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<!-- Copyright (C) 2006-2016 Free Software Foundation, Inc.
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Permission is granted to copy, distribute and/or modify this document
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under the terms of the GNU Free Documentation License, Version 1.3 or
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(see below). A copy of the license is included in the section entitled
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(a) The FSF's Front-Cover Text is:
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A GNU Manual
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<!-- Created by GNU Texinfo 5.2, http://www.gnu.org/software/texinfo/ -->
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<head>
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<title>GNU libgomp: OpenACC Library Interoperability</title>
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<meta name="description" content="GNU libgomp: OpenACC Library Interoperability">
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<meta name="keywords" content="GNU libgomp: OpenACC Library Interoperability">
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<link href="Library-Index.html#Library-Index" rel="index" title="Library Index">
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<link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
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<link href="index.html#Top" rel="up" title="Top">
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<link href="The-libgomp-ABI.html#The-libgomp-ABI" rel="next" title="The libgomp ABI">
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<body lang="en" bgcolor="#FFFFFF" text="#000000" link="#0000FF" vlink="#800080" alink="#FF0000">
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<a name="OpenACC-Library-Interoperability"></a>
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<div class="header">
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<p>
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Next: <a href="The-libgomp-ABI.html#The-libgomp-ABI" accesskey="n" rel="next">The libgomp ABI</a>, Previous: <a href="CUDA-Streams-Usage.html#CUDA-Streams-Usage" accesskey="p" rel="prev">CUDA Streams Usage</a>, Up: <a href="index.html#Top" accesskey="u" rel="up">Top</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Library-Index.html#Library-Index" title="Index" rel="index">Index</a>]</p>
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</div>
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<hr>
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<a name="OpenACC-Library-Interoperability-1"></a>
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<h2 class="chapter">8 OpenACC Library Interoperability</h2>
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<a name="Introduction-1"></a>
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<h3 class="section">8.1 Introduction</h3>
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<p>The OpenACC library uses the CUDA Driver API, and may interact with
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programs that use the Runtime library directly, or another library
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based on the Runtime library, e.g., CUBLAS<a name="DOCF2" href="#FOOT2"><sup>2</sup></a>.
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This chapter describes the use cases and what changes are
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required in order to use both the OpenACC library and the CUBLAS and Runtime
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libraries within a program.
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</p>
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<a name="First-invocation_003a-NVIDIA-CUBLAS-library-API"></a>
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<h3 class="section">8.2 First invocation: NVIDIA CUBLAS library API</h3>
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<p>In this first use case (see below), a function in the CUBLAS library is called
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prior to any of the functions in the OpenACC library. More specifically, the
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function <code>cublasCreate()</code>.
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</p>
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<p>When invoked, the function initializes the library and allocates the
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hardware resources on the host and the device on behalf of the caller. Once
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the initialization and allocation has completed, a handle is returned to the
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caller. The OpenACC library also requires initialization and allocation of
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hardware resources. Since the CUBLAS library has already allocated the
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hardware resources for the device, all that is left to do is to initialize
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the OpenACC library and acquire the hardware resources on the host.
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</p>
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<p>Prior to calling the OpenACC function that initializes the library and
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allocate the host hardware resources, you need to acquire the device number
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that was allocated during the call to <code>cublasCreate()</code>. The invoking of the
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runtime library function <code>cudaGetDevice()</code> accomplishes this. Once
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acquired, the device number is passed along with the device type as
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parameters to the OpenACC library function <code>acc_set_device_num()</code>.
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</p>
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<p>Once the call to <code>acc_set_device_num()</code> has completed, the OpenACC
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library uses the context that was created during the call to
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<code>cublasCreate()</code>. In other words, both libraries will be sharing the
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same context.
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</p>
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<div class="smallexample">
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<pre class="smallexample"> /* Create the handle */
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s = cublasCreate(&h);
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if (s != CUBLAS_STATUS_SUCCESS)
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{
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fprintf(stderr, "cublasCreate failed %d\n", s);
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exit(EXIT_FAILURE);
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}
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/* Get the device number */
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e = cudaGetDevice(&dev);
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if (e != cudaSuccess)
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{
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fprintf(stderr, "cudaGetDevice failed %d\n", e);
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exit(EXIT_FAILURE);
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}
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/* Initialize OpenACC library and use device 'dev' */
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acc_set_device_num(dev, acc_device_nvidia);
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</pre></div>
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<div align="center">Use Case 1
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</div>
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<a name="First-invocation_003a-OpenACC-library-API"></a>
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<h3 class="section">8.3 First invocation: OpenACC library API</h3>
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<p>In this second use case (see below), a function in the OpenACC library is
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called prior to any of the functions in the CUBLAS library. More specificially,
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the function <code>acc_set_device_num()</code>.
