MQTT API Reference
MQTT 3.1.1 client library

Establish a new MQTT connection.

const IotMqttConnectInfo_t * pConnectInfo,
uint32_t timeoutMs,
IotMqttConnection_t * const pMqttConnection );

This function opens a connection between a new MQTT client and an MQTT server (also called a broker). MQTT connections are established on top of transport layer protocols (such as TCP/IP), and optionally, application layer security protocols (such as TLS). The MQTT packet that establishes a connection is called the MQTT CONNECT packet. After IotMqtt_Init, this function must be called before any other MQTT library function.

If pConnectInfo->cleanSession is true, this function establishes a clean MQTT session. Subscriptions and unacknowledged PUBLISH messages will be discarded when the connection is closed.

If pConnectInfo->cleanSession is false, this function establishes (or re-establishes) a persistent MQTT session. The parameters pConnectInfo->pPreviousSubscriptions and pConnectInfo->previousSubscriptionCount may be used to restore subscriptions present in a re-established persistent session. Any restored subscriptions MUST have been present in the persistent session; this function does not send an MQTT SUBSCRIBE packet!

This MQTT library is network agnostic, meaning it has no knowledge of the underlying network protocol carrying the MQTT packets. It interacts with the network through a network abstraction layer, allowing it to be used with many different network stacks. The network abstraction layer is established per-connection, allowing every IotMqttConnection_t to use a different network stack. The parameter pNetworkInterface sets up the network abstraction layer for an MQTT connection; see the documentation on IotMqttNetworkInfo_t for details on its members.

The pConnectInfo parameter provides the contents of the MQTT CONNECT packet. Most members are defined by the MQTT spec.. The pConnectInfo->pWillInfo member provides information on a Last Will and Testament (LWT) message to be published if the MQTT connection is closed without sending a DISCONNECT packet. Unlike other PUBLISH messages, a LWT message payload is limited to 65535 bytes in length. Additionally, the retry interval and limit members of IotMqttPublishInfo_t are ignored for LWT messages. The LWT message is optional; pWillInfo may be NULL.

Unlike IotMqtt_PublishAsync, IotMqtt_SubscribeAsync, and IotMqtt_UnsubscribeAsync, this function is always blocking. Additionally, because the MQTT connection acknowledgement packet (CONNACK packet) does not contain any information on which CONNECT packet it acknowledges, only one CONNECT operation may be in progress at any time. This means that parallel threads making calls to IotMqtt_Connect will be serialized to send their CONNECT packets one-by-one.

[in]pNetworkInfoInformation on the transport-layer network connection to use with the MQTT connection.
[in]pConnectInfoMQTT connection setup parameters.
[in]timeoutMsIf the MQTT server does not accept the connection within this timeout in milliseconds, this function returns IOT_MQTT_TIMEOUT.
[out]pMqttConnectionSet to a newly-initialized MQTT connection handle if this function succeeds.
One of the following:


