Is this a correct and portable way of checking if 2 c-strings overlap in memory?

Question

Might not be the most efficient way, but is it correct and portable?

int are_overlapping(const char *a, const char *b) {   return (a + strlen(a) == b + strlen(b)); } 

To clarify: what I'm looking for is overlap in memory, not in the actual content. For example:

const char a[] = "string"; const char b[] = "another string"; are_overlapping(a, b); // should return 0 are_overlapping(a, a + 3); // should return 1 

Answer 1

Yes, your code is correct. If two strings end at the sample place they by definition overlapped - they share the same null terminator. Either both strings are identical, or one is a substring of the other.

Everything about your program is perfectly well-defined behaviour, so assuming standards-compliant compilers, it should be perfectly portable.

The relevant bit in the standard is from 6.5.9 Equality operators (emphasis mine):

Two pointers compare equal if and only if both are null pointers, both are pointers to the same object (including a pointer to an object and a subobject at its beginning) or function, both are pointers to one past the last element of the same array object, or one is a pointer to one past the end of one array object and the other is a pointer to the start of a different array object that happens to immediately follow the first array object in the address space.