We record that the given id has been invoked by appending its id to the coverage data file.
We record that the given id has been invoked by appending its id to the coverage data file. This will happen concurrently on as many threads as the application is using, but appending small amounts of data to a file is atomic on both POSIX and Windows if it is a single write of a small enough string.
http://stackoverflow.com/questions/3032482/is-appending-to-a-file-atomic-with-windows-ntfs
http://stackoverflow.com/questions/1154446/is-file-append-atomic-in-unix