Pointer comparison

Pointer comparison evaluates the memory addresses held by the pointers, not the values stored at those addresses.

Result: In C, comparison operators return an int: 1 for true and 0 for false.

Purpose: To check if two pointers point to the exact same memory location (or if both are NULL).
Syntax:
- ptr1 == ptr2: Returns 1 if addresses are identical.
- ptr1 != ptr2: Returns 1 if addresses are different.
Valid Comparisons:
- Pointers of the same type.
- One pointer and void*.
- Any pointer and NULL.

Purpose: To determine the order of pointers in memory (lower address vs. higher address).
Strict Rule (The "Same Array" Rule):
- Relational comparisons are guaranteed to be valid ONLY if both pointers point to elements within the same array (or the position just past the last element).
Behavior:
- ptr1 < ptr2: True if ptr1 points to an earlier element (lower index) than ptr2.
- Undefined Behavior: Comparing pointers to completely unrelated variables (e.g., two different int variables declared separately) is undefined and unsafe, even if it compiles.

NULL Macro: Represents a null pointer constant (defined in , , etc.).
Common Idioms:
- Explicit: if (ptr == NULL)
- Implicit: if (!ptr) (Checks if pointer is NULL)
- Implicit: if (ptr) (Checks if pointer is NOT NULL)

Type Mismatch: Comparing incompatible pointer types (e.g., int vs char) generally causes a compiler warning and is unsafe without explicit casting.
Struct Padding: Be careful when comparing pointers within structures; while struct members increase in address order, there may be padding bytes between them.