Asynchronous IO

[FIRTH at TL-20B.ARPA]

Please forgive this stupid question, since I'm sure the answer must be
obvious - it's just that I can't find it and would appreciate a pointer
(or clue)

What is the best way to achieve asynchronous IO under "vanilla" Unix?
The essential requirements are

(a) a program can issue an IO request and continue running while the IO
    is done in parallel

(b) the program can arrange that a particular routine (conventionally
    called a "completion routine") is invoked when the IO is finished

Many thanks - Robert Firth
-------