If they do, that is no longer an implementation of C. It is a dialect of C, and there are many (GNU C being the most popular), but there are real drawbacks to using dialects.

This is in contrast to the other category that exists, which is "implementation-defined".

▲

1718627440 an hour ago | parent [-]

> If they do, that is no longer an implementation of C.

This is plain wrong. Undefined behaviour, means the C standard specifies no restriction on the behaviour of the program, which is what the implementation chooses to emit. An implementation can very well choose to emit any program it pleases, including programs that encrypt your harddisk, but also programs that stick to well defined rules.

	▲	simonask 37 minutes ago \| parent [-]
		Sure, but the point is that code written against such a compiler is not C and is not portable. It is written in a dialect of C, and that comes with drawbacks. Writing C (or any language) means adhering to the standard, because that's the definition of the language.