Monday, June 1, 2009

Static Initialization Check Feature

While thinking about how to do 3rd party types for my sterilization library, I ran into an interesting problem; how to verify that boot time initialization is done correctly.

Some cases are easy, for instance, where the initialization is done by the same code author that defines the validity check. In that case the checks can just be appended to the initialization code.

The problem cases is where the two bits are separated. This is a sort of punting model where the first author "punts" by throwing out some state variable and expecting someone (a second author) to set them up correctly. In this case you have the problem of where to put the checks. If you put it in a static constructor in the module with the state variables, then it ends up running before any static constructors in the modules that could have set the values. Another option is having a test function that the second author needs to call, but they could forget. A third option would be to have a function that gets called at the top of main, err, yuck. The option I think I'll go with is to check that things are correct on the tare down and then force an immediate teardown at some point as part of the test rig.

What I'd really like is some sort of delayed assert that runs after all the static thiss but before main. Of course then I'll want something between that and main... (Yet another example of why to never have more than two levels of operation if you can avoid it) To avoid that issue it could be restricted to provably side effect free expressions. Given that in my case all I want to do is check that a global is non null this would be just fine for.

Of course, in my case what I'd really like is static whole program optimization and analysis to, where possible, rip out static constructors in favor of literal data segments and replace the checks I'm taking about with compile time checks. But now I'm just dreaming.

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