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<div class="section" id="event-queues">
<h1>Event Queues<a class="headerlink" href="#event-queues" title="Permalink to this headline"></a></h1>
<p>An event queue allows a task to serialize incoming events and simplify
event processing. Events are stored in a queue and a task removes and
processes an event from the queue. An event is processed in the context
of this task. Events may be generated by OS callouts, interrupt handlers,
and other tasks.</p>
<div class="section" id="description">
<h2>Description<a class="headerlink" href="#description" title="Permalink to this headline"></a></h2>
<p>Mynewt’s event queue model uses callback functions to process events.
Each event is associated with a callback function that is called to
process the event. This model enables a library package, that uses
events in its implementation but does not have real-time timing
requirements, to use an application event queue instead of creating a
dedicated event queue and task to process its events. The callback
function executes in the context of the task that the application
creates to manage the event queue. This model reduces an application’s
memory requirement because memory must be allocated for the task’s stack
when a task is created. A package that has real-time timing requirements
and must run at a specific task priority should create a dedicated event
queue and task to process its events.</p>
<p>In the Mynewt model, a package defines its events and implements the
callback functions for the events. A package that does not have
real-time timing requirements should use Mynewt’s default event queue
for its events. The callback function for an event from the Mynewt
default event queue is executed in the context of the application main
task. A package can, optionally, export a function that allows an
application to specify the event queue for the package to use. The
application task handler that manages the event queue simply pulls events
from the event queue and executes the event’s callback function in its context.</p>
<p>A common way that Mynewt applications or packages process events from an
event queue is to have a task that executes in an infinite loop and
calls the <a class="reference internal" href="#c.os_eventq_get" title="os_eventq_get"><code class="xref c c-func docutils literal notranslate"><span class="pre">os_eventq_get()</span></code></a> function to dequeue and return the event
from the head of the event queue. The task then calls the event callback
function to process the event. The <a class="reference internal" href="#c.os_eventq_get" title="os_eventq_get"><code class="xref c c-func docutils literal notranslate"><span class="pre">os_eventq_get()</span></code></a> function puts the
task in to the <code class="docutils literal notranslate"><span class="pre">sleeping</span></code> state when there are no events on the queue.
(See <a class="reference internal" href="../context_switch/context_switch.html"><span class="doc">Scheduler</span></a> for more
information on task execution states.) Other tasks (or interrupts) call
the <a class="reference internal" href="#c.os_eventq_put" title="os_eventq_put"><code class="xref c c-func docutils literal notranslate"><span class="pre">os_eventq_put()</span></code></a> function to add an event to the queue. The
<a class="reference internal" href="#c.os_eventq_put" title="os_eventq_put"><code class="xref c c-func docutils literal notranslate"><span class="pre">os_eventq_put()</span></code></a> function determines whether a task is blocked
waiting for an event on the queue and puts the task into the
<code class="docutils literal notranslate"><span class="pre">ready-to-run</span></code> state.</p>
<p>A task can use the <a class="reference internal" href="#c.os_eventq_run" title="os_eventq_run"><code class="xref c c-func docutils literal notranslate"><span class="pre">os_eventq_run()</span></code></a> wrapper function that calls the
<a class="reference internal" href="#c.os_eventq_get" title="os_eventq_get"><code class="xref c c-func docutils literal notranslate"><span class="pre">os_eventq_get()</span></code></a> function to dequeue an event from the queue and then
calls the event callback function to process the event.
