Linker error undefined reference to .. if I use the MQTT_CLIENT_STD_FUNCTION_CALLBACK about arduinomqttclient HOT 2 CLOSED

Hi @per1234

I appreciate your detailed explanation & tecnical dive in. I changed my code and things work fine now.

from arduinomqttclient.

Hi @ma-bg. This feature is only available when the MQTT_CLIENT_STD_FUNCTION_CALLBACK macro is defined.

Unfortunately the comment in the source code regarding this is misleading:

The comment implies that you can simply add the #define directive to the sketch:

MySketch.ino:

#define MQTT_CLIENT_STD_FUNCTION_CALLBACK
#include <ArduinoMqttClient.h>

// ...

But this is not possible. The reason is that this only defines the macro in the "translation unit" of the sketch, but not in the translation unit of MqttClient.cpp, where the macro must also be defined. If the macro is not defined in that translation unit, you get an undefined reference error.

There are two approaches to using the feature:

• Define the macro globally via a -D flag in the compilation command.
• Define the macro in the library header file.

Arduino IDE 2.x does not provide an interface for the user to define arbitrary global macros so the former is only possible for Arduino IDE users by modifying the compilation command pattern in the boards platform. So the latter approach will be the best option for Arduino IDE users:

1. Open the MqttClient.h file of your local installation of the library in any text editor.
2. Change line 36 from this:
   

   ArduinoMqttClient/src/MqttClient.h

   Line 36 in f257432

   // #define MQTT_CLIENT_STD_FUNCTION_CALLBACK

   
   To this:

   #define MQTT_CLIENT_STD_FUNCTION_CALLBACK

3. Save the file.

If you would like further assistance with the configuration of the library or the usage of this feature, you are welcome to post over on Arduino Forum:

https://forum.arduino.cc/

I'm sure we'll be able to help you out over there.

from arduinomqttclient.