The newer C features that you're most likely to feel the lack of in an older compiler are: 1) Mixed declarations and code. All variables must be declared at th

The newer C features that you're most likely to feel the lack of in an older compiler are: 1) Mixed declarations and code. All variables must be declared at the top of a basic block. 2) Along similar lines, declaring variables in control statements like for(). 3) // comments. (Hardly a critical feature, but you're probably used to having them.) Perhaps not coincidentally, I think these were all features which began their life in C++, and were backported to C in C99.