This function will register a subscription to a topic on a MQTT server.
The topic can contain wild-cards (+ and #).
The + is used as a wild-card for a single level of hierarchy.
For example, a topic of "a/b/+/c" will match messages with topic "a/b/a/c" and "a/b/b/c", but not "a/b/c" or "a/b/a/c/d"
The # is used as a wild-card for all remaining levels of hierarchy.
For example, a topic of "a/b/#" will match messages with topic "a/b/c" and "a/b/c/d", but not "a/b".
Note:
There is no information stored in the RTCU about the subscriptions made. The information persistence is defined by the implementation behavior defined by the MQTT server.
Input:
handle : INT
The handle to the MQTT connection.
topic : STRING
The topic to subscribe to.
qos : SINT (0..2)
The QoS used by the MQTT server to transfer message to the device.
Returns: INT
0
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- Subscription is registered.
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1
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- Invalid handle.
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2
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- Not connected to the MQTT server.
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3
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- Invalid parameter.
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Declaration:
FUNCTION mqttSubscribe : INT;
VAR_INPUT
handle : INT;
topic : STRING;
qos : SINT;
END_VAR;
Example:
INCLUDE rtcu.inc
VAR
mqtt : INT;
text : STRING;
rxd : mqttReceive;
rc : INT;
msg : ARRAY [1..50] OF SINT;
END_VAR;
PROGRAM example;
DebugMsg(message := "Initialize GPRS...");
gsmPower(power := ON);
gprsOpen();
WHILE NOT gprsConnected() DO
DebugMsg(message := "Waiting for cellular connection...");
Sleep(delay := 2500);
END_WHILE;
DebugMsg(message := "Initialize MQTT...");
text := strFormat(format := "RTCU_\4", v4 := boardSerialNumber());
mqtt := mqttOpen(ip := "test.mosquitto.org", port := 1883, clientid := text);
DebugFmt(message := "MQTT handle= \1", v1 := mqtt);
IF NOT mqttConnected(handle := mqtt) THEN
rc := mqttStatus(handle := mqtt);
DebugFmt(message := "Connection failed, Status=\1", v1 := rc);
END_IF;
rc := mqttSubscribe(handle := mqtt, qos := 2, topic := "RTCU/+/temperature");
DebugFmt(message := "Subscribed= \1", v1 := rc);
rxd(data := ADDR(msg), maxsize := SIZEOF(msg));
BEGIN
rc := mqttWaitEvent(timeout := 5);
IF rc > 0 THEN
DebugMsg(message := "Message received");
rxd();
DebugMsg(message := "-Topic= " + rxd.topic);
DebugFmt(message := "-Handle= \1", v1 := rxd.handle);
DebugFmt(message := "-Size= \1", v1 := rxd.size);
DebugMsg(message := "-Temperature= " + strFromMemory(src := ADDR(msg), len := rxd.size));
END_IF;
END;
END_PROGRAM;
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