no-op error in dmesg

Hi, I have one structure called foo. Address of object of foo(say objfoo) is passed to ioctl call status = ioctl(fd, arg, &objfoo); But during manipulation I am not getting ioctl error 5 (input/output error). I am more confused about error in dmesg which is *foo: no-op * What does no-op indicate? If it is no-operation, how can we relate this to foo structure? Thanks, Pritam