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| <div class="section" id="step-5-adding-system-introspection"> |
| <span id="guide:tutorial/Adding System Introspection"></span><h1>Step 5: Adding System Introspection<a class="headerlink" href="#step-5-adding-system-introspection" title="Permalink to this headline">¶</a></h1> |
| <p>Let us consider adding some code to our project that depends on features the |
| target platform may not have. For this example, we will add some code that |
| depends on whether or not the target platform has the <code class="docutils literal notranslate"><span class="pre">log</span></code> and <code class="docutils literal notranslate"><span class="pre">exp</span></code> |
| functions. Of course almost every platform has these functions but for this |
| tutorial assume that they are not common.</p> |
| <p>If the platform has <code class="docutils literal notranslate"><span class="pre">log</span></code> and <code class="docutils literal notranslate"><span class="pre">exp</span></code> then we will use them to compute the |
| square root in the <code class="docutils literal notranslate"><span class="pre">mysqrt</span></code> function. We first test for the availability of |
| these functions using the <span class="target" id="index-0-module:CheckSymbolExists"></span><a class="reference internal" href="../../module/CheckSymbolExists.html#module:CheckSymbolExists" title="CheckSymbolExists"><code class="xref cmake cmake-module docutils literal notranslate"><span class="pre">CheckSymbolExists</span></code></a> module in |
| <code class="docutils literal notranslate"><span class="pre">MathFunctions/CMakeLists.txt</span></code>. On some platforms, we will need to link to |
| the <code class="docutils literal notranslate"><span class="pre">m</span></code> library. If <code class="docutils literal notranslate"><span class="pre">log</span></code> and <code class="docutils literal notranslate"><span class="pre">exp</span></code> are not initially found, require the |
| <code class="docutils literal notranslate"><span class="pre">m</span></code> library and try again.</p> |
| <p>Add the checks for <code class="docutils literal notranslate"><span class="pre">log</span></code> and <code class="docutils literal notranslate"><span class="pre">exp</span></code> to <code class="docutils literal notranslate"><span class="pre">MathFunctions/CMakeLists.txt</span></code>, |
| after the call to <span class="target" id="index-0-command:target_include_directories"></span><a class="reference internal" href="../../command/target_include_directories.html#command:target_include_directories" title="target_include_directories"><code class="xref cmake cmake-command docutils literal notranslate"><span class="pre">target_include_directories()</span></code></a>:</p> |
| <div class="literal-block-wrapper docutils container" id="mathfunctions-cmakelists-txt-check-symbol-exists"> |
| <div class="code-block-caption"><span class="caption-text">MathFunctions/CMakeLists.txt</span><a class="headerlink" href="#mathfunctions-cmakelists-txt-check-symbol-exists" title="Permalink to this code">¶</a></div> |
| <div class="highlight-cmake notranslate"><div class="highlight"><pre><span></span><span class="nf">target_include_directories(</span><span class="nb">MathFunctions</span><span class="w"></span> |
| <span class="w"> </span><span class="no">INTERFACE</span><span class="w"> </span><span class="o">${</span><span class="nt">CMAKE_CURRENT_SOURCE_DIR</span><span class="o">}</span><span class="w"></span> |
| <span class="w"> </span><span class="nf">)</span><span class="w"></span> |
| |
| <span class="c"># does this system provide the log and exp functions?</span> |
| <span class="nf">include(</span><span class="nb">CheckSymbolExists</span><span class="nf">)</span><span class="w"></span> |
| <span class="nf">check_symbol_exists(</span><span class="nb">log</span><span class="w"> </span><span class="s">"math.h"</span><span class="w"> </span><span class="no">HAVE_LOG</span><span class="nf">)</span><span class="w"></span> |
