From: already5chosen@yahoo.com   
      
   On Fri, 09 Jan 2026 01:42:53 -0800   
   Tim Rentsch  wrote:   
      
   > highcrew  writes:   
   >   
   > > Hello,   
   > >   
   > > While I consider myself reasonably good as C programmer, I still   
   > > have difficulties in understanding undefined behavior.   
   > > I wonder if anyone in this NG could help me.   
   > >   
   > > Let's take an example.  There's plenty here:   
   > > https://en.cppreference.com/w/c/language/behavior.html   
   > > So let's focus on https://godbolt.org/z/48bn19Tsb   
   > >   
   > > For the lazy, I report it here:   
   > >   
   > >   int table[4] = {0};   
   > >   int exists_in_table(int v)   
   > >   {   
   > >       // return true in one of the first 4 iterations   
   > >       // or UB due to out-of-bounds access   
   > >       for (int i = 0; i <= 4; i++) {   
   > >           if (table[i] == v) return 1;   
   > >       }   
   > >       return 0;   
   > >   }   
   > >   
   > > This is compiled (with no warning whatsoever) into:   
   > >   
   > >   exists_in_table:   
   > >           mov     eax, 1   
   > >           ret   
   > >   table:   
   > >           .zero   16   
   > >   
   > >   
   > > Well, this is *obviously* wrong.  And sure, so is the original code,   
   > > but I find it hard to think that the compiler isn't able to notice   
   > > it, given that it is even "exploiting" it to produce very efficient   
   > > code.   
   > >   
   > > I understand the formalism:  the resulting assembly is formally   
   > > "correct", in that UB implies that anything can happen.   
   > > Yet I can't think of any situation where the resulting assembly   
   > > could be considered sensible.  The compiled function will   
   > > basically return 1 for any input, and the final program will be   
   > > buggy.   
   > >   
   > > Wouldn't it be more sensible to have a compilation error, or   
   > > at least a warning?  The compiler will be happy even with -Wall   
   > > -Wextra -Werror.   
   > >   
   > > There's plenty of documentation, articles and presentations that   
   > > explain how this can make very efficient code... but nothing   
   > > will answer this question:  do I really want to be efficiently   
   > > wrong?   
   > >   
   > > I mean, yes I would find the problem, thanks to my 100% coverage   
   > > unit testing, but couldn't the compiler give me a hint?   
   > >   
   > > Could someone drive me into this reasoning?  I know there is a lot   
   > > of thinking behind it, yet everything seems to me very incorrect!   
   > > I'm in deep cognitive dissonance here! :)  Help!   
   >   
   > The important thing to realize is that the fundamental issue here   
   > is not a technical question but a social question.  In effect what   
   > you are asking is "why doesn't gcc (or clang, or whatever) do what   
   > I want or expect?".  The answer is different people want or expect   
   > different things.  For some people the behavior described is   
   > egregiously wrong and must be corrected immediately.  For other   
   > people the compiler is acting just as they think it should,   
   > nothing to see here, just fix the code and move on to the next   
   > bug.  Different people have different priorities.   
   >   
      
   I have hard time imagining sort of people that would have objections in   
   case compiler generates the same code as today, but issues diagnostic.   
   Probably in the same style that it often produces in similar situations:   
    warning: array subscript 4 is above array bounds of 'int[4]'   
    [-Warray-bounds]   
      
   > After observing that, I think the right question is something like   
   > "Given that compilers act in these surprising ways, how should I   
   > protect my code so that it doesn't fall prey to the death-by-UB   
   > syndrome, or what can I do to diagnose a possibly death-by-UB   
   > situation when a strange bug crops up?"  I don't pretend to have   
   > good answers to these questions.  The best advice I can give   
   > (besides seeking help from others with more experience) is to be   
   > persistent, and to realize that the skills needed for combating a   
   > death-by-UB syndrome are rather different from the skills needed   
   > for regular programming.  I have been in the situation of being   
   > made responsible for finding and correcting a death-by-UB kind of   
   > symptom, and what's worse in programming environment where I   
   > didn't have a great deal of familiarity or experience.  Despite   
   > those drawbacks the bug got diagnosed and fixed, and I attribute   
   > that result mostly to tenacity and by being willing to consider   
   > unusual or unfamiliar points of view.   
      
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