io_canread returns the next file descriptor that can be read from. You have to have used io_wantread() on the file descriptor earlier, and you have to have called io_wait() or io_waituntil().
These functions then keep an internal data structure on which descriptors were reported readable by the operating system.
Please note that there is no guarantee that there still is data that can be read from the descriptor, just that there was data when io_wait() or io_waituntil() were called. Another process could have read the data before you. Look at the result from io_tryread().
If there are no more descriptors that you can write to without blocking, io_canwrite will return -1. In this case you should call io_wait() or io_waituntil() again.
You should use io_tryread(3) to read from the descriptor, not plain read(2). If you use read(2) and you get EAGAIN, call io_eagain(3).
io_wait(3), io_canwrite(3), io_eagain(3)
io_dontwantread(3), io_fd(3), io_fd_canwrite(3), io_wantread(3).