Tracking messages

Tech Talk
1

Documentation says that providing mosquitto_publish() with pointer to mid allows tracking for completion of the message sending. I am making architecture for serial message sending to prevent data loss in memory in case of power failure. So generally I am expected to take value of mid, and have some code in the publish callback comparing this message id to the arriving callbacks, and raise a flag of completion.
The problem here I can’t clear is one thread calling mosquitto_publish() and another thread performing manual mosquitto looping. Therefore callbacks are initiated in another thread than mosquitto_publish().

Example:

Thread 1:

volatile sig_atomic_t midPublish;

void somefunc(void)
{
   ...
   mosquitto_publish(..., &midPublish, ...);
   ...
}

Thread 2:

extern volatile sig_atomic_t midPublish;

void publish_callback(struct mosquitto *mosq, void *userdata, int mid)
{
    ....
    if(mid == midPublish)
    {
        ...
    }
    ...
}

The compiler says

warning: passing argument 2 of ‘mosquitto_publish’ discards ‘volatile’ qualifier from pointer target type

and it is actually correct. While atomicity can be achieve here on this platform, volatility seems can’t and I am afraid of effects of just casting &(int*)midPublish will lead to actual optimized code not to get proper values from the variables/related memory locations.

Any advice?

Edit: one of the possible solutions is to synchronize threads by one thread passing data to publish onto the MQTT thread, and then MQTT thread in its manual loop calling mosquitto_publish() with following mosquitto_loop() call therefore publish and callback will be physically separate in their execution time.