Note:</p>
<ul class="simple">
<li><p>Only one task should consume or block waiting for events from an
event queue.</p></li>
<li><p>The OS callout subsystem uses events for timer expiration notification.</p></li>
</ul>
</div>
<div class="section" id="example">
<h2>Example<a class="headerlink" href="#example" title="Permalink to this headline"></a></h2>
<p>Here is an example of using an event from the BLE host:</p>
<div class="highlight-c notranslate"><div class="highlight"><pre><span></span><span class="k">static</span><span class="w"> </span><span class="kt">void</span><span class="w"> </span><span class="nf">ble_hs_event_tx_notify</span><span class="p">(</span><span class="k">struct</span><span class="w"> </span><span class="nc">os_event</span><span class="w"> </span><span class="o">*</span><span class="n">ev</span><span class="p">);</span>
<span class="cm">/** OS event - triggers tx of pending notifications and indications. */</span>
<span class="k">static</span><span class="w"> </span><span class="k">struct</span><span class="w"> </span><span class="nc">os_event</span><span class="w"> </span><span class="n">ble_hs_ev_tx_notifications</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="p">.</span><span class="n">ev_cb</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">ble_hs_event_tx_notify</span><span class="p">,</span>
<span class="p">};</span>
</pre></div>
</div>
</div>
<div class="section" id="api">
<h2>API<a class="headerlink" href="#api" title="Permalink to this headline"></a></h2>
<dl class="c type">
<dt id="c.os_event_fn">
<span class="target" id="group___o_s_event_1ga6e572d40b307b9f40bec72281548b63e"></span><em class="property"><span class="pre">typedef</span> </em><span class="pre">void</span> <code class="sig-name descname"><span class="pre">os_event_fn</span></code><span class="sig-paren">(</span><em class="property"><span class="pre">struct</span></em> <a class="reference internal" href="#c.os_event" title="os_event"><span class="pre">os_event</span></a> <span class="pre">*</span><em><span class="pre">ev</span></em><span class="sig-paren">)</span><a class="headerlink" href="#c.os_event_fn" title="Permalink to this definition"></a><br /></dt>
<dd></dd></dl>
<dl class="c function">
<dt id="c.os_eventq_init">
<span class="target" id="group___o_s_event_1ga12330bd028b5efdb00b96f8d85ad576b"></span><span class="pre">void</span> <code class="sig-name descname"><span class="pre">os_eventq_init</span></code><span class="sig-paren">(</span><em class="property"><span class="pre">struct</span></em> <a class="reference internal" href="#c.os_eventq" title="os_eventq"><span class="pre">os_eventq</span></a><span class="pre">*</span><span class="sig-paren">)</span><a class="headerlink" href="#c.os_eventq_init" title="Permalink to this definition"></a><br /></dt>
<dd><p>Initialize the event queue. </p>
<p><dl class="simple">
<dt><strong>Parameters</strong></dt><dd><ul class="breatheparameterlist simple">
<li><p><code class="docutils literal notranslate"><span class="pre">evq</span></code>: The event queue to initialize </p></li>
</ul>
</dd>
</dl>
</p>
</dd></dl>
<dl class="c function">
<dt id="c.os_eventq_inited">
<span class="target" id="group___o_s_event_1ga21fb29b6ce469a93d87eacc2c64f7bca"></span><span class="pre">int</span> <code class="sig-name descname"><span class="pre">os_eventq_inited</span></code><span class="sig-paren">(</span><em class="property"><span class="pre">const</span></em> <em class="property"><span class="pre">struct</span></em> <a class="reference internal" href="#c.os_eventq" title="os_eventq"><span class="pre">os_eventq</span></a> <span class="pre">*</span><em><span class="pre">evq</span></em><span class="sig-paren">)</span><a class="headerlink" href="#c.os_eventq_inited" title="Permalink to this definition"></a><br /></dt>
<dd><p>Check whether the event queue is initialized. </p>
<p><dl class="simple">
<dt><strong>Parameters</strong></dt><dd><ul class="breatheparameterlist simple">
<li><p><code class="docutils literal notranslate"><span class="pre">evq</span></code>: The event queue to check </p></li>
</ul>
</dd>
</dl>
</p>
</dd></dl>
<dl class="c function">
<dt id="c.os_eventq_put">
