What is the unsigned counterpart of ptrdiff_t?

I don't think there is a formal unsigned counterpart to ptrdiff_t (no uptrdiff_t), but using size_t for it is pretty reasonable.

In POSIX, the signed counterpart to size_t is ssize_t. It is the type returned by functions such as read(), for example.

That suggests there will be few implementations where the underlying type of ssize_t and ptrdiff_t will be different.

size_t is used to represent object sizes. It was widely believed that compiler writers will not create objects with negative sizes.

Note that with ptrdiff_t` you get the difference depending on how you are comparing, so a signed type makes sense (changing this to a unsigned type for reasonable values is trivial):

5.7 Additive operators

6 [...]As with any other arithmetic overflow, if the result does not fit in the space provided, the behavior is undefined.[...]

So, you may need to create a special type for 'very large values'.