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<article class="markdown"><h1 id="valgrind">
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Valgrind
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<a class="anchor" href="#valgrind">#</a>
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</h1>
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<p>Valgrind is a suite of tools designed to provide debugging and profiling tools to make your programs more correct and detect some runtime issues (“4. Memcheck: A Memory Error Detector,” #ref-valgrind). The most used of these tools is Memcheck, which can detect many memory-related errors that are common in C and C++ programs and that can lead to crashes and unpredictable behavior (for example, unfreed memory buffers). To run Valgrind on your program:</p>
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<div class="highlight"><pre tabindex="0" style="color:#f8f8f2;background-color:#272822;-moz-tab-size:4;-o-tab-size:4;tab-size:4;"><code class="language-shell" data-lang="shell"><span style="display:flex;"><span>valgrind --leak-check<span style="color:#f92672">=</span>full --show-leak-kinds<span style="color:#f92672">=</span>all myprogram arg1 arg2
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</span></span></code></pre></div><p>Arguments are optional and the default tool that will run is Memcheck. The output will be presented in the form: the number of allocations, frees, and errors. Suppose we have a simple program like this:</p>
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<div class="highlight"><pre tabindex="0" style="color:#f8f8f2;background-color:#272822;-moz-tab-size:4;-o-tab-size:4;tab-size:4;"><code class="language-c" data-lang="c"><span style="display:flex;"><span><span style="color:#75715e">#include</span> <span style="color:#75715e"><stdlib.h></span><span style="color:#75715e">
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</span></span></span><span style="display:flex;"><span><span style="color:#75715e"></span>
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</span></span><span style="display:flex;"><span><span style="color:#66d9ef">void</span> <span style="color:#a6e22e">dummy_function</span>() {
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</span></span><span style="display:flex;"><span> <span style="color:#66d9ef">int</span><span style="color:#f92672">*</span> x <span style="color:#f92672">=</span> <span style="color:#a6e22e">malloc</span>(<span style="color:#ae81ff">10</span> <span style="color:#f92672">*</span> <span style="color:#66d9ef">sizeof</span>(<span style="color:#66d9ef">int</span>));
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</span></span><span style="display:flex;"><span> x[<span style="color:#ae81ff">10</span>] <span style="color:#f92672">=</span> <span style="color:#ae81ff">0</span>; <span style="color:#75715e">// error 1: Out of bounds write, as you can see here we write to an out of bound memory address.
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</span></span></span><span style="display:flex;"><span><span style="color:#75715e"></span>} <span style="color:#75715e">// error 2: Memory Leak, x is allocated at function exit.
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</span></span></span><span style="display:flex;"><span><span style="color:#75715e"></span>
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</span></span><span style="display:flex;"><span><span style="color:#66d9ef">int</span> <span style="color:#a6e22e">main</span>(<span style="color:#66d9ef">void</span>) {
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</span></span><span style="display:flex;"><span> <span style="color:#a6e22e">dummy_function</span>();
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</span></span><span style="display:flex;"><span> <span style="color:#66d9ef">return</span> <span style="color:#ae81ff">0</span>;
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</span></span><span style="display:flex;"><span>}
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</span></span></code></pre></div><p>This program compiles and runs with no errors. Let’s see what Valgrind will output.</p>
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<div class="highlight"><pre tabindex="0" style="color:#f8f8f2;background-color:#272822;-moz-tab-size:4;-o-tab-size:4;tab-size:4;"><code class="language-shell" data-lang="shell"><span style="display:flex;"><span><span style="color:#f92672">==</span>29515<span style="color:#f92672">==</span> Memcheck, a memory error detector
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</span></span><span style="display:flex;"><span><span style="color:#f92672">==</span>29515<span style="color:#f92672">==</span> Copyright <span style="color:#f92672">(</span>C<span style="color:#f92672">)</span> 2002-2015, and GNU GPL<span style="color:#e6db74">'d, by Julian Seward et al.
