io.c:599
static VALUE
rb_io_fsync(io)
    VALUE io;
{
#ifdef HAVE_FSYNC
    OpenFile *fptr;
    FILE *f;

    GetOpenFile(io, fptr);
    rb_io_check_writable(fptr);
    f = GetWriteFile(fptr);

    io_fflush(f, fptr);
    if (fsync(fileno(f)) < 0)
	rb_sys_fail(fptr->path);
    return INT2FIX(0);
#else
    rb_notimplement();
    return Qnil;		/* not reached */
#endif
}
