> In ye olden times, the C standard was considered guidelines rather than a ruleset, undefined behavior was closer to implementation-defined behavior than dark

> In ye olden times, the C standard was considered guidelines rather than a ruleset, undefined behavior was closer to implementation-defined behavior than dark magic, and optimizers were stupid enough to make that distinction irrelevant. On a majority of platforms, dereferencing a null pointer compiled and behaved exactly like dereferencing a value at address 0. Let me unpack that for you. Old compilers didn't recognise undefined behaviour, and so compiled the code that triggered undefined behaviour in exactly the same way they compiled all other code. The result was implementation defined, as the article says. Modern compilers can recognise undefined behaviour. When they recognise it they don't warn the programmer "hey, you are doing something non-portable here". Instead they may take advantage of it in any way they damned well please. Most of those ways will be contrary to what the programmer is expecting, consequently yielding a buggy program. But not in all circumstances. The icing on the cake is some undefined behaviour (like dereferencing null pointers) is tolerated (ie treated in the old way), and some not. In fact most large C programs will rely on undefined behaviour of some sort, such as what happens when integers overflow or signed is converted to unsigned. Despite that, what is acceptable undefined behaviour and what is not isn't defined by the standard, or anywhere else really. So the behaviour of most large C programs is it legally allowed to to change if you use a different compiler, a different version of the same compiler, or just different optimisation flags. Consequently most C programs depend on the compiler writers do the same thing with some undefined behaviour, despite there being no guarantees that will happen. This state of affairs, which is to say having a language standard that doesn't standardise major features of the language, is apparently considered perfectly acceptable by the C standards committee.