> I can't understand what undefined behaviors C has, because it is the most simple and defined language I do know of. "Undefined behaviour" is a very simple an

> I can't understand what undefined behaviors C has, because it is the most simple and defined language I do know of. "Undefined behaviour" is a very simple and well-defined term when it comes to languages like C. Undefined behaviour is code who's behaviour is specified to be arbitrary. And languages like C is famous for having them. (I'm assuming that you were being honest about not knowing about undefined behaviour means in C, and not trying to re-frame the word to mean something else.) An example is overflow on signed integers. Strictly speaking it does not help that you know "how computers work" since you have no guarantees about what will happen if an overflow happens in your program. You can't assume that a wrap-around will happen and base your program on that, since that might be a faulty assumption on some compilers. Undefined behaviour allows the compiler to assume that it never happens and introduce optimizations based on that assumption. For example, use of initialized variables is undefined. This allows the compiler to avoid having to initialize variables (like arrays) to some default value when they are declared.