<span class="target" id="group___o_s_event_1ga00853fd6e0f5bbf39e3930ee70665755"></span><span class="pre">void</span> <code class="sig-name descname"><span class="pre">os_eventq_put</span></code><span class="sig-paren">(</span><em class="property"><span class="pre">struct</span></em> <a class="reference internal" href="#c.os_eventq" title="os_eventq"><span class="pre">os_eventq</span></a><span class="pre">*</span>, <em class="property"><span class="pre">struct</span></em> <a class="reference internal" href="#c.os_event" title="os_event"><span class="pre">os_event</span></a><span class="pre">*</span><span class="sig-paren">)</span><a class="headerlink" href="#c.os_eventq_put" title="Permalink to this definition"></a><br /></dt>
<dd><p>Put an event on the event queue. </p>
<p><dl class="simple">
<dt><strong>Parameters</strong></dt><dd><ul class="breatheparameterlist simple">
<li><p><code class="docutils literal notranslate"><span class="pre">evq</span></code>: The event queue to put an event on </p></li>
<li><p><code class="docutils literal notranslate"><span class="pre">ev</span></code>: The event to put on the queue </p></li>
</ul>
</dd>
</dl>
</p>
</dd></dl>
<dl class="c function">
<dt id="c.os_eventq_get_no_wait">
<span class="target" id="group___o_s_event_1ga5142a879bb51db18fa7ff725a81e13d0"></span><em class="property"><span class="pre">struct</span></em> <a class="reference internal" href="#c.os_event" title="os_event"><span class="pre">os_event</span></a> <span class="pre">*</span><code class="sig-name descname"><span class="pre">os_eventq_get_no_wait</span></code><span class="sig-paren">(</span><em class="property"><span class="pre">struct</span></em> <a class="reference internal" href="#c.os_eventq" title="os_eventq"><span class="pre">os_eventq</span></a> <span class="pre">*</span><em><span class="pre">evq</span></em><span class="sig-paren">)</span><a class="headerlink" href="#c.os_eventq_get_no_wait" title="Permalink to this definition"></a><br /></dt>
<dd><p>Poll an event from the event queue and return it immediately. </p>
<p>If no event is available, don’t block, just return NULL.</p>
<p><dl class="simple">
<dt><strong>Return</strong></dt><dd><p>Event from the queue, or NULL if none available. </p>
</dd>
</dl>
</p>
</dd></dl>
<dl class="c function">
<dt id="c.os_eventq_get">
<span class="target" id="group___o_s_event_1ga5d4a42f82a5fd7c2911ad4cb61a78045"></span><em class="property"><span class="pre">struct</span></em> <a class="reference internal" href="#c.os_event" title="os_event"><span class="pre">os_event</span></a> <span class="pre">*</span><code class="sig-name descname"><span class="pre">os_eventq_get</span></code><span class="sig-paren">(</span><em class="property"><span class="pre">struct</span></em> <a class="reference internal" href="#c.os_eventq" title="os_eventq"><span class="pre">os_eventq</span></a><span class="pre">*</span><span class="sig-paren">)</span><a class="headerlink" href="#c.os_eventq_get" title="Permalink to this definition"></a><br /></dt>
<dd><p>Pull a single item from an event queue. </p>
<p>This function blocks until there is an item on the event queue to read.</p>
<p><dl class="simple">
<dt><strong>Return</strong></dt><dd><p>The event from the queue </p>
</dd>
<dt><strong>Parameters</strong></dt><dd><ul class="breatheparameterlist simple">
<li><p><code class="docutils literal notranslate"><span class="pre">evq</span></code>: The event queue to pull an event from</p></li>
</ul>
</dd>
</dl>
</p>
</dd></dl>
<dl class="c function">
<dt id="c.os_eventq_run">
<span class="target" id="group___o_s_event_1ga5ccd457fe11fddc18f30e9155167948f"></span><span class="pre">void</span> <code class="sig-name descname"><span class="pre">os_eventq_run</span></code><span class="sig-paren">(</span><em class="property"><span class="pre">struct</span></em> <a class="reference internal" href="#c.os_eventq" title="os_eventq"><span class="pre">os_eventq</span></a> <span class="pre">*</span><em><span class="pre">evq</span></em><span class="sig-paren">)</span><a class="headerlink" href="#c.os_eventq_run" title="Permalink to this definition"></a><br /></dt>
<dd><p>Pull a single item off the event queue and call it’s event callback. </p>
<p><dl class="simple">
<dt><strong>Parameters</strong></dt><dd><ul class="breatheparameterlist simple">
