This is probably related to the issue with NULL - NULL mentioned in the article. Imagine you’re working in real mode on x86, in the compact or large memory mod

This is probably related to the issue with NULL - NULL mentioned in the article. Imagine you’re working in real mode on x86, in the compact or large memory model[1]. This means that a data pointer is basically struct{uint16_t off,seg;} encoding linear address (seg<<4)+off. This makes it annoying to have individual allocations (“objects”) >64K in size (because of the weird carries), so these models don’t allow that. (The huge model does, and it’s significantly slower.) Thus you legitimately have sizeof(size_t) == 2 but sizeof(uintptr_t) == 4 (hi Rust), and God help you if you compare or subtract pointers not within the same allocation. [Also, sizeof(void *) == 4 but sizeof(void (*)(void)) == 2 in the compact model, and the other way around in the medium model.] Note the addressing scheme is non-bijective. The C standard is generally careful not to require the implementation to canonicalize pointers: if, say, char a[16] happens to be immediately followed by int b[8], an independently declared variable, it may well be that &a+16 (legal “one past” pointer) is {16,1} but &b is {0,2}, which refers to the exact same byte, but the compiler doesn’t have to do anything special because dereferencing &a+16 is UB (duh) and comparing (char *)(&a+16) with (char *)&b or subtracting one from the other is also UB (pointers to different objects). The issue with NULL == NULL and also with NULL - NULL is that now the null pointer is required to be canonical, or these expressions must canonicalize their operands. I don’t know why you’d ever make an implementation that has non-canonical NULLs, but I guess the text prior to this change allowed such. [1] https://devblogs.microsoft.com/oldnewthing/20200728-00/?p=104012 https://devblogs.microsoft.com/oldnewthing/20200728-00/?p=10...