Can a one past the element pointer that compares equal to a valid pointer be dereferenced?

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In C18 we have:

§ 6.5.9p10

Two pointers compare equal if and only if both are null pointers, both are pointers to the same object(including a pointer to an object and a subobject at its beginning) or function, both are pointers to one past the last element of the same array object, or one is a pointer to one past the end of one array object and the other is a pointer to the start of a different array object that happens to immediately follow the first array object in the address space.

So for int a[4][2] = {{1, 2}, {3, 4}, {5, 6}, {7, 8}}, we can have a[1] == a[0] + 2.

Can that by consequence guarantee that *(a[1]) == *(a[0] + 2)?

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1 Answers

It is not allowed to dereference such a pointer, even if it compares equal to another valid pointer.

Section 6.5.6p8 regarding the + operator states:

When an expression that has integer type is added to or subtracted from a pointer, the result has the type of the pointer operand. If the pointer operand points to an element of an array object, and the array is large enough, the result points to an element offset from the original element such that the difference of the subscripts of the resulting and original array elements equals the integer expression. In other words, if the expression P points to the i-th element of an array object, the expressions (P)+N (equivalently, N+(P)) and (P)-N (where N has the value n) point to, respectively, the i+n-th and i−n-th elements of the array object, provided they exist. Moreover, if the expression P points to the last element of an array object, the expression (P)+1 points one past the last element of the array object, and if the expression Q points one past the last element of an array object,the expression (Q)-1 points to the last element of the array object. If both the pointer operand and the result point to elements of the same array object, or one past the last element of the array object, the evaluation shall not produce an overflow; otherwise, the behavior is undefined. If the result points one past the last element of the array object, it shall not be used as the operand of a unary * operator that is evaluated

On a related note, some compilers have the concept of pointer provenance, meaning it internally keeps track of the source of a pointer. A consequence of this is that if two unrelated variables are adjacent in memory, comparing the address of one to one-past the address of the other will always evaluate to false, even if the addresses are the same.

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