cwitty@newtonlabs.com (Carl R. Witty) wrote,

"Manuel M. T. Chakravarty" <chak@cse.unsw.edu.au> writes:

...

Array and list indicies are quite different beats from pointer differences. To get into a problem with array indicies, we would need an array (and thus, heap) that is big enough to requires such an index. In contrast, pointers are not constrained be point into the heap. For, example with mmap(), I can easily get two pointers that are much further apart than what would fit into the real (even the virtual) memory of the concrete hardware. In other words, in x86 Linux, your address space is already 3GB. We don't need any new fancy hardware for 30 bit pointer differences to have the potential to get us into trouble.

Why would you want to take the difference of two pointers that point into different objects?

For example, if you construct a representation of the mapped areas in the address space of a process (ie, by reading /proc/<pid>/maps on a Linux system). You may do that, among othe things, to find a suitable memory area for mapping anonymous memory pages. Manuel PS: If you think that this is a useless thing to do, check out http://www.cse.unsw.edu.au/~cs9243/assigns/ass2/ You need this if you want to implement a Distributed Shared Memory system as a pure user-space library.