21–30 of 233 posts
> That's the whole list. No, I didn't forget any items. Yes, seriously.
I’m a little disappointed there isn’t an empty or in the middle there. I imagine screen readers would announce something like “list, zero items”.
14. Okay, but if the line with UB is unreachable (dead) code, then it's as if the UB wasn't there.
15. If the line with UB is unreachable code, then the program won't crash because of the UB.
16. If the line with UB is unreachable code, then the program will at least stop running somehow and at some point.
This is nonsense. The line with UB has to be reached for the execution to be undefined.
> Undefined behavior: Anything is allowed to happen, and you might no longer have a computer left after it all happens. and > The moment your program contains UB, all bets are off. Does this include that a compiler, when it can prove that a program it compiles contains UB, erase your disk as part of the compilation process? i.e. without you ever running the compiled program.
The C specification doesn't say anything about what a compiler may or may not do during compilation. It only specifies how the compiled program should behave. So, yes, the C specification allows a standard-conforming compiler to erase your disk during compilation. It even allows it to do that if your code doesn't contain UB.
> "If my program contains UB, and the compiler produced a binary that does X, is that a compiler bug?" It's not a compiler bug. True, if the language spec is the only thing in the universe you care about. But there are better and worse things compilers can do in the presence of UB, and the big one is warning you about it when it possibly can . There's another big discussion on the front page about signed integer over…
GCC should add -Wundefined-behaviour flag that will unconditionally print "warning: this program might or not have UB" as the first diagnostic.
> 14. Okay, but if the line with UB is unreachable (dead) code, then it's as if the UB wasn't there. This one came as a surprise to me. Is this really (unconditionally) true? And if so, why? Seems trivial to me to ensure that UB that will never under any circumstance execute will not affect the functioning of an otherwise correct program.
13. But if the line with UB isn't executed, then the program will work normally as if the UB wasn't there. 14. Okay, but if the line with UB is unreachable (dead) code, then it's as if the UB wasn't there. 15. If the line with UB is unreachable code, then the program won't crash because of the UB. 16. If the line with UB is unreachable code, then the program will at least stop running somehow and at some point. This…
> "If my program contains UB, and the compiler produced a binary that does X, is that a compiler bug?" It's not a compiler bug. True, if the language spec is the only thing in the universe you care about. But there are better and worse things compilers can do in the presence of UB, and the big one is warning you about it when it possibly can . There's another big discussion on the front page about signed integer over…
> there are better and worse things compilers can do in the presence of UB, and the big one is warning you about it when it possibly can. GCC should add -Wundefined-behaviour flag that will unconditionally print "warning: this program might or not have UB" as the first diagnostic.
It is not explained why points 31 to 36 are valid, unlike HDL synthesis tools, C compilers are pretty deterministic, same input same output, except timestamps, etc. If an UB adopt X behavior one time, a reproducible build will take same X behavior the next time. Of course I am not negating the fact that this undefined in the first instance nor condoning the UB usage.
I am always amazed by articles like these. This guy thoroughly covers this topic. I cannot say I have ever written anything that ran into or caused undefined behavior. And I have programmed for a long time. Is this just an interesting topic or do people actively push the compiler to see what it will do? Maybe I am a boring/simple programmer. :)
But how would you know? :)
I am always amazed by articles like these. This guy thoroughly covers this topic. I cannot say I have ever written anything that ran into or caused undefined behavior. And I have programmed for a long time. Is this just an interesting topic or do people actively push the compiler to see what it will do? Maybe I am a boring/simple programmer. :)
I suspect every C programmer invokes undefined behavior in their first 50 hours of writing C.