__gnu_cxx::stdio_sync_filebuf(3cxx) on Linux
The text 825bab4623e7, as Debian 10 (Buster) 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>.