For those of us who are in the no-ABI-breakage business (I used to be when I worked on JavaScriptCore, which is a framework on Apple platforms and folks dynlink

For those of us who are in the no-ABI-breakage business (I used to be when I worked on JavaScriptCore, which is a framework on Apple platforms and folks dynlink to it), any behavior change that causes an app that has nontrivial number of users to break is considered an ABI break. Some function in your library used to return 42 and now returns 43, and an app with 10,000 users asserted that you returned 42? That's an ABI break. There are more obvious examples: renaming functions that someone linked to? ABI break. Change the size of a struct in a public header? ABI break, usually (there are mitigations for that one). The list goes on and Hyrum's law very much applies. Another way to define ABI compatibility is that folks who promise it are taking the same exact stance with respect to folks who link against them as the stance that Linus takes regarding not breaking userspace: https://lkml.org/lkml/2012/12/23/75 https://lkml.org/lkml/2012/12/23/75 If I promise you ABI compat, and then I make a change and your app breaks, then I'm promising that it's my fault and not yours.