IotMqtt_GetSubscriptionPacketSize

Calculate the size and "Remaining length" of a SUBSCRIBE or UNSUBSCRIBE packet generated from the given parameters.

IotMqttError_t IotMqtt_GetSubscriptionPacketSize( IotMqttOperationType_t type,
                                                  const IotMqttSubscription_t * pSubscriptionList,
                                                  size_t subscriptionCount,
                                                  size_t * pRemainingLength,
                                                  size_t * pPacketSize );

Parameters

[in]	type	Either IOT_MQTT_SUBSCRIBE or IOT_MQTT_UNSUBSCRIBE.
[in]	pSubscriptionList	User-provided array of subscriptions.
[in]	subscriptionCount	Size of pSubscriptionList.
[out]	pRemainingLength	Output for calculated "Remaining length" field.
[out]	pPacketSize	Output for calculated total packet size.

Returns

IOT_MQTT_SUCCESS if the packet is within the length allowed by MQTT 3.1.1 spec; IOT_MQTT_BAD_PARAMETER otherwise. If this function returns IOT_MQTT_BAD_PARAMETER, the output parameters should be ignored.

Note

This call is part of serializer API used for implementing light-weight MQTT client.

Example

// Example code below shows how IotMqtt_GetSubscriptionPacketSize() should be used to calculate
// the size of subscribe or unsubscribe request.

IotMqttError_t xResult;
IotMqttSubscription_t xMQTTSubscription[ 1 ];
size_t xRemainingLength = 0;
size_t xPacketSize = 0;

// Initialize Subscribe parameters. Details are out of scope for this example.
// It will involve setting QOS, topic filter  and topic filter length.
 _initializeSubscribe( xMQTTSubscription );

xResult = IotMqtt_GetSubscriptionPacketSize( IOT_MQTT_SUBSCRIBE,
                                             xMQTTSubscription,
                                             sizeof( xMQTTSubscription ) / sizeof( IotMqttSubscription_t ),
                                             &xRemainingLength, &xPacketSize );
IotMqtt_Assert( xResult == IOT_MQTT_SUCCESS );

// Application should allocate buffer with size == xPacketSize or use static buffer
// with size >= xPacketSize to serialize connect request.