What makes it possible for glibc malloc to compare pointers from different “objects”?

Question

Comparing pointers with a relational operator (e.g. <, <=, >= or >) is only defined by the C standard when th

Answer 1

The authors of the C Standard recognized that there are some segmented-memory hardware platforms where an attempt to perform a relational comparison between objects in different segments might behave strangely. Rather than say that such platforms could not efficiently accommodate efficient C implementations, the authors of the Standard allow such implementations to do anything they see fit if an attempt is made to compare pointers to objects that might be in different segments.

For the authors of the Standard to have said that comparisons between disjoint objects should only exhibit strange behavior on such segmented-memory systems that can't efficiently yield consistent behavior would have been seen as implying that such systems were inferior to platforms where relational comparisons between arbitrary pointers will yield a consistent ranking, and the authors of the Standard went out of their way to avoid such implications. Instead, they figured that since there was no reason for implementations targeting commonplace platforms to do anything weird with such comparisons, such implementations would handle them sensibly whether the Standard mandated them or not.

Unfortunately, some people who are more interested in making a compiler that conforms to the Standard than in making one that's useful have decided that any code which isn't written to accommodate the limitations of hardware that has been obsolete for decades should be considered "broken". They claim that their "optimizations" allow programs to be more efficient than would otherwise be possible, but in many cases the "efficiency" gains are only significant in cases where a compiler omits code which is actually necessary. If a programmer works around the compiler's limitations, the resulting code may end up being less efficient than if the compiler hadn't bothered with the "optimization" in the first place.