1.3.1 What is the Objective C Runtime?

The Objective C Runtime Library provides C functions and data structures required to execute an Objective C program.

The GNU Objective C Runtime Library offers everything NeXT’s runtime does, including Categories, Protocols, ‘+poseAs:’, thread-safety, class initialization on demand, delayed loading of classes, and initialization of static instances (such as @""-style string objects).

It also has several differences over NeXT’s implementation: