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<section class="tex2jax_ignore mathjax_ignore" id="exercise-0-hello-world">
<h1>Exercise 0: Hello World<a class="headerlink" href="#exercise-0-hello-world" title="Link to this heading">#</a></h1>
<p>This exercise shows how to build and run a simple example program with SLEPc.</p>
<div class="admonition note">
<p class="admonition-title">Note</p>
<p>The description below related to directories and the use of the <code class="docutils notranslate"><span class="pre">PETSC_ARCH</span></code> variable will be different in the case of a prefix-based installation.</p>
</div>
<section id="compiling">
<h2>Compiling<a class="headerlink" href="#compiling" title="Link to this heading">#</a></h2>
<p>SLEPc needs the following environment variables to be set:</p>
<p><code class="docutils notranslate"><span class="pre">SLEPC_DIR</span></code> - the location of SLEPc</p>
<p><code class="docutils notranslate"><span class="pre">PETSC_DIR</span></code> - the location of PETSc</p>
<p><code class="docutils notranslate"><span class="pre">PETSC_ARCH</span></code> - the architecture being used</p>
<p>Make sure that you have them correctly set.</p>
<p>Like in PETSc, a makefile is necessary to compile a SLEPc program. Paste this simple example into a file named <code class="docutils notranslate"><span class="pre">makefile</span></code> in your working directory:</p>
<div class="highlight-make notranslate"><div class="highlight"><pre><span></span><span class="nf">hello</span><span class="o">:</span><span class="w"> </span><span class="n">hello</span>.<span class="n">o</span>
<span class="w"> </span>-<span class="si">${</span><span class="nv">CLINKER</span><span class="si">}</span><span class="w"> </span>-o<span class="w"> </span>hello<span class="w"> </span>hello.o<span class="w"> </span><span class="si">${</span><span class="nv">SLEPC_SYS_LIB</span><span class="si">}</span>
<span class="w"> </span><span class="si">${</span><span class="nv">RM</span><span class="si">}</span><span class="w"> </span>hello.o
<span class="cp">include ${SLEPC_DIR}/lib/slepc/conf/slepc_common</span>
</pre></div>
</div>
<p>Also place the following source code into a file named <code class="docutils notranslate"><span class="pre">hello.c</span></code> in the same directory:</p>
<div class="highlight-c notranslate"><div class="highlight"><pre><span></span><span class="k">static</span><span class="w"> </span><span class="kt">char</span><span class="w"> </span><span class="n">help</span><span class="p">[]</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="s">"Simple Hello World example program in SLEPc</span><span class="se">\n</span><span class="s">"</span><span class="p">;</span>
<span class="cp">#include</span><span class="w"> </span><span class="cpf"><slepcsys.h></span>
<span class="kt">int</span><span class="w"> </span><span class="nf">main</span><span class="p">(</span><span class="kt">int</span><span class="w"> </span><span class="n">argc</span><span class="p">,</span><span class="w"> </span><span class="kt">char</span><span class="w"> </span><span class="o">**</span><span class="n">argv</span><span class="p">)</span>
<span class="p">{</span>
<span class="w"> </span><span class="n"><a href="https://petsc.org/release/manualpages/Sys/PetscFunctionBeginUser/">PetscFunctionBeginUser</a></span><span class="p">;</span>
<span class="w"> </span><span class="n"><a href="https://petsc.org/release/manualpages/Sys/PetscCall/">PetscCall</a></span><span class="p">(</span><span class="n"><a href="../../manualpages/Sys/SlepcInitialize.html">SlepcInitialize</a></span><span class="p">(</span><span class="o">&</span><span class="n">argc</span><span class="p">,</span><span class="o">&</span><span class="n">argv</span><span class="p">,(</span><span class="kt">char</span><span class="o">*</span><span class="p">)</span><span class="mi">0</span><span class="p">,</span><span class="n">help</span><span class="p">));</span>
