sensor_task.cpp

void sensor_task(void) {
  //  Background task to receive and process sensor data.
  //  This task will be reused by all sensors: temperature, humidity, altitude.
  SensorContext *context = NULL;  //  Declared outside the task to prevent cross-initialisation error in C++.
  MsgQ_t queue; Evt_t event;  //  TODO: Workaround for msg_post() in C++.
  task_open();  //  Start of the task. Must be matched with task_close().
  for (;;) {  //  Run the sensor processing code forever. So the task never ends.
    //  This code is executed by multiple sensors. We use a global semaphore to prevent 
    //  concurrent access to the single shared I2C Bus on Arduino Uno.
    sem_wait(i2cSemaphore);  //  Wait until no other sensor is using the I2C Bus. Then lock the semaphore.
    //  We have to fetch the context pointer again after the wait.
    context = (SensorContext *) task_get_data();
    
    //  Prepare a display message for copying the sensor data.
    DisplayMsg msg;
    msg.super.signal = context->sensor->info.id;  //  Set the ID.
    memset(msg.name, 0, maxSensorNameSize + 1);  //  Zero the name array.
    strncpy(msg.name, context->sensor->info.name, maxSensorNameSize);  //  Set the sensor name e.g. tmp
    
    //  Poll for the sensor data and copy into the display message.
    msg.count = context->sensor->info.poll_sensor_func(msg.data, maxSensorDataSize);
    //  Do we have new data?
    if (msg.count > 0) {
      //  If we have new data, send the message. Note: When posting a message, its contents are cloned into the message queue.
      msg_post(context->display_task_id, msg);
    }
    //  We are done with the I2C Bus.  Release the semaphore so that another task can fetch the sensor data.
    sem_signal(i2cSemaphore);
    //  Wait a short while before polling the sensor again.
    task_wait(context->sensor->info.poll_interval);
  }
  task_close();  //  End of the task. Should never come here.
}