I think I've found the problemo:
fd(4) states that devfs only maintains the standard I/O descriptors by default, and points to fdescfs (5).
So that provides a solution to my problem, if not a totally portable one :-/
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Thou shalt check the array bounds of all strings (indeed, all arrays), for surely where thou typest ``foo'' someone someday shall type ``supercalifragilisticexpialidocious''.