<span class="w"> </span><span class="n"><a href="https://petsc.org/release/manualpages/Sys/PetscCall/">PetscCall</a></span><span class="p">(</span><span class="n"><a href="https://petsc.org/release/manualpages/Sys/PetscPrintf/">PetscPrintf</a></span><span class="p">(</span><span class="n"><a href="https://petsc.org/release/manualpages/Sys/PETSC_COMM_WORLD/">PETSC_COMM_WORLD</a></span><span class="p">,</span><span class="s">"Hello world</span><span class="se">\n</span><span class="s">"</span><span class="p">));</span>
<span class="w"> </span><span class="n"><a href="https://petsc.org/release/manualpages/Sys/PetscCall/">PetscCall</a></span><span class="p">(</span><span class="n"><a href="../../manualpages/Sys/SlepcFinalize.html">SlepcFinalize</a></span><span class="p">());</span>
<span class="w"> </span><span class="k">return</span><span class="w"> </span><span class="mi">0</span><span class="p">;</span>
<span class="p">}</span>
</pre></div>
</div>
<p>Compile the program with the following command:</p>
<div class="highlight-console notranslate"><div class="highlight"><pre><span></span><span class="gp">$ </span>make<span class="w"> </span>hello
</pre></div>
</div>
</section>
<section id="source-code-details">
<h2>Source Code Details<a class="headerlink" href="#source-code-details" title="Link to this heading">#</a></h2>
<p>Examine the source code of the sample program. The following comments will help you understand the code thoroughly.</p>
<p><strong>Header File</strong> : All SLEPc programs must include a header file with all the necessary definitions. In this simple example the file <a class="reference external" href="https://slepc.upv.es/release/include/slepcsys.h.html" target="_blank">slepcsys.h</a> (the most basic SLEPc header) is enough because no solver components such as <code class="docutils notranslate"><span class="pre"><a href="../../manualpages/EPS/EPS.html">EPS</a></span></code> are being used.</p>
<div class="admonition note">
<p class="admonition-title">Note</p>
<p>SLEPc header files automatically include some PETSc header files.</p>
</div>
<p><strong>Library Initialization</strong>: All SLEPc programs must begin with a call to <code class="docutils notranslate"><span class="pre"><a href="../../manualpages/Sys/SlepcInitialize.html">SlepcInitialize</a>()</span></code>, which in turn initializes PETSc and MPI. Similarly, at the end of the program <code class="docutils notranslate"><span class="pre"><a href="../../manualpages/Sys/SlepcFinalize.html">SlepcFinalize</a>()</span></code> must be called for library cleanup.</p>
<p><strong>Input/Output</strong>: In this example, we do input/output via a call to a PETSc function, <a class="reference external" href="https://petsc.org/release/manualpages/Sys/PetscPrintf/" title="(in PETSc v3.24)"><span class="xref std std-doc">PetscPrintf</span></a>(). Remember that in parallel programs input/output cannot be done simply via C standard library functions. Note that in SLEPc programs we can freely use any PETSc function.</p>
<p><strong>Error Checking</strong>: All SLEPc routines return an integer indicating whether an error has occurred during the call. The PETSc macro <a class="reference external" href="https://petsc.org/release/manualpages/Sys/PetscCall/" title="(in PETSc v3.24)"><span class="xref std std-doc">PetscCall</span></a>() checks the return value and calls the PETSc error handler upon error detection. All function calls should be wrapped around <a class="reference external" href="https://petsc.org/release/manualpages/Sys/PetscCall/" title="(in PETSc v3.24)"><span class="xref std std-doc">PetscCall</span></a>() to enable a complete error traceback.</p>
</section>
<section id="running-the-program">
<h2>Running the Program<a class="headerlink" href="#running-the-program" title="Link to this heading">#</a></h2>
<p>SLEPc programs are executed as any other MPI program. Note that this typically differs from one system to another. To run the program with only one process, in some systems you can launch it as a normal program:</p>
<div class="highlight-console notranslate"><div class="highlight"><pre><span></span><span class="gp">$ </span>./hello
</pre></div>
</div>
<p>but in other systems this would not work. Check the documentation of your system. Standard MPI implementations provide the <code class="docutils notranslate"><span class="pre">mpiexec</span></code> command to launch the applications</p>
