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undefined behaviors in C++</a></li><li><a href="/cpp/unit-testing-in-c.html" class="sidebar-link">Unit Testing in C++</a></li><li><a href="/cpp/c-debugging-and-debug-prevention-tools-techniques.html" class="sidebar-link">C++ Debugging and Debug-prevention Tools & Techniques</a></li><li><a href="/cpp/optimization-in-c.html" class="sidebar-link">Optimization in C++</a></li><li><a href="/cpp/optimization.html" class="sidebar-link">Optimization</a></li><li><a href="/cpp/profiling.html" class="sidebar-link">Profiling</a></li><li><a href="/cpp/refactoring-techniques.html" class="sidebar-link">Refactoring Techniques</a></li><li><a href="/cpp/internationalization-in-c.html" class="sidebar-link">Internationalization in C++</a></li><li><a href="/cpp/contributors.html" class="sidebar-link">The Contributors</a></li></ul></section></li></ul> </aside> <main class="page"> <div class="theme-default-content content__default"><h1 id="atomic-types"><a href="#atomic-types" class="header-anchor">#</a> Atomic Types</h1> <h2 id="multi-threaded-access"><a href="#multi-threaded-access" class="header-anchor">#</a> Multi-threaded Access</h2> <p>An atomic type can be used to safely read and write to a memory location shared between two threads.</p> <p>A Bad example that is likely to cause a data race:</p> <div class="language-cpp extra-class"><pre class="language-cpp"><code><span class="token macro property"><span class="token directive-hash">#</span><span class="token directive keyword">include</span> <span class="token string"><thread></span></span>
<span class="token macro property"><span class="token directive-hash">#</span><span class="token directive keyword">include</span> <span class="token string"><iostream></span></span>
<span class="token comment">//function will add all values including and between 'a' and 'b' to 'result'</span>
<span class="token keyword">void</span> <span class="token function">add</span><span class="token punctuation">(</span><span class="token keyword">int</span> a<span class="token punctuation">,</span> <span class="token keyword">int</span> b<span class="token punctuation">,</span> <span class="token keyword">int</span> <span class="token operator">*</span> result<span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token keyword">for</span> <span class="token punctuation">(</span><span class="token keyword">int</span> i <span class="token operator">=</span> a<span class="token punctuation">;</span> i <span class="token operator"><=</span> b<span class="token punctuation">;</span> i<span class="token operator">++</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token operator">*</span>result <span class="token operator">+=</span> i<span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token punctuation">}</span>
<span class="token keyword">int</span> <span class="token function">main</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token comment">//a primitive data type has no thread safety</span>
<span class="token keyword">int</span> shared <span class="token operator">=</span> <span class="token number">0</span><span class="token punctuation">;</span>
<span class="token comment">//create a thread that may run parallel to the 'main' thread</span>
<span class="token comment">//the thread will run the function 'add' defined above with paramters a = 1, b = 100, result = &shared</span>
<span class="token comment">//analogous to 'add(1,100, &shared);'</span>
std<span class="token double-colon punctuation">::</span>thread <span class="token function">addingThread</span><span class="token punctuation">(</span>add<span class="token punctuation">,</span> <span class="token number">1</span><span class="token punctuation">,</span> <span class="token number">100</span><span class="token punctuation">,</span> <span class="token operator">&</span>shared<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token comment">//attempt to print the value of 'shared' to console</span>
<span class="token comment">//main will keep repeating this until the addingThread becomes joinable</span>
<span class="token keyword">while</span> <span class="token punctuation">(</span><span class="token operator">!</span>addingThread<span class="token punctuation">.</span><span class="token function">joinable</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token comment">//this may cause undefined behavior or print a corrupted value</span>
<span class="token comment">//if the addingThread tries to write to 'shared' while the main thread is reading it</span>
std<span class="token double-colon punctuation">::</span>cout <span class="token operator"><<</span> shared <span class="token operator"><<</span> std<span class="token double-colon punctuation">::</span>endl<span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token comment">//rejoin the thread at the end of execution for cleaning purposes</span>
addingThread<span class="token punctuation">.</span><span class="token function">join</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token keyword">return</span> <span class="token number">0</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
</code></pre></div><p>The above example may cause a corrupted read and can lead to undefined behavior.</p> <p>An example with thread safety:</p> <div class="language-cpp extra-class"><pre class="language-cpp"><code><span class="token macro property"><span class="token directive-hash">#</span><span class="token directive keyword">include</span> <span class="token string"><atomic></span></span>
