io_fd - prepare descriptor for io_wait

       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 - prepare descriptor for io_wait

       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_fd(3)

#include<libowfat/io.h>

       int io_fd(int64 fd);