C & undefined behaviour - was Re: tumble under BSD

Mouse mouse at Rodents-Montreal.ORG
Sat Apr 2 17:58:10 CDT 2016

> Anyone interested in C and UB will want to read most of John Regehr's
> http://blog.regehr.org/ - it hosts some of the best material on UB.

Unfortunately Mr. (I'm assuing this is appropriate given "John") Regehr
is falling into the same trap he's trying to warn against: basically,
assuming that the mental model he has is the only one that could ever
possibly be relevant.

For example, consider the "Comparisons of Pointers to Unrelated
Objects" example:

# define SQLITE_WITHIN(P,S,E) \
    ((uintptr_t)(P)>=(uintptr_t)(S) && \

He writes that "[c]omparisons between unrelated objects destroy
determinism because the allocator makes no guarantees about their
relative locations".  No.  Comparisons between unrelated objects
destroy determinism because pointers into different objects may not be
comparable!  He's assuming the "the entire address space is a single
array of bytes (perhaps with holes)" memory model is the only possible
one.  He needs to talk with someone who wrote large-model 8086 code -
or someone who's used the Lisp Machine C compiler I heard of that
represents pointers as <array,index> pairs.

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