<span class="token macro property"><span class="token directive-hash">#</span><span class="token directive keyword">include</span> <span class="token string"><thread></span></span>
<span class="token macro property"><span class="token directive-hash">#</span><span class="token directive keyword">include</span> <span class="token string"><iostream></span></span>
<span class="token comment">//function will add all values including and between 'a' and 'b' to 'result'</span>
<span class="token keyword">void</span> <span class="token function">add</span><span class="token punctuation">(</span><span class="token keyword">int</span> a<span class="token punctuation">,</span> <span class="token keyword">int</span> b<span class="token punctuation">,</span> std<span class="token double-colon punctuation">::</span>atomic<span class="token operator"><</span><span class="token keyword">int</span><span class="token operator">></span> <span class="token operator">*</span> result<span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token keyword">for</span> <span class="token punctuation">(</span><span class="token keyword">int</span> i <span class="token operator">=</span> a<span class="token punctuation">;</span> i <span class="token operator"><=</span> b<span class="token punctuation">;</span> i<span class="token operator">++</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token comment">//atomically add 'i' to result</span>
result<span class="token operator">-></span><span class="token function">fetch_add</span><span class="token punctuation">(</span>i<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token punctuation">}</span>
<span class="token keyword">int</span> <span class="token function">main</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token comment">//atomic template used to store non-atomic objects</span>
std<span class="token double-colon punctuation">::</span>atomic<span class="token operator"><</span><span class="token keyword">int</span><span class="token operator">></span> shared <span class="token operator">=</span> <span class="token number">0</span><span class="token punctuation">;</span>
<span class="token comment">//create a thread that may run parallel to the 'main' thread</span>
<span class="token comment">//the thread will run the function 'add' defined above with paramters a = 1, b = 100, result = &shared</span>
<span class="token comment">//analogous to 'add(1,100, &shared);'</span>
std<span class="token double-colon punctuation">::</span>thread <span class="token function">addingThread</span><span class="token punctuation">(</span>add<span class="token punctuation">,</span> <span class="token number">1</span><span class="token punctuation">,</span> <span class="token number">10000</span><span class="token punctuation">,</span> <span class="token operator">&</span>shared<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token comment">//print the value of 'shared' to console</span>
<span class="token comment">//main will keep repeating this until the addingThread becomes joinable</span>
<span class="token keyword">while</span> <span class="token punctuation">(</span><span class="token operator">!</span>addingThread<span class="token punctuation">.</span><span class="token function">joinable</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token comment">//safe way to read the value of shared atomically for thread safe read</span>
std<span class="token double-colon punctuation">::</span>cout <span class="token operator"><<</span> shared<span class="token punctuation">.</span><span class="token function">load</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token operator"><<</span> std<span class="token double-colon punctuation">::</span>endl<span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token comment">//rejoin the thread at the end of execution for cleaning purposes</span>
addingThread<span class="token punctuation">.</span><span class="token function">join</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token keyword">return</span> <span class="token number">0</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
</code></pre></div><p>The above example is safe because all <code>store()</code> and <code>load()</code> operations of the <code>atomic</code> data type protect the encapsulated <code>int</code> from simultaneous access.</p> <h4 id="syntax"><a href="#syntax" class="header-anchor">#</a> Syntax</h4> <ul><li>std::atomic<T></T></li> <li>std::atomic_flag</li></ul> <h4 id="remarks"><a href="#remarks" class="header-anchor">#</a> Remarks</h4> <p><code>std::atomic</code> allows atomic access to a TriviallyCopyable type, it is implementation-dependent if this is done via atomic operations or by using locks. The only guaranteed lock-free atomic type is <code>std::atomic_flag</code>.</p></div> <footer class="page-edit"><div class="edit-link"><a href="https://github.com/devtut/generate/edit/master/docs/cpp/atomic-types.md" target="_blank" rel="noopener noreferrer">Edit this page on GitHub</a> <span><svg xmlns="http://www.w3.org/2000/svg" aria-hidden="true" focusable="false" x="0px" y="0px" viewBox="0 0 100 100" width="15" height="15" class="icon outbound"><path fill="currentColor" d="M18.8,85.1h56l0,0c2.2,0,4-1.8,4-4v-32h-8v28h-48v-48h28v-8h-32l0,0c-2.2,0-4,1.8-4,4v56C14.8,83.3,16.6,85.1,18.8,85.1z"></path> <polygon fill="currentColor" points="45.7,48.7 51.3,54.3 77.2,28.5 77.2,37.2 85.2,37.2 85.2,14.9 62.8,14.9 62.8,22.9 71.5,22.9"></polygon></svg> <span class="sr-only">(opens new window)</span></span></div> <!----></footer> <div class="page-nav"><p class="inner"><span class="prev">
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