Negative offset is used often to access fields in parent struct having pointer to a field only. For example, to implement garbage collection or string type.
Re:Essential C (2003) [pdf]
by a1369209993 on Thursday September 03, 2020 @03:43PM
(#24367979)
That should be:
Parent* p = container_of(field,Parent,pa_somefield);
access(p->pa_otherfield);
#define container_of(FIELD,TYPE,MEMB) ({ \
const typeof( ((TYPE*)0)->MEMB )* _mptr = (FIELD); \
(TYPE*)( (char*)_mptr - __builtin_offsetof(TYPE,MEMB) ); \
})
But p - 2 is not the same as p + -2, and it's not clear in your example whether the former suffices or the latter is required. I can definitely imagine examples where the latter is required--certainly C clearly accommodates this usage--but IME it's nonetheless a rare scenario and not something that could, alone, justify always using signed offsets. Pointers are intrinsically neither signed nor unsigned; it's how you use them that matters.
In C an object can be larger than PTRDIFF_MAX, a real possibility in modern 32-bit environments. (Some libc's have been modified to fail malloc invocations that large, but mmap can suffice.) Because pointer subtraction is represented as ptrdiff_t, the expression &a[n] - a could produce undefined behavior where n is > PTRDIFF_MAX. But a + n is well defined behavior for all positive n (signed or unsigned) as long as the size of a is >= n.
There's an asymmetry between pointer-pointer arithmetic and pointer-integer arithmetic; they behave differently and have different semantics. Pointers are a powerful concept, but like most powerful concepts the abstraction can leak and produce aberrations. I realize opinions vary on whether to prefer signed vs unsigned indices and object sizes (IME, the camps tend to split into C vs C++ programers), but the choice shouldn't be predicated on the semantics of C pointers because those semantics alone don't favor one over the other.