How to test if pointer inside an array?

Question

I would like to test if myCurrentPtr points inside my array a.

_B represents the number of values in a.
So, a + _B shall point the latest value of the array.

#define _B ((uint8_t)5)
volatile uint8_t a[5] = {0, 1, 2, 3, 4}; //`a` is a pointer to the first array element

if (myCurrentPtr > (a + _B)) {
    printf("Out of bounds!");
}

Doesn't compile. Do you have any idea?

Whereas,

...
if (myCurrentPtr > (a + 5)) {
    printf("Out of bounds!");
}

compiles just fine.

Aren't the two exactly the same after preprocessing?

Answer 1

How to test if pointer inside an array?

Code can use >=, >, <, <= between two object pointers p,q is they are in the same array (or just one passed the end of the array). Else code is undefined behavior. C does not have a portable way to test in/outside the array.

The below code is poor

if (myCurrentPtr == (a + _B)) {                            // Defined behavior
  printf("pointer just passed a[]\n"); 
} else if (myCurrentPtr >= a && myCurrentPtr < (a + _B)) { // Undefined behavior
  printf("pointer in array\n");        
} else {
  printf("pointer outside array\n");
}

Code could explicitly compare one at a time with ==, != with myCurrentPtr and each element of a[]. Likely this is unsatisfactorily slow, yet reliable.

// Dependable, well defined, but slow.
found = false;
for (int i=0; i<5; i++) {
  if (myCurrentPtr == &a[i]) {
    found = true;
    break;
  }
}

Other approaches rely on iffy code.

// Iffy code - depending on memory model, may work, may not.
uintptr_t mcp = (uintptr_t) myCurrentPtr;
uintptr_t ia =  (uintptr_t) a;
uintptr_t ia5 = (uintptr_t) &a[5];

if (mcp >= ia && mcp < ia5) {         // Not highly portable
  printf("pointer in array\n");        
} else {
  printf("pointer just passed a[]\n");
}

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The best approach to "How to test if pointer inside an array?" is to re-form the problem. OP did not post why this test is needed. Good code typically can re-work the issue and not use this test.