Compile time
1. The program need
not satisfy any invariants. In fact, it needn't be a well-formed program at
all. You could feed this HTML to the compiler and watch it barf...
2. What can go wrong
at compile time:
·
Syntax errors
·
Type checking errors
·
(Rarely) compiler crashes
3. If the compiler
succeeds, what do we know?
·
The program was well formed---a meaningful program in whatever
language.
·
It's possible to start running the program. (The program might
fail immediately, but at least we can try.)
4.
What are the inputs and outputs?
·
Input was the program being compiled, plus any header files,
interfaces, libraries, or other voodoo that it needed to import in order to get compiled.
·
Output is hopefully assembly code or relocatable object code or
even an executable program. Or if something goes wrong, output is a bunch of
error messages.
Run time
1. We know nothing
about the program's invariants---they are whatever the programmer put in.
Run-time invariants are rarely enforced by the compiler alone; it needs help
from the programmer.
2.
What can go wrong are run-time errors:
·
Division by zero
·
Dereferencing a null pointer
·
Running out of memory
Also there can be errors that are detected by the program
itself:
·
Trying to open a file that isn't there
·
Trying find a web page and discovering that an alleged URL is
not well formed
3. If run-time
succeeds, the program finishes (or keeps going) without crashing.
4.
Inputs and outputs are entirely up to the programmer. Files,
windows on the screen, network packets, jobs sent to the printer, you name it.
If the program launches missiles, that's an output, and it happens only at run
time :-)