<li><p><code class="docutils literal notranslate"><span class="pre">evq</span></code>: The event queue to pull the item off. </p></li>
</ul>
</dd>
</dl>
</p>
</dd></dl>
<dl class="c function">
<dt id="c.os_eventq_poll">
<span class="target" id="group___o_s_event_1ga75902d07b868e820357767c4aae2e175"></span><em class="property"><span class="pre">struct</span></em> <a class="reference internal" href="#c.os_event" title="os_event"><span class="pre">os_event</span></a> <span class="pre">*</span><code class="sig-name descname"><span class="pre">os_eventq_poll</span></code><span class="sig-paren">(</span><em class="property"><span class="pre">struct</span></em> <a class="reference internal" href="#c.os_eventq" title="os_eventq"><span class="pre">os_eventq</span></a><span class="pre">*</span><span class="pre">*</span>, <span class="pre">int</span>, <a class="reference internal" href="../time/os_time.html#c.os_time_t" title="os_time_t"><span class="pre">os_time_t</span></a><span class="sig-paren">)</span><a class="headerlink" href="#c.os_eventq_poll" title="Permalink to this definition"></a><br /></dt>
<dd><p>Poll the list of event queues specified by the evq parameter (size nevqs), and return the “first” event available on any of the queues. </p>
<p>Event queues are searched in the order that they are passed in the array.</p>
<p><dl class="simple">
<dt><strong>Return</strong></dt><dd><p>An event, or NULL if no events available </p>
</dd>
<dt><strong>Parameters</strong></dt><dd><ul class="breatheparameterlist simple">
<li><p><code class="docutils literal notranslate"><span class="pre">evq</span></code>: Array of event queues </p></li>
<li><p><code class="docutils literal notranslate"><span class="pre">nevqs</span></code>: Number of event queues in evq </p></li>
<li><p><code class="docutils literal notranslate"><span class="pre">timo</span></code>: Timeout, forever if OS_WAIT_FOREVER is passed to poll.</p></li>
</ul>
</dd>
</dl>
</p>
</dd></dl>
<dl class="c function">
<dt id="c.os_eventq_remove">
<span class="target" id="group___o_s_event_1ga74c23f0cc51953fa974cfb09e9bb646a"></span><span class="pre">void</span> <code class="sig-name descname"><span class="pre">os_eventq_remove</span></code><span class="sig-paren">(</span><em class="property"><span class="pre">struct</span></em> <a class="reference internal" href="#c.os_eventq" title="os_eventq"><span class="pre">os_eventq</span></a><span class="pre">*</span>, <em class="property"><span class="pre">struct</span></em> <a class="reference internal" href="#c.os_event" title="os_event"><span class="pre">os_event</span></a><span class="pre">*</span><span class="sig-paren">)</span><a class="headerlink" href="#c.os_eventq_remove" title="Permalink to this definition"></a><br /></dt>
<dd><p>Remove an event from the queue. </p>
<p><dl class="simple">
<dt><strong>Parameters</strong></dt><dd><ul class="breatheparameterlist simple">
<li><p><code class="docutils literal notranslate"><span class="pre">evq</span></code>: The event queue to remove the event from </p></li>
<li><p><code class="docutils literal notranslate"><span class="pre">ev</span></code>: The event to remove from the queue </p></li>
</ul>
</dd>
</dl>
</p>
</dd></dl>
<dl class="c function">
<dt id="c.os_eventq_dflt_get">
<span class="target" id="group___o_s_event_1gacb92d16784e50c39fc62c92748d3a72d"></span><em class="property"><span class="pre">struct</span></em> <a class="reference internal" href="#c.os_eventq" title="os_eventq"><span class="pre">os_eventq</span></a> <span class="pre">*</span><code class="sig-name descname"><span class="pre">os_eventq_dflt_get</span></code><span class="sig-paren">(</span><span class="pre">void</span><span class="sig-paren">)</span><a class="headerlink" href="#c.os_eventq_dflt_get" title="Permalink to this definition"></a><br /></dt>
<dd><p>Retrieves the default event queue processed by OS main task. </p>
<p><dl class="simple">
<dt><strong>Return</strong></dt><dd><p>The default event queue. </p>
</dd>
</dl>
</p>
</dd></dl>
<dl class="c macro">
<dt id="c.OS_EVENT_QUEUED">
<span class="target" id="group___o_s_event_1gaf8776b7d4a3a3374e89562eefb3f3df2"></span><code class="sig-name descname"><span class="pre">OS_EVENT_QUEUED</span></code><span class="sig-paren">(</span><em><span class="pre">__ev</span></em><span class="sig-paren">)</span><a class="headerlink" href="#c.OS_EVENT_QUEUED" title="Permalink to this definition"></a><br /></dt>