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</p>
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<p>In the use case presented here, the function <code>acc_set_device_num()</code>
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is used to both initialize the OpenACC library and allocate the hardware
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resources on the host and the device. In the call to the function, the
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call parameters specify which device to use and what device
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type to use, i.e., <code>acc_device_nvidia</code>. It should be noted that this
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is but one method to initialize the OpenACC library and allocate the
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appropriate hardware resources. Other methods are available through the
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use of environment variables and these will be discussed in the next section.
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</p>
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<p>Once the call to <code>acc_set_device_num()</code> has completed, other OpenACC
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functions can be called as seen with multiple calls being made to
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<code>acc_copyin()</code>. In addition, calls can be made to functions in the
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CUBLAS library. In the use case a call to <code>cublasCreate()</code> is made
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subsequent to the calls to <code>acc_copyin()</code>.
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As seen in the previous use case, a call to <code>cublasCreate()</code>
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initializes the CUBLAS library and allocates the hardware resources on the
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host and the device. However, since the device has already been allocated,
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<code>cublasCreate()</code> will only initialize the CUBLAS library and allocate
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the appropriate hardware resources on the host. The context that was created
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as part of the OpenACC initialization is shared with the CUBLAS library,
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similarly to the first use case.
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</p>
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<div class="smallexample">
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<pre class="smallexample"> dev = 0;
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acc_set_device_num(dev, acc_device_nvidia);
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/* Copy the first set to the device */
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d_X = acc_copyin(&h_X[0], N * sizeof (float));
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if (d_X == NULL)
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{
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fprintf(stderr, "copyin error h_X\n");
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exit(EXIT_FAILURE);
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}
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/* Copy the second set to the device */
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d_Y = acc_copyin(&h_Y1[0], N * sizeof (float));
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if (d_Y == NULL)
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{
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fprintf(stderr, "copyin error h_Y1\n");
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exit(EXIT_FAILURE);
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}
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/* Create the handle */
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s = cublasCreate(&h);
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if (s != CUBLAS_STATUS_SUCCESS)
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{
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fprintf(stderr, "cublasCreate failed %d\n", s);
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exit(EXIT_FAILURE);
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}
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/* Perform saxpy using CUBLAS library function */
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s = cublasSaxpy(h, N, &alpha, d_X, 1, d_Y, 1);
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if (s != CUBLAS_STATUS_SUCCESS)
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{
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fprintf(stderr, "cublasSaxpy failed %d\n", s);
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exit(EXIT_FAILURE);
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}
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/* Copy the results from the device */
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acc_memcpy_from_device(&h_Y1[0], d_Y, N * sizeof (float));
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</pre></div>
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<div align="center">Use Case 2
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</div>
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<a name="OpenACC-library-and-environment-variables"></a>
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<h3 class="section">8.4 OpenACC library and environment variables</h3>
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<p>There are two environment variables associated with the OpenACC library
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that may be used to control the device type and device number:
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<code>ACC_DEVICE_TYPE</code> and <code>ACC_DEVICE_NUM</code>, respecively. These two
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environement variables can be used as an alternative to calling
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<code>acc_set_device_num()</code>. As seen in the second use case, the device
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type and device number were specified using <code>acc_set_device_num()</code>.
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If however, the aforementioned environment variables were set, then the
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call to <code>acc_set_device_num()</code> would not be required.
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</p>
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<p>The use of the environment variables is only relevant when an OpenACC function
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is called prior to a call to <code>cudaCreate()</code>. If <code>cudaCreate()</code>
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is called prior to a call to an OpenACC function, then you must call
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<code>acc_set_device_num()</code><a name="DOCF3" href="#FOOT3"><sup>3</sup></a>
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</p>
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<div class="footnote">
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<hr>
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<h4 class="footnotes-heading">Footnotes</h4>
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<h3><a name="FOOT2" href="#DOCF2">(2)</a></h3>
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<p>See section 2.26,
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"Interactions with the CUDA Driver API" in
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"CUDA Runtime API", Version 5.5, and section 2.27, "VDPAU
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Interoperability", in "CUDA Driver API", TRM-06703-001, Version 5.5,
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for additional information on library interoperability.</p>
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<h3><a name="FOOT3" href="#DOCF3">(3)</a></h3>
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<p>More complete information
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about <code>ACC_DEVICE_TYPE</code> and <code>ACC_DEVICE_NUM</code> can be found in
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sections 4.1 and 4.2 of the <a href="http://www.openacc.org/">OpenACC</a>
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Application Programming Interfaceâ, Version 2.0.</p>
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</div>
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<hr>
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<div class="header">
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<p>
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Next: <a href="The-libgomp-ABI.html#The-libgomp-ABI" accesskey="n" rel="next">The libgomp ABI</a>, Previous: <a href="CUDA-Streams-Usage.html#CUDA-Streams-Usage" accesskey="p" rel="prev">CUDA Streams Usage</a>, Up: <a href="index.html#Top" accesskey="u" rel="up">Top</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Library-Index.html#Library-Index" title="Index" rel="index">Index</a>]</p>
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