These rules are intended to answer awkward questions.
"w",
"t" and "p".
"r"
or "rw".
"w" descriptors are automatically changed to "r"
descriptors.
If the function you are passing the descriptor to uses WIO (see
§3.8.6), then "p" descriptors become "t".
If the function you are passing the descriptor to uses PIO, then "p"
descriptors are unchanged.