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
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