=== MQTT 

==== mg_set_protocol_mqtt()

[source,c]
----
void mg_set_protocol_mqtt(struct mg_connection *nc);
----
Attach built-in MQTT event handler to the given connection.

The user-defined event handler will receive following extra events:

- MG_EV_MQTT_CONNACK
- MG_EV_MQTT_PUBLISH
- MG_EV_MQTT_PUBACK
- MG_EV_MQTT_PUBREC
- MG_EV_MQTT_PUBREL
- MG_EV_MQTT_PUBCOMP
- MG_EV_MQTT_SUBACK 

==== mg_send_mqtt_handshake()

[source,c]
----
void mg_send_mqtt_handshake(struct mg_connection *nc, const char *client_id);
----
Send MQTT handshake. 

==== mg_send_mqtt_handshake_opt()

[source,c]
----
void mg_send_mqtt_handshake_opt(struct mg_connection *nc, const char *client_id,
                                struct mg_send_mqtt_handshake_opts);
----
Send MQTT handshake with optional parameters. 

==== mg_mqtt_publish()

[source,c]
----
void mg_mqtt_publish(struct mg_connection *nc, const char *topic,
                     uint16_t message_id, int flags, const void *data,
                     size_t len);
----
Publish a message to a given topic. 

==== mg_mqtt_subscribe()

[source,c]
----
void mg_mqtt_subscribe(struct mg_connection *nc,
                       const struct mg_mqtt_topic_expression *topics,
                       size_t topics_len, uint16_t message_id);
----
Subscribe to a bunch of topics. 

==== mg_mqtt_unsubscribe()

[source,c]
----
void mg_mqtt_unsubscribe(struct mg_connection *nc, char **topics,
                         size_t topics_len, uint16_t message_id);
----
Unsubscribe from a bunch of topics. 

==== mg_mqtt_disconnect()

[source,c]
----
void mg_mqtt_disconnect(struct mg_connection *nc);
----
Send a DISCONNECT command. 

==== mg_mqtt_connack()

[source,c]
----
void mg_mqtt_connack(struct mg_connection *nc, uint8_t return_code);
----
Send a CONNACK command with a given `return_code`. 

==== mg_mqtt_puback()

[source,c]
----
void mg_mqtt_puback(struct mg_connection *nc, uint16_t message_id);
----
Send a PUBACK command with a given `message_id`. 

==== mg_mqtt_pubrec()

[source,c]
----
void mg_mqtt_pubrec(struct mg_connection *nc, uint16_t message_id);
----
Send a PUBREC command with a given `message_id`. 

==== mg_mqtt_pubrel()

[source,c]
----
void mg_mqtt_pubrel(struct mg_connection *nc, uint16_t message_id);
----
Send a PUBREL command with a given `message_id`. 

==== mg_mqtt_pubcomp()

[source,c]
----
void mg_mqtt_pubcomp(struct mg_connection *nc, uint16_t message_id);
----
Send a PUBCOMP command with a given `message_id`. 

==== mg_mqtt_suback()

[source,c]
----
void mg_mqtt_suback(struct mg_connection *nc, uint8_t *qoss, size_t qoss_len,
                    uint16_t message_id);
----
Send a SUBACK command with a given `message_id`
and a sequence of granted QoSs. 

==== mg_mqtt_unsuback()

[source,c]
----
void mg_mqtt_unsuback(struct mg_connection *nc, uint16_t message_id);
----
Send a UNSUBACK command with a given `message_id`. 

==== mg_mqtt_ping()

[source,c]
----
void mg_mqtt_ping(struct mg_connection *nc);
----
Send a PINGREQ command. 

==== mg_mqtt_pong()

[source,c]
----
void mg_mqtt_pong(struct mg_connection *nc);
----
Send a PINGRESP command. 

==== mg_mqtt_next_subscribe_topic()

[source,c]
----
int mg_mqtt_next_subscribe_topic(struct mg_mqtt_message *msg,
                                 struct mg_str *topic, uint8_t *qos, int pos);
----
Extract the next topic expression from a SUBSCRIBE command payload.

Topic expression name will point to a string in the payload buffer.
Return the pos of the next topic expression or -1 when the list
of topics is exhausted.