this app was developed and works fine under ios 5.0, but crashes under ios 4.3

Question

I developed an iPhone app under iOS 5.0, and it works fine. But when it comes to iOS 4.3(Base SDK = latest iOS 5.0, compiler = Apple LLVM 3.0, Deployment Target = iOS 4.3),

Answer 1

At runtime, lots of methods are called that are not in your code, but which are called behind the scenes as a result of the API calls you have made.

I would focus not on the method that is being called, but on why the object it is sent to is unable to respond to the selector. The object could have been cast as the wrong type, and so is not inheriting the right methods. (In the code snippet you show, you don't explicitly cast AVAudioSession *audio_session.) The other direction is to check that you're not using some other API call that is iOS 5 only, which in the background is calling this method and thus generating the error.

Finally, if you're only recently changed your build target to include iOS 4.3, you may simply need to do a clean build (Product > Clean) so that it compiles iOS 4.3-compatible code.