Sometimes a default function will be of no help for your particular resource manager. For example, iofunc_read_default() and iofunc_write_default() functions implement /dev/null — they do all the work of returning 0 bytes (EOF) or swallowing all the message bytes (respectively).
You'll want to do something in those handlers (unless your resource manager doesn't support the _IO_READ or _IO_WRITE messages).
Note that even in such cases, there are still helper functions you can use: iofunc_read_verify() and iofunc_write_verify().