__gnu_cxx::stdio_sync_filebuf(3cxx) on Linux
The text 7c62b70ba95b, as Debian 9 (Stretch) shipped it.
Provides a layer of compatibility for C. This GNU extension provides extensions for working with standard C FILE*'s. It must be instantiated by the user with the type of character used in the file stream, e.g., stdio_filebuf<char>.