Comment by lelanthran

4 hours ago

> The point of the post is to say it's not possible to avoid them. Or at least, no human since the invention of C in 1972 has.

What are you talking about? UB was coined only in the first C standard, in 1989. Prior to that there was no "If you do this, anything can happen". It was "If you do this, that will happen".

3 comments

lelanthran

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professoretc  3 hours ago

More like, "if you do this, what happens depends on your particular combination of hardware, operating system, and compiler. Don't ask us."

  • nickez  2 hours ago

    No, that would be implementation defined.

    • tekne  17 minutes ago

      No, that's actually UB. The important bit here is "compiler defined" -- UB means the compiler is allowed to assume it never happens while compiling.

      Consider, for example, an implementation defined function f() -- which can also diverge/crash horribly, etc.

      If I write

          if p {
      print("p is true")
    } else {
      g()
    }

    if p {
      f()
    }

      Then either we: - print p is true and execute f - do nothing

      This is true regardless of if f immediately crashes the computer, nasal demons, whatever -- that's implementation defined.

      UB means f may never happen.

      And that means the compiler may optimize this to just:

          g()

      Notice the difference here -- the print never happens!, and g always happens.

      You can see why this is concerning when you write code like

          if dry_run {
      print("would run rm -rf /")
    } else {
      run("rm -rf /")
    }

    if dry_run {
      // oops: some_debug_string is NULL and will segfault!
      print(some_debug_string);
    }