When According to section 6.5.6 of C99, when two pointers are subtracted, both must point to elements of the same array object or to one past the last element of the array object; the result is the difference of the subscripts of the two array elements. Otherwise the operation results in undefined behavior (see bullet 45 of Appendix J). This restriction exists because pointer subtraction in C produces the number of objects between the two pointers, not the number of bytes.
Similarly, comparing pointers using the relational operators <, <=, >=, and > gives the positions of the pointers relative to each other. Subtracting or comparing pointers that do not refer to the same array can result results in erroneous behaviorundefined behavior (see bullet 46 of Appendix J).
Comparing pointers using the equality operators == and != has well-defined semantics regardless of whether or not either of the pointers is null, points into the same object, one past the last element of an array object, or function.
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