Comment by sparkie
14 hours ago
C offers a different solution to the problem in Annex K of the standard. It provides a type `rsize_t`, which like `size_t` is unsigned, and has the same bit width, but where `RSIZE_MAX` is recommended to be `SIZE_MAX >> 1` or smaller. You perform bounds checking as `<= RSIZE_MAX` to ensure that a value used for indexing is not in the range that would be considered negative if converted to a signed integer. A negative value provided where `rsize_t` is expected would fail the check `<= RSIZE_MAX`.
IMO, this is a better approach than using signed types for indexing, but AFAIK, it's not included in GCC/glibc or gnulib. It's an optional extension and you're supposed to define `__STDC_WANT_LIB_EXT1__` to use it.
I don't know if any compiler actually supports it. It came from Microsoft and was submitted for standardization, but ISO made some changes from Microsoft's own implementation.
https://www.open-std.org/JTC1/SC22/WG14/www/docs/n1173.pdf#p...
This is an interesting middle ground. As ncruces pointed out in a sibling comment, the sign bit in a pointer cannot be set without contradicting the ptrdiff_t type. That makes this seem like a reasonable approach to storing sizes.