Earlier quoted context omitted.
I said "in C". You're talking "in Linux" (or glibc/whatever). Which, as I already said, plays by its own rules and defies C. It's broken by design.
So if I malloc 2 MB or 2 GB or whatever in a C program running on Linux, but I have not yet either read from or written to that memory, then what's the state? Has the C library forced Linux to actually allocate it, or has it not? Or does it depend, and if so, on what?
It depends on the overcommit setting. By default it's on, and that indicates Linux doesn't promise to back it with a physical page. Only the virtual address range is allocated (i.e. the only guarantee is that future allocations within your process won't return addresses from that range). This implies that if you try to write to it, your write might segfault due to OOM. If overcommit is turned off, then Linux promises it will be backed with a physical page if you try to it, meaning your write won't segfault due to OOM. Aside from these, I think everything else is an implementation detail, but generally OSes map unwritten pages to the same zero page as an optimization, and then when a write occurs they back it with a physical page.