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PODo I need to synchronize std::condition_variable/condition_variable_any::notify_one
 

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  1. PODo I need to synchronize std::condition_variable/condition_variable_any::notify_one
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    <p>Do I need to synchronize <code>std::condition_variable/condition_variable_any::notify_one</code>?</p> <p>As far as I can see, if lost of notifications is acceptable - it is OK to call <code>notify_one</code> not protected (by mutex for example).</p> <p>For instance, I saw following usage patterns (sorry, don't remember where):</p> <pre><code>{ { lock_guard&lt;mutex&gt; l(m); // do work } c.notify_one(); } </code></pre> <hr> <p>But, I inspected libstdc++ sources, and I see:</p> <p><a href="https://github.com/ooPo/gcc-4.7.0-PS3/blob/5d9addb08be92a040b8a8675c9e5a231ce37d705/libstdc%2B%2B-v3/src/c%2B%2B11/condition_variable.cc">condition_variable::notify_one</a></p> <pre><code>void condition_variable::notify_one() noexcept { int __e = __gthread_cond_signal(&amp;_M_cond); // XXX not in spec // EINVAL if (__e) __throw_system_error(__e); } </code></pre> <p>and <a href="https://github.com/ooPo/gcc-4.7.0-PS3/blob/5d9addb08be92a040b8a8675c9e5a231ce37d705/libstdc%2B%2B-v3/include/std/condition_variable">condition_variable_any::notify_one</a>:</p> <pre><code>void condition_variable_any::notify_one() noexcept { lock_guard&lt;mutex&gt; __lock(_M_mutex); _M_cond.notify_one(); } </code></pre> <p>And here is layout of condition_variable_any:</p> <pre><code>class condition_variable_any { condition_variable _M_cond; mutex _M_mutex; // data end </code></pre> <p>I.e. it is just thin wrapper around condition_variable+mutex.</p> <p>So, questions:</p> <ol> <li>Is it thread-safe to not protect <code>notify_one</code> by mutex for either <code>condition_variable_any</code> or <code>condition_variable</code>?</li> <li>Why implementation of condition_variable_any uses additional mutex?</li> <li>Why implementation of <code>condition_variable_any::notify_one</code> and <code>condition_variable::notify_one</code> differs? Maybe <code>condition_variable::notify_one</code> requires manual protection but <code>condition_variable_any::notify_one</code> doesn't? Is it libstdc++ bug?</li> </ol>
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