When AppendFile(), and WriteFile() have errors, it stops program using exit(1).
But in this case, it does not call fclose(), fsync().
So, it might cause problems such as losing file content, or leave broken file.

This functions should be designed carefully because it regards to the log files.

How about using return() instead of exit()?
At least, exit() should be called after close().

By the way, how should we share program issues? Should we use this github?