Tell us about the Objective-C runtime.
What the runtime is
The Objective-C runtime is a C library (libobjc, headers <objc/runtime.h>, <objc/message.h>) that implements the language's object model at execution time: classes, objects, selectors, message sending. The compiler translates the [obj msg] syntax into runtime function calls, and all "which code runs" decisions are postponed until launch. That is what the "dynamism" of Obj-C means.
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