<dd><p>Return whether or not the given event is queued. </p>
</dd></dl>
<dl class="c struct">
<dt id="c.os_event">
<span class="target" id="structos__event"></span><em class="property"><span class="pre">struct</span> </em><code class="sig-name descname"><span class="pre">os_event</span></code><a class="headerlink" href="#c.os_event" title="Permalink to this definition"></a><br /></dt>
<dd><em>#include &lt;os_eventq.h&gt;</em><p>Structure representing an OS event. </p>
<p>OS events get placed onto the event queues and are consumed by tasks. </p>
<div class="breathe-sectiondef docutils container">
<p class="breathe-sectiondef-title rubric" id="breathe-section-title-public-members">Public Members</p>
<dl class="c var">
<dt id="c.os_event.ev_queued">
<span class="target" id="structos__event_1a22704177c6946c3bbb9f30dd8dca1b68"></span><span class="pre">uint8_t</span> <code class="sig-name descname"><span class="pre">ev_queued</span></code><a class="headerlink" href="#c.os_event.ev_queued" title="Permalink to this definition"></a><br /></dt>
<dd><p>Whether this OS event is queued on an event queue. </p>
</dd></dl>
<dl class="c var">
<dt id="c.os_event.ev_cb">
<span class="target" id="structos__event_1adce337a55ab01bf317fcd9048b4fb97d"></span><a class="reference internal" href="#c.os_event_fn" title="os_event_fn"><span class="pre">os_event_fn</span></a> <span class="pre">*</span><code class="sig-name descname"><span class="pre">ev_cb</span></code><a class="headerlink" href="#c.os_event.ev_cb" title="Permalink to this definition"></a><br /></dt>
<dd><p>Callback to call when the event is taken off of an event queue. </p>
<p>APIs, except for <a class="reference internal" href="#group___o_s_event_1ga5ccd457fe11fddc18f30e9155167948f"><span class="std std-ref">os_eventq_run()</span></a>, assume this callback will be called by the user. </p>
</dd></dl>
<dl class="c var">
<dt id="c.os_event.ev_arg">
<span class="target" id="structos__event_1affb49e14dcf2b9e1b6f10bac3da788f1"></span><span class="pre">void</span> <span class="pre">*</span><code class="sig-name descname"><span class="pre">ev_arg</span></code><a class="headerlink" href="#c.os_event.ev_arg" title="Permalink to this definition"></a><br /></dt>
<dd><p>Argument to pass to the event queue callback. </p>
</dd></dl>
</div>
</dd></dl>
<dl class="c struct">
<dt id="c.os_eventq">
<span class="target" id="structos__eventq"></span><em class="property"><span class="pre">struct</span> </em><code class="sig-name descname"><span class="pre">os_eventq</span></code><a class="headerlink" href="#c.os_eventq" title="Permalink to this definition"></a><br /></dt>
<dd><em>#include &lt;os_eventq.h&gt;</em><div class="breathe-sectiondef docutils container">
<p class="breathe-sectiondef-title rubric" id="breathe-section-title-public-members">Public Members</p>
<dl class="c var">
<dt id="c.os_eventq.evq_owner">
<span class="target" id="structos__eventq_1a4f44bc0985277390484d7b5cb6b0e35c"></span><em class="property"><span class="pre">struct</span></em> <a class="reference internal" href="../task/task.html#c.os_task" title="os_task"><span class="pre">os_task</span></a> <span class="pre">*</span><code class="sig-name descname"><span class="pre">evq_owner</span></code><a class="headerlink" href="#c.os_eventq.evq_owner" title="Permalink to this definition"></a><br /></dt>
<dd><p>Pointer to task that “owns” this event queue. </p>
</dd></dl>
<dl class="c var">
<dt id="c.os_eventq.evq_task">
<span class="target" id="structos__eventq_1afba1a5b1c24f170c4fe65c186a9257f8"></span><em class="property"><span class="pre">struct</span></em> <a class="reference internal" href="../task/task.html#c.os_task" title="os_task"><span class="pre">os_task</span></a> <span class="pre">*</span><code class="sig-name descname"><span class="pre">evq_task</span></code><a class="headerlink" href="#c.os_eventq.evq_task" title="Permalink to this definition"></a><br /></dt>
<dd><p>Pointer to the task that is sleeping on this event queue, either NULL, or the owner task. </p>
</dd></dl>
</div>
</dd></dl>
</div>
</div>
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