> But only user code is required to be well-formed and have defined behavior, system headers could just as well be written in Forth as long as the actual behavi

> But only user code is required to be well-formed and have defined behavior, system headers could just as well be written in Forth as long as the actual behavior under the compiler they come with is correct. To understand this better, realize that, ultimately, a standard is a contract. A standard says, in part, "If you refrain from doing these things, the system promises to do these things and not do these other things here." "Undefined behavior", then, is invoked by code that strays from the standard by doing things the standard makes no guarantees about, like casting a value of type pointer-to-double to type pointer-to-char and dereferencing the result; the behavior of the compiler and the code the compiler emits is then undefined, which is to say its behavior is not guaranteed by the standard. And, yes, a conformant C compiler is perfectly within its rights to delete code that it can tell invokes undefined behavior.