Regarding error codes, TurboFloat Server returns 0 on success and 1 on failure. We don't return other error codes because the types of errors/success outnumber 128. Hence the error logs to dig deeper into the type of error or success.
We do plan on adding a library that you can use to control the TurboFloat Server, but that's coming after the TFS 4.0 release.
Also, If I am killing TFS with SIGKILL (9) signal, it returns error code (127 which could be -1).
Don't do that. SIGKILL kills the process and the process cannot do anything about it. It can't shutdown cleanly. It just exits immediately. whether it's in the middle of something or not. That return code is not from the TurboFloat Server. I'd have to look into it more to tell you exactly where the return code is coming from (whether it's from SIGKILL, or it's a result of SIGKILL "crashing" the process closed).
Use SIGTERM to cleanly terminate the TurboFloat Server instance.