Hi Dennis,
Great that you have found some existing code to play with. And kudos to @gigertron for sharing your code with the community. However, the code on GitHub was written for the EyeX Engine developer preview 0.8.3 which is not supported anymore.
In EyeX Engine 0.8.14 the API related to callbacks has been changed to be aligned for all kinds of callbacks. To commit a snapshot, use txCommitSnapshotAsync. To get a result code, use txGetAsyncDataResultCode. Please check out the MinimalGazeDataStream.c file in the EyeX SDK C++ sample and update Gigertron’s code accordingly. Bonus points if you fork and push on GitHub 🙂