The tree makes noise, but we need a test to listen for it

Many years ago I was on the ABI stability team at Sun, and we actively tried to test all the "must be true" assertions in library interfaces like this. The easiest were changes in the nature of parameters: the hardest were changes in the meanings of otherwise unchanged parameters.

Ones like this we would have considered either "uninterpreted opaque cookie" (good) or "must be zero" (bad). Finding the latter usually resulted in version-number changes, weird backward-compatibility tests to see if anyone had stolen the upper half for anything and debates about how to change the documentation.

As a side effect, we tried to write a static test or shared-library test we could run against any apps that were being compatibility-tested by their vendors, and we recorded the difference in our porting database, so that if another vendor or an old SunOS system used it, it would get fixed in a port.

The latter, IMHO, was the really valuable part: if Hockey-PUX had the bug, we'd refrain from reproducing it on applications ported to Solaris. Linux rarely has such bugs: other vendors (including ourselves) weren't as fortunate.

--dave