Warning: invalid WbDeviceTag in API function call

[omichel]

This warning is popping up in the Webots console when I execute my controller program, what is the problem?

[omichel]

A WbDeviceTag is an abstract reference (or handle) used to identify a simulated device in Webots. Any WbDeviceTag must be obtained from the wb_robot_get_device function. Then, it is used to specify a device in various Webots function calls. Webots issues this warning when the WbDeviceTag passed to a Webots function appears not to correspond to a known device. This can happen mainly for three reasons:

1. The WbDeviceTag is 0 and thus invalid because it was not found by the wb_robot_get_device function call. Indeed, the wb_robot_get_device function returns 0, if it cannot not find a device with the specified name in the robot. Note that the name specified in the argument of the wb_robot_get_device function must correspond to the name field of the device, not to the VRML97 DEF name!
2. Your controller code is mixing up two types of WbDeviceTags, for example because it uses the WbDeviceTag of a Camera in a wb_distance_sensor_* function. Here is an example of what is wrong:

#include <webots/robot.h>
#include <webots/camera.h>
#include <webots/distance_sensor.h>

#define TIME_STEP 32

int main() {
  wb_robot_init();
  WbDeviceTag camera = wb_robot_get_device("camera");
  wb_camera_enable(camera, TIME_STEP);
  ...
  double value = wb_distance_sensor_get_value(camera);  // WRONG!
  ...
}

3. The WbDeviceTag may also be invalid because it is used before initialization with the wb_robot_get_device function call, or because it is not initialized at all, or because it is corrupted by a programming error in the controller code. Here is such an example:

    #include <webots/robot.h>
    #include <webots/camera.h>
    #include <webots/distance_sensor.h>

    #define TIME_STEP 32

    int main() {
      wb_robot_init();
      WbDeviceTag distance_sensor, camera = wb_robot_get_device("camera");
      wb_camera_enable(camera, TIME_STEP);
      wb_distance_sensor_enable(distance_sensor, TIME_STEP);  // WRONG!
      ...
    }