[dockd]

I would go one step farther: The documentation will say it is undefined behavior but the compiler doesn't have to. Here's an example from the man page for sprintf

  sprintf(buf, "%s some further text", buf);

If you miss that section of the manual, your code may work, leading you to think the behavior is defined.

Then you will have interesting arguments with other programmers about what exactly is undefined behavior, e.g. what happens for

  sprintf(buf, "%d %d", f(i), i++);

[krylon]

I remember reading a blog post a couple of years back on undefined behavior from the perspective of someone building a compiler. The way the standard defines undefined behavior (pun not intended), a compiler writer can basically assume undefined behavior never occurs and stay compliant with the standard.

This offers the door to some optimizations, but also allows compiler writers to reduce the complexity in the compiler itself in some places.

I'm being very vague here, because I have no actual experience with compiler internals, nor that level of language-lawyer pedantry. The blog's name was "Embedded in academia", I think, you can probably still find the blog and the particular post if it sounds interesting.

[kllrnohj]

Yeah a decent chunk of UB is about reducing the burden on the compiler. Null derefs being an obvious such example. If it was defined behavior, the compiler would be endlessly adding & later attempting to optimize-away null checks. Which isn't something anyone actually wants when reaching for C/C++.

Similarly with C/C++ it's not actually possible for the compiler to ensure you don't access a pointer past the end of the array - the array size often isn't "known" in a way the compiler can understand.

[lmm]

> Which isn't something anyone actually wants when reaching for C/C++.

Disagree. I think a lot of people want some kind of "cross-platform assembler" (i.e. they want e.g. null deref to trap on architectures where it traps, and silently succeed on architectures where it succeeds), and get told C is this, which it very much isn't.

[pjmlp]

Except every other sane systems programming language does indeed do null checks, even those older than C, but they didn't come with UNIX, so here we are.

[Sohcahtoa82]

I'll tell you what happens when someone writes:

      sprintf(buf, "%d %d", f(i), i++);

They get told to rewrite it.

[Koshkin]

Good point, actually. Many cases of undefined behavior are clearly visible to an experienced C programmer when they review someone else’s code.

[pjmlp]

By whom? Most places still don't do proper code reviews or unit testing.