Adding properties to PUBLISH packet. about emqtt5 HOT 4 CLOSED

I also tried your example like that

 // Please do not move the line below as it must outlive the packet
        Protocol::MQTT::V5::Property<uint32> maxProp(Protocol::MQTT::V5::PacketSizeMax, 2048);
        Protocol::MQTT::V5::Property<Protocol::MQTT::Common::DynamicStringPair> userProp(Protocol::MQTT::V5::UserProperty, Protocol::MQTT::Common::DynamicStringPair("key", "value"));
        //Protocol::MQTT::V5::ControlPacket<Protocol::MQTT::V5::CONNECT> packet;


        // Check if we have a max packet size property and if not, append one to let the server know our limitation (if any)
        //packet.props.append(&maxProp); // It'll fail silently if it already exists
        props.append(&userProp); // It'll fail silently if it already exists

Same behaviour

from emqtt5.

For your code, the issue is that you're making a pointer that's pointing to an object that's deleted. The prop instance in your if block doesn't survive after the if block. So the Properties object in publish in now referencing a dangling object.

You have two solutions to your issue.

Solution 1: Still use the property that's allocated on the stack, you'll need to move the declaration in the same scope as the usage, like this:

Protocol::MQTT::V5::registerAllProperties();
Network::Client::MQTTv5::Properties properties;
Network::Client::MQTTv5::DynamicBinDataView c_data(correlation_data.length(), (const uint8_t*)correlation_data.c_str());
Protocol::MQTT::V5::Property<Network::Client::MQTTv5::DynamicBinDataView> prop(Protocol::MQTT::V5::CorrelationData, c_data);
if (!correlation_data.empty()) {
       if (!properties.append(&prop))
           printf("PROP APPEND FAILED!\n");
       else
           printf("PROP APPEND SUCCESS!\n");
}
client.publish(publish_topic, (const uint8*) publish_message,
                                           strlen(publish_message), retain_published_message, QoS, (uint16)0U, &properties))

This is valid because std::string::c_str returns a valid zero terminated string if empty (see here). It would be also valid if it returned nullptr since you're using a view and a view can be null.
The property prop will be destructed at end of the function scope, so after publish used it and no pointer is dangling in that case.

Solution 2: Use a property that's allocated on the heap instead
Write like this:

Protocol::MQTT::V5::registerAllProperties();
Network::Client::MQTTv5::Properties properties;
if (!correlation_data.empty()) {
       Network::Client::MQTTv5::DynamicBinDataView c_data(correlation_data.length(), (const uint8_t*)correlation_data.c_str());
       auto * prop = new Protocol::MQTT::V5::Property<Network::Client::MQTTv5::DynamicBinDataView> (Protocol::MQTT::V5::CorrelationData, c_data, true);
       if (!properties.append(prop))
           printf("PROP APPEND FAILED!\n");
       else
           printf("PROP APPEND SUCCESS!\n");
}
client.publish(publish_topic, (const uint8*) publish_message,
                                           strlen(publish_message), retain_published_message, QoS, (uint16)0U, &properties))

Notice the , true in the end of the Property constructor that tells the library to delete the instance when the Properties is being destructed.

Hope that helps!

from emqtt5.

Sorry for the inconvenience, i'm idiot
Completely forgot about scope and if

from emqtt5.

That's ok, the interface in eMQTT5 is designed for embedded development, so when a pointer is used in the methods signature, it often doesn't expect a heap allocated object (or you have to explicitly tell it that it's a pointer to the heap). Dealing with pointers on the stack that self destruct is not usual for a C++ developer, I can easily understand it's disturbing.

from emqtt5.