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<h2 id="respond-to-sync-and-reset-events">Respond to <em>sync</em> and <em>reset</em> events</h2>
<h3 id="sync">sync</h3>
<p>The NimBLE stack is inoperable while the host and controller are out of sync.
In a combined host-controller app, the sync happens immediately at startup.
When the host and controller are separate, sync typically occurs in under a
second after the application starts. An application learns when sync is
achieved by configuring the host's <em>sync callback</em>: <code>ble_hs_cfg.sync_cb</code>. The
host calls the sync callback whenever sync is acquired. The sync callback has
the following form:</p>
<div class="codehilite" style="background: #ffffff"><pre style="line-height: 125%;"><span></span><code>typedef void ble_hs_sync_fn(void);
</code></pre></div>
<p>Because the NimBLE stack begins in the unsynced state, the application should
delay all BLE operations until the sync callback has been called.</p>
<h3 id="reset">reset</h3>
<p>Another event indicated by the host is a <em>controller reset</em>. The NimBLE stack
resets itself when a catstrophic error occurs, such as loss of communication
between the host and controller. Upon resetting, the host drops all BLE
connections and loses sync with the controller. After a reset, the application
should refrain from using the host until sync is again signalled via the sync
callback.</p>
<p>An application learns of a host reset by configuring the host's <em>reset
callback</em>: <code>ble_hs_cfg.reset_cb</code>. This callback has the following form:</p>
<div class="codehilite" style="background: #ffffff"><pre style="line-height: 125%;"><span></span><code>typedef void ble_hs_reset_fn(int reason);
</code></pre></div>
<p>The <code>reason</code> parameter is a
<a href="../../../network/ble/ble_hs/ble_hs_return_codes/">NimBLE host return code</a>.</p>
<h3 id="example">Example</h3>
<p>The following example demonstrates the configuration of the sync and reset
callbacks.</p>
<div class="codehilite" style="background: #ffffff"><pre style="line-height: 125%;"><span></span><code><span style="color: #633820">#include</span> <span style="color: #177500">&quot;sysinit/sysinit.h&quot;</span>
<span style="color: #633820">#include</span> <span style="color: #177500">&quot;console/console.h&quot;</span>
<span style="color: #633820">#include</span> <span style="color: #177500">&quot;host/ble_hs.h&quot;</span>
<span style="color: #A90D91">static</span> <span style="color: #A90D91">void</span>
<span style="color: #000000">on_sync</span>(<span style="color: #A90D91">void</span>)
{
<span style="color: #177500">/* Begin advertising, scanning for peripherals, etc. */</span>
}
<span style="color: #A90D91">static</span> <span style="color: #A90D91">void</span>
<span style="color: #000000">on_reset</span>(<span style="color: #A90D91">int</span> <span style="color: #000000">reason</span>)
{
<span style="color: #000000">console_printf</span>(<span style="color: #C41A16">&quot;Resetting state; reason=%d\n&quot;</span>, <span style="color: #000000">reason</span>);
}
<span style="color: #A90D91">int</span>
<span style="color: #000000">main</span>(<span style="color: #A90D91">void</span>)
{
<span style="color: #177500">/* Initialize all packages. */</span>
<span style="color: #000000">sysinit</span>();
<span style="color: #000000">ble_hs_cfg</span>.<span style="color: #000000">sync_cb</span> <span style="color: #000000">=</span> <span style="color: #000000">on_sync</span>;
<span style="color: #000000">ble_hs_cfg</span>.<span style="color: #000000">reset_cb</span> <span style="color: #000000">=</span> <span style="color: #000000">on_reset</span>;
<span style="color: #177500">/* As the last thing, process events from default event queue. */</span>
<span style="color: #A90D91">while</span> (<span style="color: #1C01CE">1</span>) {
<span style="color: #000000">os_eventq_run</span>(<span style="color: #000000">os_eventq_dflt_get</span>());
}
}
</code></pre></div>
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