1callbacks(3)               Library Functions Manual               callbacks(3)
2
3
4

NAME

6       callbacks - Callbacks
7
8       You must not call a function from this API from within a callback
9       otherwise a deadlock might result. The only exception to this is the
10       ability to call connect within the connection lost callback, to allow a
11       reconnect.
12
13       When using MQTT 5.0, you can also call connect from within the
14       disconnected callback, which is invoked when the MQTT server sends a
15       disconnect packet. This server behaviour is allowed in MQTT 5.0, but
16       not in MQTT 3.1.1, so the disconnected callback will never be invoked
17       if you use MQTT 3.1.1.
18
19       In particular, you must make a publish call within the message arrived
20       callback. These restrictions are all lifted in the MQTTAsync API.
21
22       If no callbacks are assigned, this will include the message arrived
23       callback. This could be done if the application is a pure publisher,
24       and does not subscribe to any topics. If however messages are received,
25       and no message arrived callback is set, or receive not called, then
26       those messages will accumulate and take up memory, as there is no place
27       for them to be delivered. It is up to the application to protect
28       against this situation.
29
30
31
32Paho MQTT C Client Library      Thu Jan 19 2023                   callbacks(3)