a class="citerefentry" href="clEnqueueWriteBuffer.html"><span class="citerefentry"><span class="refentrytitle">clEnqueueWriteBuffer</span></span></a>,
            <a class="citerefentry" href="clEnqueueWriteBufferRect.html"><span class="citerefentry"><span class="refentrytitle">clEnqueueWriteBufferRect</span></span></a>,
            <a class="citerefentry" href="clEnqueueReadBufferRect.html"><span class="citerefentry"><span class="refentrytitle">clEnqueueReadBufferRect</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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