Attention people who think UB is fine

After my post on undefined behavior in C some people insisted that it’s demonstrably fine, we know what the code will do, and at worst we’ll get a segfault. This is false. Here’s an example. C code #include<utility> // Let's pretend this is a memory mapped device controlling // motors and gears and stuff. volatile int device_command = 0; void serve_hot_chocolate() { device_command = 42; // Oh no, we spilled some hot chocolate, invoking UB. // // The definition of std::unreachable() is: // Invokes undefined behavior at a given point. std::unreachable(); } void explode() { device_command = 3; } Example assembly output With GCC 16.2 with -O1. "serve_hot_chocolate()": mov DWORD PTR "device_command"[rip], 42 "explode()": mov DWORD PTR "device_command"[rip], 3 ret "device_command": .zero 4 Ruh roh. Exploding chocolate! The function explode() will be executed, even though it was never called! godbolt link Another example: dereferencing a null pointer Some people insisted that dereferencing a null pointer is fine. It’ll just try to access address zero! And these people supposedly know what happens when you try: if nothing is mapped there, you’ll get a segfault. Again, not true. Let’s say we change std::unreachable() to int* ptr = nullptr; *ptr = 99; in the previous example. Now we get this: "serve_hot_chocolate()": mov DWORD PTR "device_command"[rip], 42 mov DWORD PTR ds:0, 0 ud2 Curious, eh? First it writes zero, not 99, to memory address zero. Then it crashes the program using an invalid instruction. The number 99 never appears in the generated program. But the null pointer write does. Presumably the null pointer write is done in order to improve debugger error messages. Now let’s check clang with godbolt: "serve_hot_chocolate()": mov dword ptr [rip + "device_command"], 42 "explode()": mov dword ptr [rip + "device_command"], 3 ret "device_command": .long 0 Again we get exploding chocolate. Let’s do some pointer arithmetic on the null pointer Ok, so what happens if we add ptr += 10. That’s UB for three reasons: using a null pointer for anything other than comparison against other pointers, even if an object would be valid at the null pointer (which it’s not), creating a pointer that points outside that object, or if an array, beyond one-past the array, is UB, even if that were valid (which it’s not), we then dereference the address. Let’s try it. "serve_hot_chocolate": mov DWORD PTR "device_command"[rip], 42 mov DWORD PTR ds:40, 99 …