<div class="highlight-console notranslate"><div class="highlight"><pre><span></span><span class="gp">$ </span>mpiexec<span class="w"> </span>-n<span class="w"> </span><span class="m">4</span><span class="w"> </span>./hello
</pre></div>
</div>
<p>In SLEPc (and PETSc) there are a lot of options (run-time parameters) to control program behavior. These options are usually equivalent to function calls, so the user can test its effect without changing the source code (this will be illustrated in the next exercises). To show which options are available in a program use:</p>
<div class="highlight-console notranslate"><div class="highlight"><pre><span></span><span class="gp">$ </span>./hello<span class="w"> </span>-help
</pre></div>
</div>
</section>
<section id="support-for-debugging-and-complex-numbers">
<h2>Support for Debugging and Complex Numbers<a class="headerlink" href="#support-for-debugging-and-complex-numbers" title="Link to this heading">#</a></h2>
<p>The support for debugging capabilities, complex scalar arithmetic, and other features is managed by SLEPc and PETSc by means of different <em>architectures</em> , represented by different values of the <code class="docutils notranslate"><span class="pre">PETSC_ARCH</span></code> variable. In a given system, you can typically find several versions of SLEPc and PETSc, each of them built with different configuration options. For instance, suppose the following values are available:</p>
<ul class="simple">
<li><p><code class="docutils notranslate"><span class="pre">arch-linux-gnu-c-opt</span></code>: built with compiler optimization</p></li>
<li><p><code class="docutils notranslate"><span class="pre">arch-linux-gnu-c-debug</span></code>: built with debugging support</p></li>
<li><p><code class="docutils notranslate"><span class="pre">arch-linux-gnu-c-opt-complex</span></code>: optimized with complex scalars</p></li>
<li><p><code class="docutils notranslate"><span class="pre">arch-linux-gnu-c-debug-complex</span></code>: debug with complex scalars</p></li>
</ul>
<div class="admonition note">
<p class="admonition-title">Note</p>
<p>In order to learn about the particular architectures available in your system, type <code class="docutils notranslate"><span class="pre">ls</span> <span class="pre">$SLEPC_DIR</span></code>. There should be a subdirectory for each allowed value of <code class="docutils notranslate"><span class="pre">PETSC_ARCH</span></code>.</p>
</div>
<p>When using an architecture with support for complex scalars, all scalar values are complex instead of real. Try compiling the example program for complex numbers:</p>
<div class="highlight-console notranslate"><div class="highlight"><pre><span></span><span class="gp">$ </span>make<span class="w"> </span><span class="nv">PETSC_ARCH</span><span class="o">=</span>arch-linux-gnu-c-opt-complex<span class="w"> </span>hello
</pre></div>
</div>
<p>When using the debug versions some options are available to support debugging. For example</p>
<div class="highlight-console notranslate"><div class="highlight"><pre><span></span><span class="gp">$ </span>./hello<span class="w"> </span>-start_in_debugger
</pre></div>
</div>
<p>opens the program in a debugger stopped at the <code class="docutils notranslate"><span class="pre"><a href="../../manualpages/Sys/SlepcInitialize.html">SlepcInitialize</a>()</span></code> function.</p>
<p>Other useful options are: <code class="docutils notranslate"><span class="pre">-info</span></code> to get informative messages about progress of the calculations, <code class="docutils notranslate"><span class="pre">-malloc_info</span></code> to print memory usage at end of run, <code class="docutils notranslate"><span class="pre">-log_trace</span> <span class="pre">[filename]</span></code> to get a full trace of the execution (in a file), <code class="docutils notranslate"><span class="pre">-malloc_dump</span></code> to list memory blocks not freed at the end of the program, and <code class="docutils notranslate"><span class="pre">-log_view</span></code> to get a summary including performance results.</p>
</section>
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