---
tag: docs
type: other
title: Subscription Callback
---

If the client is used to subscribe to topics, a callback function must be provided in the constructor. This function is called when new messages arrive at the client.

The callback function has the following signature:

    void callback(const char[] topic, byte* payload, unsigned int length)

##### Parameters

 - <span class="methodparams">topic</span> <span class="methodparamstype">const char[]</span> - the topic the message arrived on
 - <span class="methodparams">payload</span> <span class="methodparamstype">byte[]</span> - the message payload
 - <span class="methodparams">length</span> <span class="methodparamstype">unsigned int</span> - the length of the message payload


Internally, the client uses the same buffer for both inbound and outbound
messages. After the callback function returns, or if a call to either `publish`
or subscribe is made from within the callback function, the `topic`
and `payload` values passed to the function will be overwritten. The application
should create its own copy of the values if they are required after the callback returns.