xcode 4.0.2 won't rebuild older projects for iPhone

I have installed xcode 4.0.2. I open my old project writtent for xcode 3. I try to build and get several errors of type: "Apple开发者_JAVA百科 Mach-O Linker *Id) Error" Lots of symbols such as _OBJC_CLASS_$_NSAutoReleasePool are not known. It says that they are "Undefined symbols for i386"

I had this same issue opening an old project with a newer version of XCode / newer operating system. I had a project written in 10.5 and Xcode 3 and when I tried to compile and run in 10.6 I had very similar errors. Chances are that you did not update your target's Build Settings. Go into XCode and click on your target (the blueprint image at the top that has your projects name) then make sure you change all the values to what your system now has under "Build Settings", make sure you change "Base SDK", "Compiler for C/C++/Objective C","iOS deployment target" and "Mac os x Deployment target" to what you are trying to build for / the latest your system has. For most of these settings you can chose "Compiler Default" and that will put you in the clear.

Happy Coding!

Try to clean it and see if all your frameworks are all still there.