How does objc_msgSend work internally in Objective-C runtime?

objc_msgSend is the core runtime function that performs dynamic method dispatch using selectors and method lookup tables.

When you call a method like [obj doSomething], the compiler converts it into objc_msgSend(obj, @selector(doSomething)). The runtime then performs fast path lookup in the class's cache. If not found, it searches method lists up the inheritance chain. If still not found, it triggers dynamic resolution or message forwarding. This system enables Objective-C's dynamic behavior but introduces overhead compared to static dispatch languages.