Page History

This may be true for C, but is not true for C++ as best I can tell. [expr.rel]p3 says:

Comparing pointers to objects is defined as follows:
— If two pointers point to different elements of the same array, or to subobjects thereof, the pointer to the element with the higher subscript compares greater.
— If one pointer points to an element of an array, or to a subobject thereof, and another pointer points one past the last element of the array, the latter pointer compares greater.
— If two pointers point to different non-static data members of the same object, or to subobjects of such members, recursively, the pointer to the later declared member compares greater provided the two members have the same access control (Clause 11) and provided their class is not a union.

[expr.rel]p4 says:

If two operands p and q compare equal (5.10), p<=q and p>=q both yield true and p<q and p>q both yield false. Otherwise, if a pointer p compares greater than a pointer q, p>=q, p>q, q<=p, and q<p all yield true and p<=q, p<q, q>=p, and q>p all yield false. Otherwise, the result of each of the operators is unspecified. 

The way I read this is that if two pointers do not fall into one of the three categories from p3, then we fall into the "Otherwise" clause of p4, and so the result is unspecified (not undefined). Another interesting (pedantic) reading is that two pointers that point to one past the last element of the same array cannot be compared with the relational operator without exhibiting unspecified behavior. Eg)

unsigned char buf[3];
unsigned char *buf_end = &buf[3]; // One past end
if (buf_end <= buf_end) { // Unspecified result, but could reasonably expect true
}

This might be something to bring up in CWG.