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A program that doesn't communicate with the outside world in some way is useless although very safe. Such a program might almost be in solitary confinement. A prisoner in solitary confinement is safe in the sense that he cannot hurt other people but he is equally of no use to society either.
So for a program to be useful it must communicate. And if the program is written in a safe way so that it does not have internal dangers, it is largely futile if its communication with the world is unsafe. So safety in communication is important since it is here that the program truly has a useful effect.
It is perhaps worth recalling from the introduction that we characterized the difference between safety-critical and security-critical systems as that the former is where the program must not harm the world whereas the latter is where the world must not harm the program. So communication is the ultimate lynchpin of both safety and security.
Read Chapter 9 in full
Note: All chapters of this booklet will, in time, be available on the Ada 2005 home page.
- Ch.9 - Safe Communication - (688 KB)