MQTT_GetSubscribePacketSize

Get packet size and Remaining Length of an MQTT SUBSCRIBE packet.

MQTTStatus_t MQTT_GetSubscribePacketSize( const MQTTSubscribeInfo_t * pSubscriptionList,
                                          size_t subscriptionCount,
                                          size_t * pRemainingLength,
                                          size_t * pPacketSize );

This function must be called before MQTT_SerializeSubscribe in order to get the size of the MQTT SUBSCRIBE packet that is generated from the list of MQTTSubscribeInfo_t. The size of the MQTTFixedBuffer_t supplied to MQTT_SerializeSubscribe must be at least pPacketSize. The provided pSubscriptionList is valid for serialization with MQTT_SerializeSubscribe only if this function returns MQTTSuccess. The remaining length returned in pRemainingLength and the packet size returned in pPacketSize are valid only if this function returns MQTTSuccess.

Parameters

[in]	pSubscriptionList	List of MQTT subscription info.
[in]	subscriptionCount	The number of elements in pSubscriptionList.
[out]	pRemainingLength	The Remaining Length of the MQTT SUBSCRIBE packet.
[out]	pPacketSize	The total size of the MQTT SUBSCRIBE packet.

Returns

MQTTBadParameter if the packet would exceed the size allowed by the MQTT spec; MQTTSuccess otherwise.

Example

// Variables used in this example.
MQTTStatus_t status;
MQTTSubscribeInfo_t subscriptionList[ NUMBER_OF_SUBSCRIPTIONS ] = { 0 };
size_t remainingLength = 0, packetSize = 0;
// This is assumed to be a list of filters we want to subscribe to.
const char * filters[ NUMBER_OF_SUBSCRIPTIONS ];
 
// Set each subscription.
for( int i = 0; i < NUMBER_OF_SUBSCRIPTIONS; i++ )
{
     subscriptionList[ i ].qos = MQTTQoS0;
     // Each subscription needs a topic filter.
     subscriptionList[ i ].pTopicFilter = filters[ i ];
     subscriptionList[ i ].topicFilterLength = strlen( filters[ i ] );
}
 
// Get the size requirement for the subscribe packet.
status = MQTT_GetSubscribePacketSize(
     &subscriptionList[ 0 ], NUMBER_OF_SUBSCRIPTIONS, &remainingLength, &packetSize
);
 
if( status == MQTTSuccess )
{
     // The application should allocate or use a static #MQTTFixedBuffer_t
     // of size >= packetSize to serialize the subscribe request.
}