| <span class="nf">check_symbol_exists(</span><span class="nb">exp</span><span class="w"> </span><span class="s">"math.h"</span><span class="w"> </span><span class="no">HAVE_EXP</span><span class="nf">)</span><span class="w"></span> |
| <span class="nf">if(NOT</span> <span class="nf">(</span><span class="no">HAVE_LOG</span><span class="w"> </span><span class="no">AND</span><span class="w"> </span><span class="no">HAVE_EXP</span><span class="nf">))</span><span class="w"></span> |
| <span class="w"> </span><span class="nf">unset(</span><span class="no">HAVE_LOG</span><span class="w"> </span><span class="no">CACHE</span><span class="nf">)</span><span class="w"></span> |
| <span class="w"> </span><span class="nf">unset(</span><span class="no">HAVE_EXP</span><span class="w"> </span><span class="no">CACHE</span><span class="nf">)</span><span class="w"></span> |
| <span class="w"> </span><span class="nf">set(</span><span class="no">CMAKE_REQUIRED_LIBRARIES</span><span class="w"> </span><span class="s">"m"</span><span class="nf">)</span><span class="w"></span> |
| <span class="w"> </span><span class="nf">check_symbol_exists(</span><span class="nb">log</span><span class="w"> </span><span class="s">"math.h"</span><span class="w"> </span><span class="no">HAVE_LOG</span><span class="nf">)</span><span class="w"></span> |
| <span class="w"> </span><span class="nf">check_symbol_exists(</span><span class="nb">exp</span><span class="w"> </span><span class="s">"math.h"</span><span class="w"> </span><span class="no">HAVE_EXP</span><span class="nf">)</span><span class="w"></span> |
| <span class="w"> </span><span class="nf">if(</span><span class="no">HAVE_LOG</span><span class="w"> </span><span class="no">AND</span><span class="w"> </span><span class="no">HAVE_EXP</span><span class="nf">)</span><span class="w"></span> |
| <span class="w"> </span><span class="nf">target_link_libraries(</span><span class="nb">MathFunctions</span><span class="w"> </span><span class="no">PRIVATE</span><span class="w"> </span><span class="nb">m</span><span class="nf">)</span><span class="w"></span> |
| <span class="w"> </span><span class="nf">endif()</span><span class="w"></span> |
| <span class="nf">endif()</span><span class="w"></span> |
| </pre></div> |
| </div> |
| </div> |
| <p>If available, use <span class="target" id="index-0-command:target_compile_definitions"></span><a class="reference internal" href="../../command/target_compile_definitions.html#command:target_compile_definitions" title="target_compile_definitions"><code class="xref cmake cmake-command docutils literal notranslate"><span class="pre">target_compile_definitions()</span></code></a> to specify |
| <code class="docutils literal notranslate"><span class="pre">HAVE_LOG</span></code> and <code class="docutils literal notranslate"><span class="pre">HAVE_EXP</span></code> as <code class="docutils literal notranslate"><span class="pre">PRIVATE</span></code> compile definitions.</p> |
| <div class="literal-block-wrapper docutils container" id="mathfunctions-cmakelists-txt-target-compile-definitions"> |
| <div class="code-block-caption"><span class="caption-text">MathFunctions/CMakeLists.txt</span><a class="headerlink" href="#mathfunctions-cmakelists-txt-target-compile-definitions" title="Permalink to this code">¶</a></div> |
| <div class="highlight-cmake notranslate"><div class="highlight"><pre><span></span><span class="nf">if(</span><span class="no">HAVE_LOG</span><span class="w"> </span><span class="no">AND</span><span class="w"> </span><span class="no">HAVE_EXP</span><span class="nf">)</span><span class="w"></span> |
| <span class="w"> </span><span class="nf">target_compile_definitions(</span><span class="nb">MathFunctions</span><span class="w"></span> |
| <span class="w"> </span><span class="no">PRIVATE</span><span class="w"> </span><span class="s">"HAVE_LOG"</span><span class="w"> </span><span class="s">"HAVE_EXP"</span><span class="nf">)</span><span class="w"></span> |
