How to compare C pointers?

Question

Recently, I wrote some code to compare pointers like this:

if(p1+len < p2)

however, some staff said that I should write like this:

Answer 1

As Dietrich already said, comparing unrelated pointers is dangerous, and could be considered as undefined behavior.

Given that two pointers are within the range 0 to 2GB (on a 32-bit Windows system), subtracting the 2 pointers will give you a value between -2^31 and +2^31. This is exactly the domain of a signed 32-bit integer. So in this case it does seem to make sense to subtract two pointers because the result will always be within the domain you would expect.

However, if the LargeAddressAware flag is enabled in your executable (this is Windows-specific, don't know about Unix), then your application will have an address space of 3GB (when run in 32-bit Windows with the /3G flag) or even 4GB (when run on a 64-bit Windows system). If you then start to subtract two pointers, the result could be outside the domain of a 32-bit integer, and your comparison will fail.

I think this is one of the reasons why the address space was originally divided in 2 equal parts of 2GB, and the LargeAddressAware flag is still optional. However, my impression is that current software (your own software and the DLL's you're using) seem to be quite safe (nobody subtracts pointers anymore, isn't it?) and my own application has the LargeAddressAware flag turned on by default.