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</span></span></span><span style="display:flex;"><span><span style="color:#e6db74">==29515== Using Valgrind-3.11.0 and LibVEX; rerun with -h for copyright info
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</span></span></span><span style="display:flex;"><span><span style="color:#e6db74">==29515== Command: ./a
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</span></span></span><span style="display:flex;"><span><span style="color:#e6db74">==29515==
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</span></span></span><span style="display:flex;"><span><span style="color:#e6db74">==29515== Invalid write of size 4
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</span></span></span><span style="display:flex;"><span><span style="color:#e6db74">==29515== at 0x400544: dummy_function (in /home/rafi/projects/exocpp/a)
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</span></span></span><span style="display:flex;"><span><span style="color:#e6db74">==29515== by 0x40055A: main (in /home/rafi/projects/exocpp/a)
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</span></span></span><span style="display:flex;"><span><span style="color:#e6db74">==29515== Address 0x5203068 is 0 bytes after a block of size 40 alloc'</span>d
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</span></span><span style="display:flex;"><span><span style="color:#f92672">==</span>29515<span style="color:#f92672">==</span> at 0x4C2DB8F: malloc <span style="color:#f92672">(</span>in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so<span style="color:#f92672">)</span>
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</span></span><span style="display:flex;"><span><span style="color:#f92672">==</span>29515<span style="color:#f92672">==</span> by 0x400537: dummy_function <span style="color:#f92672">(</span>in /home/rafi/projects/exocpp/a<span style="color:#f92672">)</span>
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</span></span><span style="display:flex;"><span><span style="color:#f92672">==</span>29515<span style="color:#f92672">==</span> by 0x40055A: main <span style="color:#f92672">(</span>in /home/rafi/projects/exocpp/a<span style="color:#f92672">)</span>
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</span></span><span style="display:flex;"><span><span style="color:#f92672">==</span>29515<span style="color:#f92672">==</span>
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</span></span><span style="display:flex;"><span><span style="color:#f92672">==</span>29515<span style="color:#f92672">==</span>
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</span></span><span style="display:flex;"><span><span style="color:#f92672">==</span>29515<span style="color:#f92672">==</span> HEAP SUMMARY:
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</span></span><span style="display:flex;"><span><span style="color:#f92672">==</span>29515<span style="color:#f92672">==</span> in use at exit: <span style="color:#ae81ff">40</span> bytes in <span style="color:#ae81ff">1</span> blocks
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</span></span><span style="display:flex;"><span><span style="color:#f92672">==</span>29515<span style="color:#f92672">==</span> total heap usage: <span style="color:#ae81ff">1</span> allocs, <span style="color:#ae81ff">0</span> frees, <span style="color:#ae81ff">40</span> bytes allocated
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</span></span><span style="display:flex;"><span><span style="color:#f92672">==</span>29515<span style="color:#f92672">==</span>
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</span></span><span style="display:flex;"><span><span style="color:#f92672">==</span>29515<span style="color:#f92672">==</span> LEAK SUMMARY:
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</span></span><span style="display:flex;"><span><span style="color:#f92672">==</span>29515<span style="color:#f92672">==</span> definitely lost: <span style="color:#ae81ff">40</span> bytes in <span style="color:#ae81ff">1</span> blocks
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</span></span><span style="display:flex;"><span><span style="color:#f92672">==</span>29515<span style="color:#f92672">==</span> indirectly lost: <span style="color:#ae81ff">0</span> bytes in <span style="color:#ae81ff">0</span> blocks
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</span></span><span style="display:flex;"><span><span style="color:#f92672">==</span>29515<span style="color:#f92672">==</span> possibly lost: <span style="color:#ae81ff">0</span> bytes in <span style="color:#ae81ff">0</span> blocks
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</span></span><span style="display:flex;"><span><span style="color:#f92672">==</span>29515<span style="color:#f92672">==</span> still reachable: <span style="color:#ae81ff">0</span> bytes in <span style="color:#ae81ff">0</span> blocks
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</span></span><span style="display:flex;"><span><span style="color:#f92672">==</span>29515<span style="color:#f92672">==</span> suppressed: <span style="color:#ae81ff">0</span> bytes in <span style="color:#ae81ff">0</span> blocks
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</span></span><span style="display:flex;"><span><span style="color:#f92672">==</span>29515<span style="color:#f92672">==</span> Rerun with --leak-check<span style="color:#f92672">=</span>full to see details of leaked memory
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</span></span><span style="display:flex;"><span><span style="color:#f92672">==</span>29515<span style="color:#f92672">==</span>
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</span></span><span style="display:flex;"><span><span style="color:#f92672">==</span>29515<span style="color:#f92672">==</span> For counts of detected and suppressed errors, rerun with: -v
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</span></span><span style="display:flex;"><span><span style="color:#f92672">==</span>29515<span style="color:#f92672">==</span> ERROR SUMMARY: <span style="color:#ae81ff">1</span> errors from <span style="color:#ae81ff">1</span> contexts <span style="color:#f92672">(</span>suppressed: <span style="color:#ae81ff">0</span> from 0<span style="color:#f92672">)</span>
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</span></span></code></pre></div><p><strong>Invalid write:</strong> It detected our heap block overrun, writing outside of an allocated block.</p>
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<p><strong>Definitely lost:</strong> Memory leak — you probably forgot to free a memory block.</p>
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<p>Valgrind is a effective tool to check for errors at runtime. C is special when it comes to such behavior, so after compiling your program you can use Valgrind to fix errors that your compiler may miss and that usually happens when your program is running.</p>
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<p>For more information, you can refer to the manual<sup id="fnref:1"><a href="#fn:1" class="footnote-ref" role="doc-noteref">1</a></sup></p>
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<hr>
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<li id="fn:1">
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<p>“4. Memcheck: A Memory Error Detector.” n.d. <em>Valgrind</em>.
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<a href="http://valgrind.org/docs/manual/mc-manual.html">http://valgrind.org/docs/manual/mc-manual.html</a>. <a href="#fnref:1" class="footnote-backref" role="doc-backlink">↩︎</a></p>
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</main>
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</body>
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</html>
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