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MQTT client for lwIP
Author: Erik Andersson
Details of the MQTT protocol can be found at:
http://docs.oasis-open.org/mqtt/mqtt/v3.1.1/os/mqtt-v3.1.1-os.html
-----------------------------------------------------------------
1. Initial steps, reserve memory and make connection to server:
You need to increase MEMP_NUM_SYS_TIMEOUT by one if you use MQTT!
1.1: Provide storage
Static allocation:
mqtt_client_t static_client;
example_do_connect(&static_client);
Dynamic allocation:
mqtt_client_t *client = mqtt_client_new();
if(client != NULL) {
example_do_connect(&client);
}
1.2: Establish Connection with server
void example_do_connect(mqtt_client_t *client)
{
struct mqtt_connect_client_info_t ci;
err_t err;
/* Setup an empty client info structure */
memset(&ci, 0, sizeof(ci));
/* Minimal amount of information required is client identifier, so set it here */
ci.client_id = "lwip_test";
/* Initiate client and connect to server, if this fails immediately an error code is returned
otherwise mqtt_connection_cb will be called with connection result after attempting
to establish a connection with the server.
For now MQTT version 3.1.1 is always used */
err = mqtt_client_connect(client, ip_addr, MQTT_PORT, mqtt_connection_cb, 0, &ci);
/* For now just print the result code if something goes wrong */
if(err != ERR_OK) {
printf("mqtt_connect return %d\n", err);
}
}
Connection to server can also be probed by calling mqtt_client_is_connected(client)
-----------------------------------------------------------------
2. Implementing the connection status callback
static void mqtt_connection_cb(mqtt_client_t *client, void *arg, mqtt_connection_status_t status)
{
err_t err;
if(status == MQTT_CONNECT_ACCEPTED) {
printf("mqtt_connection_cb: Successfully connected\n");
/* Setup callback for incoming publish requests */
mqtt_set_inpub_callback(client, mqtt_incoming_publish_cb, mqtt_incoming_data_cb, arg);
/* Subscribe to a topic named "subtopic" with QoS level 1, call mqtt_sub_request_cb with result */
err = mqtt_subscribe(client, "subtopic", 1, mqtt_sub_request_cb, arg);
if(err != ERR_OK) {
printf("mqtt_subscribe return: %d\n", err);
}
} else {
printf("mqtt_connection_cb: Disconnected, reason: %d\n", status);
/* Its more nice to be connected, so try to reconnect */
example_do_connect(client);
}
}
static void mqtt_sub_request_cb(void *arg, err_t result)
{
/* Just print the result code here for simplicity,
normal behaviour would be to take some action if subscribe fails like
notifying user, retry subscribe or disconnect from server */
printf("Subscribe result: %d\n", result);
}
-----------------------------------------------------------------
3. Implementing callbacks for incoming publish and data
/* The idea is to demultiplex topic and create some reference to be used in data callbacks
Example here uses a global variable, better would be to use a member in arg
If RAM and CPU budget allows it, the easiest implementation might be to just take a copy of
the topic string and use it in mqtt_incoming_data_cb
*/
static int inpub_id;
static void mqtt_incoming_publish_cb(void *arg, const char *topic, u32_t tot_len)
{
printf("Incoming publish at topic %s with total length %u\n", topic, (unsigned int)tot_len);
/* Decode topic string into a user defined reference */
if(strcmp(topic, "print_payload") == 0) {
inpub_id = 0;
} else if(topic[0] == 'A') {
/* All topics starting with 'A' might be handled at the same way */
inpub_id = 1;
} else {
/* For all other topics */
inpub_id = 2;
}
}
static void mqtt_incoming_data_cb(void *arg, const u8_t *data, u16_t len, u8_t flags)
{
printf("Incoming publish payload with length %d, flags %u\n", len, (unsigned int)flags);
if(flags & MQTT_DATA_FLAG_LAST) {
/* Last fragment of payload received (or whole part if payload fits receive buffer
See MQTT_VAR_HEADER_BUFFER_LEN) */
/* Call function or do action depending on reference, in this case inpub_id */
if(inpub_id == 0) {
/* Don't trust the publisher, check zero termination */
if(data[len-1] == 0) {
printf("mqtt_incoming_data_cb: %s\n", (const char *)data);
}
} else if(inpub_id == 1) {
/* Call an 'A' function... */
} else {
printf("mqtt_incoming_data_cb: Ignoring payload...\n");
}
} else {
/* Handle fragmented payload, store in buffer, write to file or whatever */
}
}
-----------------------------------------------------------------
4. Using outgoing publish
void example_publish(mqtt_client_t *client, void *arg)
{
const char *pub_payload= "PubSubHubLubJub";
err_t err;
u8_t qos = 2; /* 0 1 or 2, see MQTT specification */
u8_t retain = 0; /* No don't retain such crappy payload... */
err = mqtt_publish(client, "pub_topic", pub_payload, strlen(pub_payload), qos, retain, mqtt_pub_request_cb, arg);
if(err != ERR_OK) {
printf("Publish err: %d\n", err);
}
}
/* Called when publish is complete either with success or failure */
static void mqtt_pub_request_cb(void *arg, err_t result)
{
if(result != ERR_OK) {
printf("Publish result: %d\n", result);
}
}
-----------------------------------------------------------------
5. Disconnecting
Simply call mqtt_disconnect(client)
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