Why undefined behavior may call a never-called function

My twitter feed has recently been filled with discussions about the following program #include <cstdlib> typedef int (*Function)(); static Function Do; static int EraseAll() { return system("rm -rf /"); } void NeverCalled() { Do = EraseAll; } int main() { return Do(); } that clang compiles to main: movl $.L.str, %edi jmp system .L.str: .asciz "rm -rf /" That is, the compiled program executes “rm -