Strange XCode error when switching between XCode 3 and 4 - fixed

I have a machine at home running XCode 4 and a VMware virtual machine on my laptop running XCode 3. When I have to load the projects on different version on XCode, it gives me some strange error.

Opening XCode 4 saved projects on XCode 3 is easy, just change the "Project Properties" and choose the correct SDK.

But when I tried to open XCode 3 saved projects on XCode 4, as below it gives me tones of error "LLVM GCC 4.2 Error" "Operator '<' has no left operand", and the file that got those error is "usr/include/AvailabilityInternal.h" which I never heard of...

Took me a while to work out the fix. As shown below, I have to change the "Base SDK" to "Latest iOS(iOs 4.3)" and then it start working...

Posted in: troubleshooting skills,tutorial