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<title>clEnqueueReadBuffer</title>
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<div class="refentry">
<a id="idm1"></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="_top">cl_int</a>
<strong class="fsfunc">clEnqueueReadBuffer</strong>
(</code>
<td><a xmlns="http://www.w3.org/1999/xhtml" class="link" href="abstractDataTypes.html" target="_top">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="_top">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="_top">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="_top">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="_top">size_t</a> <var xmlns="http://www.w3.org/1999/xhtml" class="pdparam">cb</var>, </td>
</tr>
<tr valign="top">
<td> </td>
<td><a xmlns="http://www.w3.org/1999/xhtml" class="link" href="scalarDataTypes.html" target="_top">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="_top">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="_top">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="_top">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. 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">cb </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.
</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.
</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 have finished execution before the read
command begins execution</li>
<li class="listitem" style="list-style-type: disc">The buffer object is not mapped</li>
<li class="listitem" style="list-style-type: disc">The buffer object is 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">cb</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> 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_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_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-1.0.pdf#page=54" 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>
</p>
</div>
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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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