| <span class="nf">endif()</span><span class="w"></span> |
| </pre></div> |
| </div> |
| </div> |
| <p>If <code class="docutils literal notranslate"><span class="pre">log</span></code> and <code class="docutils literal notranslate"><span class="pre">exp</span></code> are available on the system, then we will use them to |
| compute the square root in the <code class="docutils literal notranslate"><span class="pre">mysqrt</span></code> function. Add the following code to |
| the <code class="docutils literal notranslate"><span class="pre">mysqrt</span></code> function in <code class="docutils literal notranslate"><span class="pre">MathFunctions/mysqrt.cxx</span></code> (don't forget the |
| <code class="docutils literal notranslate"><span class="pre">#endif</span></code> before returning the result!):</p> |
| <div class="literal-block-wrapper docutils container" id="mathfunctions-mysqrt-cxx-ifdef"> |
| <div class="code-block-caption"><span class="caption-text">MathFunctions/mysqrt.cxx</span><a class="headerlink" href="#mathfunctions-mysqrt-cxx-ifdef" title="Permalink to this code">¶</a></div> |
| <div class="highlight-c++ notranslate"><div class="highlight"><pre><span></span><span class="cp">#if defined(HAVE_LOG) && defined(HAVE_EXP)</span> |
| <span class="kt">double</span> <span class="n">result</span> <span class="o">=</span> <span class="n">exp</span><span class="p">(</span><span class="n">log</span><span class="p">(</span><span class="n">x</span><span class="p">)</span> <span class="o">*</span> <span class="mf">0.5</span><span class="p">);</span> |
| <span class="n">std</span><span class="o">::</span><span class="n">cout</span> <span class="o"><<</span> <span class="s">"Computing sqrt of "</span> <span class="o"><<</span> <span class="n">x</span> <span class="o"><<</span> <span class="s">" to be "</span> <span class="o"><<</span> <span class="n">result</span> |
| <span class="o"><<</span> <span class="s">" using log and exp"</span> <span class="o"><<</span> <span class="n">std</span><span class="o">::</span><span class="n">endl</span><span class="p">;</span> |
| <span class="cp">#else</span> |
| <span class="kt">double</span> <span class="n">result</span> <span class="o">=</span> <span class="n">x</span><span class="p">;</span> |
| </pre></div> |
| </div> |
| </div> |
| <p>We will also need to modify <code class="docutils literal notranslate"><span class="pre">mysqrt.cxx</span></code> to include <code class="docutils literal notranslate"><span class="pre">cmath</span></code>.</p> |
| <div class="literal-block-wrapper docutils container" id="mathfunctions-mysqrt-cxx-include-cmath"> |
| <div class="code-block-caption"><span class="caption-text">MathFunctions/mysqrt.cxx</span><a class="headerlink" href="#mathfunctions-mysqrt-cxx-include-cmath" title="Permalink to this code">¶</a></div> |
| <div class="highlight-c++ notranslate"><div class="highlight"><pre><span></span><span class="cp">#include</span> <span class="cpf"><cmath></span><span class="cp"></span> |
| </pre></div> |
| </div> |
| </div> |
| <p>Run the <span class="target" id="index-0-manual:cmake(1)"></span><a class="reference internal" href="../../manual/cmake.1.html#manual:cmake(1)" title="cmake(1)"><code class="xref cmake cmake-manual docutils literal notranslate"><span class="pre">cmake</span></code></a> executable or the |
| <span class="target" id="index-0-manual:cmake-gui(1)"></span><a class="reference internal" href="../../manual/cmake-gui.1.html#manual:cmake-gui(1)" title="cmake-gui(1)"><code class="xref cmake cmake-manual docutils literal notranslate"><span class="pre">cmake-gui</span></code></a> to configure the project and then build it |
| with your chosen build tool and run the Tutorial executable.</p> |
| <p>Which function gives better results now, <code class="docutils literal notranslate"><span class="pre">sqrt</span></code> or <code class="docutils literal notranslate"><span class="pre">mysqrt</span></code>?</p> |
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