On Thu, Aug 11, 2011 at 7:13 PM, Joe Buck <joe.b...@synopsys.com> wrote: > On Thu, Aug 11, 2011 at 09:05:19AM -0700, Florian Merz wrote: >> If I remember the standard correctly, pointer subtraction is valid if both >> pointers point to elements of the same array or to one past the last element >> of the array. According to this 0x80000000 - 0x7FFFFFFF should be a valid >> pointer subtraction with the result 0x00000001. >> >> But if the subtraction is treated as a signed, this would be an signed >> integer >> overflow, as we subtract INT_MAX from INT_MIN, which surely must overflow, >> and >> the result therefore would be undefined. > > It is true that the C and C++ languages make signed integer overflow > undefined, but that's for actual integer types as declared by the user. > For pointers, though the subtraction has to be signed (because, for two > pointers, either can can come later in the address space), this signed > subtraction has to be defined to work in a two's complement fashion (so > the wraparound in your example case works reliably).
Of course GCC can't (yet) do both at the same time. Thus we have to use unsigned arithmetic when we want two's complement arithmetic. Richard. > >
