I think it would be reasonable to change the specification for io_read to say that the offset parameter must be exactly (off_t)-1 for a nonseekable io object, and that otherwise it must fail with ESPIPE. We might need to audit the existing uses to make sure there aren't uses that would violate this new rule, but that's probably not too hard to find and fix up.
Under POSIX, any negative argument to pread is invalid and so the pread function can diagnose that without making any RPC. Then, pread will call io_read with its nonnegative argument, and this will get ESPIPE from the io server for a nonseekable object. Thanks, Roland
