I don't think the standard (C99) uses the term "undefined" in its description of the gets() function. (Also gets has been removed from the standard in C11.) But

I don't think the standard (C99) uses the term "undefined" in its description of the gets() function. (Also gets has been removed from the standard in C11.) But maybe it uses the term to describe what happens when you change memory that shouldn't be changed. Well, what do you think the C standard should say in such a case, without knowledge of the system you're running on? There are of course some functions where it would have been nice to allow passing in an invalid value and the function just returning its normal error or "no" return value. E.g. isalnum might be nicer if it didn't crash for input > 255. (But perhaps it's not possible in all implementations without adding some other constraints that didn't exist before.)