memset, memcpy, and memmove, etc all depend on UB. memset and memcpy read and write padding bytes, which is UB. If you try to say memset/cpy is a "primitive op

memset, memcpy, and memmove, etc all depend on UB. memset and memcpy read and write padding bytes, which is UB. If you try to say memset/cpy is a "primitive operation", then you're still stuck with the "correct" struct initialization (memset, bzero) being UB if you did something like: void f(bool condition) { if (condition) { struct structwithpadding thing; bzero(&thing, sizeof(thing)); } else { printf("wat\n"); } } The ludicrously broad definition of what UB apparent allows, this can be "optimized" to void f(bool condition) { printf("wat\n"); } Because bzero() writes to padding, and that is UB, which means the condition==true branch "can't happen" \o/ memmove falls to UB much requires pointer comparisons between pointers to different objects, which is UB.