This can be done in a few ways. The most simple would be:
return (EOK);
But you'll often see:
return (_RESMGR_NPARTS(0));
Note that in neither case are you causing the MsgSend() to return with a 0. The value that the MsgSend() returns is the value passed to the _IO_SET_READ_NBYTES(), _IO_SET_WRITE_NBYTES(), and other similar macros. These two were used in the read and write samples above.