int io_fd(int64 fd);
If you want to use io_canread() and io_canwrite() on a descriptor, you have to use io_wait() on it first, and io_wait() has to know which descriptors you are interested in. Use io_fd() for this.
io_pipe and io_socketpair already call io_fd for you.
Waiting on descriptors only works for sockets, fifos and pipes. It may also work on devices and TTYs, but that is platform dependent -- you should not rely on that. It does not work on files.
io_fd returns 1 on success, 0 on error.
io_wait(3), io_wantread(3), io_canread(3), io_eagain(3), io_nonblock(3)
io_dontwantread(3), io_dontwantwrite(3), io_fd_canwrite(3), io_fd_flags(3), io_wait(3), io_waituntil(3), io_wantread(3), io_wantwrite(3).