At some point, the question of "well-defined C program" is philosophical. Let's get down to brass tacks: If someone sat down and wrote this, would it happily co

At some point, the question of "well-defined C program" is philosophical. Let's get down to brass tacks: If someone sat down and wrote this, would it happily compile? Would it compile under some compilers and configurations but not others? Would it throw warnings instead of errors under some configurations? Which of these configurations are the default configurations? Which non-default configurations are considered best practices? How complex are the best practices, and how widespread is knowledge about them? In short, how much does the concept of a "well-defined C program" differ from the concept of a "C program" as implemented in practice? Can we say that most C programs, or even a medium-sized percentage of C programs, are well-defined?