is session.</p><p>Each message is sent with an increasing index; this returns the index for the next message.</p></dd></dl><dl><dt class="spec value" id="val-outbound_group_session_key_length"><a href="#val-outbound_group_session_key_length" class="anchor"></a><code><span class="keyword">val</span> outbound_group_session_key_length : <span><a href="../Types/OutboundGroupSession/index.html#type-t">Types.OutboundGroupSession.t</a> Ctypes_static.ptr</span> <span>&#45;&gt;</span> Unsigned.size_t</code></dt><dd>