What is the difference between flush() and sync() in regard to fstream buffers?

basic_ostream::flush This is a non-virtual function which writes uncommited changes to the underlying buffer. In case of error, it sets an error flag in the used stream object. This is because the return value is a reference to the stream itself, to allow chaining.

basic_filebuf::sync This is a virtual function which writes all pending changes to the underlying file and returns an error code to signal success or failure.

endl This, when applied to an ostream, writes an '\n' to the stream and then calls flush on that stream.

So, essentially: flush is a more general function for any stream, whereas sync is explicitly bound to a file. flush is non-virtual, whereas sync is virtual. This changes how they can be used via pointers (to base class) in the case of inheritance. Furthermore, they differ in how they report errors.

sync is a member of input streams, all unread characters are cleared from the buffer. flush is a member of output streams and buffered output is passed down to the kernel.