// An initialized and connected network connection.
IotNetworkConnection_t pNetworkConnection;
// Parameters to MQTT connect.
// Example network abstraction types.
IotNetworkServerInfo_t serverInfo = { ... };
IotNetworkCredentials_t credentialInfo = { ... };
IotNetworkInterface_t networkInterface = { ... };
// Example using a generic network implementation.
networkInfo.createNetworkConnection = true;
networkInfo.u.setup.pNetworkServerInfo = &serverInfo;
networkInfo.u.setup.pNetworkCredentialInfo = &credentialInfo;
networkInfo.pNetworkInterface = &networkInterface;
// Set the members of the connection info (password and username not used).
connectInfo.cleanSession = true;
connectInfo.keepAliveSeconds = 30;
connectInfo.pClientIdentifier = "uniqueclientidentifier";
connectInfo.clientIdentifierLength = 22;
// Set the members of the will info (retain and retry not used).
willInfo.qos = IOT_MQTT_QOS_1;
willInfo.pTopicName = "will/topic/name";
willInfo.topicNameLength = 15;
willInfo.pPayload = "MQTT client unexpectedly disconnected.";
willInfo.payloadLength = 38;
// Set the pointer to the will info.
connectInfo.pWillInfo = &willInfo;
// Call CONNECT with a 5 second block time. Should return
// IOT_MQTT_SUCCESS when successful.
IotMqttError_t result = IotMqtt_Connect( &networkInfo,
&mqttConnection );
if( result == IOT_MQTT_SUCCESS )
// Do something with the MQTT connection...
// Clean up and close the MQTT connection once it's no longer needed.
IotMqtt_Disconnect( mqttConnection, 0 );
const IotNetworkInterface_t * pNetworkInterface
The network functions used by the new MQTT connection.
Definition: iot_mqtt_types.h:990
Initializer for IotMqttConnectInfo_t.
Definition: iot_mqtt_types.h:1061
uint16_t keepAliveSeconds
Period of keep-alive messages. Set to 0 to disable keep-alive.
Definition: iot_mqtt_types.h:674
bool cleanSession
Whether this connection is a clean session.
Definition: iot_mqtt_types.h:632
void IotMqtt_Disconnect(IotMqttConnection_t mqttConnection, uint32_t flags)
Closes an MQTT connection and frees resources.
Definition: iot_mqtt_api.c:1327
Return codes of MQTT functions.
Definition: iot_mqtt_types.h:103
const void * pPayload
Payload of PUBLISH.
Definition: iot_mqtt_types.h:405
MQTT operation completed successfully.
Definition: iot_mqtt_types.h:119
uint16_t topicNameLength
Length of IotMqttPublishInfo_t.pTopicName.
Definition: iot_mqtt_types.h:403
Initializer for IotMqttNetworkInfo_t.
Definition: iot_mqtt_types.h:1057
const char * pTopicName
Topic name of PUBLISH.
Definition: iot_mqtt_types.h:402
struct _mqttConnection * IotMqttConnection_t
Opaque handle of an MQTT connection.
Definition: iot_mqtt_types.h:66
Initializer for IotMqttConnection_t.
Definition: iot_mqtt_types.h:1069
Infomation on the transport-layer network connection for the new MQTT connection.
Definition: iot_mqtt_types.h:930
IotMqttQos_t qos
QoS of message. Must be 0 or 1.
Definition: iot_mqtt_types.h:399
union IotMqttNetworkInfo_t::@5 u
Valid member depends of IotMqttNetworkInfo_t.createNetworkConnection.
uint16_t clientIdentifierLength
Length of IotMqttConnectInfo_t.pClientIdentifier.
Definition: iot_mqtt_types.h:677
size_t payloadLength
Length of IotMqttPublishInfo_t.pPayload. For LWT messages, this is limited to 65535.
Definition: iot_mqtt_types.h:406
bool createNetworkConnection
Whether a new network connection should be created.
Definition: iot_mqtt_types.h:947
Definition: iot_mqtt_types.h:306
Information on a PUBLISH message.
Definition: iot_mqtt_types.h:397
Initializer for IotMqttPublishInfo_t.
Definition: iot_mqtt_types.h:1063
const IotMqttPublishInfo_t * pWillInfo
A message to publish if the new MQTT connection is unexpectedly closed.
Definition: iot_mqtt_types.h:672
const char * pClientIdentifier
MQTT client identifier.
Definition: iot_mqtt_types.h:676
IotMqttError_t IotMqtt_Connect(const IotMqttNetworkInfo_t *pNetworkInfo, const IotMqttConnectInfo_t *pConnectInfo, uint32_t timeoutMs, IotMqttConnection_t *const pMqttConnection)
Establish a new MQTT connection.
Definition: iot_mqtt_api.c:975
Information on a new MQTT connection.
Definition: iot_mqtt_types.h:585