Forces a write of all user-space buffered data for the given this.
Will block during the operation. Closing the stream will implicitly cause a flush.
This function is optional for inherited classes.
cancellable is not null, then the operation can be cancelled by triggering the cancellable
object from another thread. If the operation was cancelled, the error g_io_error_cancelled will be returned.
optional cancellable object
true on success, false on error