What is a non-fragile ABI?

Question

It may seem implied that everyone knows what a "Non Fragile ABI" is - considering the frequency and matter-of-fact-nature to which it is referred to - within Xcode. For example...

Subscript requires size of interface node which is not constant in non-fragile ABI

or

Select the Objective-C ABI version to use. Available versions are 1 (legacy "fragile" ABI), 2, (non-fragile ABI 1), and 3 (non-fragile ABI 2).

That said... What is a non-fragile ABI? (and why isn't it called something less-abstract / explained more clearly?)

Answer 1

The non-fragile ABI refers to the ability to add instance variables to a class without requiring recompilation of all subclasses.

I.e. in v1 (there really aren't true versions of ObjC), if Apple were to add an instance variable to, say, NSView (on Cocoa, 32 bit), then every subclass of NSView (or subclass of subclass) would have to be recompiled or they would blow up. v2 and v3 fix this.

It is explained in detail in this weblog post.

The documentation you are referring to is in the llvm/clang man page. Pretty rare place to be for most developers most of the time; unless you are writing a Makefile that is driving the compiler directly, there isn't much reason to read that page (unless spelunking -- which is quite educational, of course).

It is written in the style of a Unix man page and, no surprise, is a bit... obtuse. For almost all tasks, it is best to stick to the higher level documentation. I.e. the Xcode build settings documentation is general quite a bit less obtuse.