It's perhaps worth saaying that 30 years ago code in the wild might well not have compiled and run correctly on a different system, and is more likely to today. There tended to be more non-standard features and ignoring of the standard -- specifically storage association and order of evaluation rules, which people who hadn't read the standard would swear weren't in it. Also, there are a few things which have been dropped from the standard, though not necessarily from compilers.