There's also behavior which is undefined for hardware reasons. An example is what the C standard calls "trap representations": Bit patterns which fit into the

There's also behavior which is undefined for hardware reasons. An example is what the C standard calls "trap representations": Bit patterns which fit into the space occupied by a specific type, but which will cause a hardware trap (exception, interrupt, what have you) if you actually store them in a variable of that type. The only type which cannot have trap representations is unsigned char. Basically, what it amounts to is this: C compilers don't compile to a runtime, they compile to raw machine code with, perhaps, a standard library. If you do something the hardware doesn't like when your program runs, well, the C compiler is long gone by that point and the C standard makes no guarantees. More prosaically, storing to a location beyond the end of an array might not cause a segfault. It might corrupt some other array, it might cause a hardware crash, it might even corrupt the program's machine code. Because C is explicitly a language for embedded hardware, with no MMUs, no W^X protection, and no OSes, the C standard can say very little about such things.