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<title>clEnqueueReadBuffer</title>
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<a id="idp58732576"></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>
Refers to the 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 non-blocking.
</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 event 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 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 or if size is 0.
</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> greater than
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="http://www.khronos.org/registry/cl/specs/opencl-1.2.pdf#page=72" 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 © 2007-2011 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
permit persons to whom the Materials are furnished to do so, subject to
the condition that this copyright notice and permission notice shall be included
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