How to check whether a pointer is pointing to an element inside an array?

Question

The following function intends to check whether the pointer b is pointing to the array a[0],...,a[len-1] ?

bool between(int *a, int len, int *b)
{
    return (a <= b) && (b < a + len);
}

From reading https://en.cppreference.com/w/c/language/operator_comparison the function invokes undefined behaviour. If so, what is the right way to do this? Why does the standard disallow this?

Answer 1

Comparing two unrelated pointers (ie pointers that don't point to members of the same array object or struct) using the comparison operators < , <= , > , and >= do indeed invoke undefined behavior as described in the linked page as well as section 6.5.8 of the C standard .

As to why this is disallowed, not all implementations have a flat memory model, and unrelated objects need not reside in areas of memory where performing a comparison makes sense.

So your function would return true in cases where b points to a member of a , false when it points one past the last member of a , and would invoke undefined behavior otherwise.

It is however allowed to compare unrelated pointers using == or != . So you can circumvent the limitation on comparison operators by looping through the array and using equality operators to compare the target pointer with each element:

bool between(int *a, int len, int *b)
{
    int i;
    for (i=0; i<len; i++) {
        if (a+i == b) {
            return true;
        }
    }
    return false;
}

While this is not a efficient as a ranged check, it is the only compliant way to do so.

Of course, it would be better to construct your program in such a way that such a comparison is not necessary. A program that is allowing a pointer to inside an array to fall outside of the array is already invoking undefined behavior, so fixing that would eliminate the need for such a function.

Note however that it is allowed to increment a pointer to one element past the end of an array and perform comparisons on that pointer (although it cannot be dereferenced).

Answer 2

In addition to @dbush good answer, code can do math on the array side a to achieve a potential quicker positive result.

By attempting a binary search first, if the outcome of between() is true and the mapping of pointers to integers are in a common implementation, the true value will be found in O(log(n)) time.

If b is not found in the array via the binary search, a linear is still needed to cope with unusual mapping of pointers to integers.

For large arrays, like n=1,000,000 the log2(1,000,000) steps of doing a bsearch is insignificant. For small arrays, best to proceed directly to the loop.

 // When optional, but common, uintptr_t available
int fcmp(const void *key, const void *array_element) {
  if (key == array_element) {
    return 0;
  }
  uintptr_t ai = (uintptr_t) array_element;
  uintptr_t ki = (uintptr_t) key;
  return (ai > ki) - (ai < ki);
}

// Best to use `size_t` for array indexing
bool between(int *a, size_t a_sz, int *b) {
  if (bsearch(b, a, a_sz, sizeof *a, fcmp)) {
    return true;
  }
  for (size_t i = 0; i < a_sz; i++) {
    if (a + i == b) {
      return true;
    }
  }
  return false;
}