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The registerable I/O channels are objects of the ‘file’ type.
See The File Library.
I/O polling best applies to files operating in nonblocking mode and files operating in other modes may not be allowed for registration. File operation mode tests are performed also before data transfer.
See Nonblocking I/O.
The registered ‘file’ objects are retained and they will survive even if no other reference is made to them. They may be deregistered explicitly and they will be deregisted automatically in certain events.
Input files are registered together with a function and the arguments for the function. When input is available, the recorded function is called.
Output files are polled until drained, then automatically deregistered.