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

Technically, p1 and p2 must be pointers into the same array. If they are not in the same array, the behaviour is undefined.

For the addition version, the type of len can be any integer type.

For the difference version, the result of the subtraction is ptrdiff_t, but any integer type will be converted appropriately.

Within those constraints, you can write the code either way; neither is more correct. In part, it depends on what problem you're solving. If the question is 'are these two elements of the array more than len elements apart', then subtraction is appropriate. If the question is 'is p2 the same element as p1[len] (aka p1 + len)', then the addition is appropriate.

In practice, on many machines with a uniform address space, you can get away with subtracting pointers to disparate arrays, but you might get some funny effects. For example, if the pointers are pointers to some structure type, but not parts of the same array, then the difference between the pointers treated as byte addresses may not be a multiple of the structure size. This may lead to peculiar problems. If they're pointers into the same array, there won't be a problem like that — that's why the restriction is in place.