C memory object and value semantics : the space of de facto and ISO standards

2 Abstract Pointers 7 2.1 Pointer Provenance . . . . . . . . . . . . . 7 2.1.1 Q1. Must the pointer used for a memory access have the right provenance, i.e. be derived from the pointer to the original allocation (with undefined behaviour otherwise)? (This lets compilers do provenance-based alias analysis) . . 7 2.1.2 Q2. Can equality testing on pointers be affected by pointer provenance information? . . . . . . . . . . . . 10 2.1.3 GCC and ISO C11 differ on the result of a == comparison on a onepast pointer . . . . . . . . . . . . . 14 2.2 Pointer provenance via integer types . . . . 14 2.2.1 Q3. Can one make a usable pointer via casts to intptr t and back? . . 14 2.2.2 Q4. Can one make a usable pointer via casts to unsigned long and back? . . . . . . . . . . . . . . . . 14 2.2.3 Q5. Must provenance information be tracked via casts to integer types and integer arithmetic? . . . . . . . 16 2.2.4 Q6. Can one use bit manipulation and integer casts to store information in unused bits of pointers? . . . 21 2.2.5 Q7. Can equality testing on integers that are derived from pointer values be affected by their provenance? . . 22 2.3 Pointers involving multiple provenances . . 23 2.3.1 Q8. Should intra-object pointer subtraction give provenance-free integer results? . . . . . . . . . . . . . 23 2.3.2 Q9. Can one make a usable offset between two separately allocated objects by inter-object subtraction (using either pointer or integer arithmetic), to make a usable pointer to the second by adding the offset to the first? . . . . . . . . . . . . . . . 26 2.3.3 Q10. Presuming that one can have valid pointers with multiple provenances, does an inter-object pointer subtraction give a value with explicitly-unknown provenance or something more specific? . . . . . . 30 2.3.4 Q11. Is the XOR linked list idiom supported? . . . . . . . . . . . . . 32 2.3.5 Q12. For arithmetic over provenanced integer values, is the provenance of the result invariant under plus/minus associativity? . . . . . . 33 2.3.6 Multiple provenance semantics summarised . . . . . . . . . . . . . 35 2.4 Pointer provenance via pointer representation copying . . . . . . . . . . . . . . . . . 35 2.4.1 Q13. Can one make a usable copy of a pointer by copying its representation bytes using the library memcpy? 35 2.4.2 Q14. Can one make a usable copy of a pointer by copying its representation bytes (unchanged) in user code? 35 2.4.3 Q15. Can one make a usable copy of a pointer by copying its representation bytes by user code that indirectly computes the identity function on those bytes? . . . . . . . . . 36