Generally a good writeup, but the article seems a bit confused about undefined behavior.

> What is the dreaded UB? I think the best way to understand it is to remember that, for any running program, there are FATES WORSE THAN DEATH. If something goes wrong in your program, immediate termination is great actually!

This has nothing to do with UB. UB is what it says on the tin, it's something for which no definition is given in the execution semantics of the language, whether intentionally or unintentionally. It's basically saying, "if this happens, who knows". Here's an example in C:

    int x = 555;
    long long *l = (long long*)&x;
    x = 123;
    printf("%d\n", *l);

This is a violation of the strict aliasing rule, which is undefined behavior. Unless it's compiled with no optimizations, or -fno-strict-aliasing which effectively disables this rule, the compiler is "free to do whatever it wants". Effectively though, it'll just print out 555 instead of 123. All undefined behavior is just stuff like this. The compiler output deviates from the expected input, and only maybe. You can imagine this kind of thing gets rather tricky with more aggressive optimizations, but this potential deviation is all that occurs.

Race conditions, silent bugs, etc. can occur as the result of the compiler mangling your code thanks to UB, but so can crashes and a myriad of other things. It's also possible UB is completely harmless, or even beneficial. It's really hard to reason about that kind of thing though. Optimizing compilers can be really hard to predict across a huge codebase, especially if you aren't a compiler dev yourself. That unpredictability is why we say it's bad. If you're compiling code with something like TCC instead of clang, it's a completely different story.

That's it. That's all there is to UB.