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:

if(p2-p1 > len) 

to be safe. Here,p1 and p2 are char * pointers,len is an integer. I have no idea about that.Is that right?

EDIT1: of course,p1 and p2 pointer to the same memory object at begging.

EDIT2：just one min ago,I found the bogo of this question in my code(about 3K lines),because len is so big that p1+len can't store in 4 bytes of pointer,so p1+len < p2 is true.But it shouldn't in fact,so I think we should compare pointers like this in some situation:

if(p2 < p1 || (uint32_t)p2-p1 > (uint32_t)len) 

Answer 1

In general, you can only safely compare pointers if they're both pointing to parts of the same memory object (or one position past the end of the object). When p1, p1 + len, and p2 all conform to this rule, both of your if-tests are equivalent, so you needn't worry. On the other hand, if only p1 and p2 are known to conform to this rule, and p1 + len might be too far past the end, only if(p2-p1 > len) is safe. (But I can't imagine that's the case for you. I assume that p1 points to the beginning of some memory-block, and p1 + len points to the position after the end of it, right?)

What they may have been thinking of is integer arithmetic: if it's possible that i1 + i2 will overflow, but you know that i3 - i1 will not, then i1 + i2 < i3 could either wrap around (if they're unsigned integers) or trigger undefined behavior (if they're signed integers) or both (if your system happens to perform wraparound for signed-integer overflow), whereas i3 - i1 > i2 will not have that problem.

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Edited to add: In a comment, you write "len is a value from buff, so it may be anything". In that case, they are quite right, and p2 - p1 > len is safer, since p1 + len may not be valid.