What is short-circuit evaluation and why is p && *p a common pattern?

&& and || stop evaluating the moment the result is decided, so p && *p safely tests p first and only dereferences it when it's non-NULL.

How it works:

• && (AND): If left side is false, right side is never evaluated
• || (OR): If left side is true, right side is never evaluated

Why? Because the result is already determined:

false && anything  →  false (no need to check 'anything')
true  || anything  →  true  (no need to check 'anything')

The p && *p pattern - safe pointer dereferencing:

// If p is NULL, *p would crash. But this is SAFE:
if (p && *p) {
    // Only reaches here if p is non-NULL AND *p is non-zero
}

// Because:
// 1. First checks: p != NULL?
// 2. If p is NULL (false), stops immediately - never dereferences!
// 3. Only if p is valid does it check *p

Logical operators always return 0 or 1:

// !nonzero = 0
!0x41    → 0x00
// !zero = 1
!0x00    → 0x01
// Double negation normalizes to 0 or 1
!!0x41   → 0x01

// Both non-zero = true
0x69 && 0x55  → 0x01
// At least one non-zero = true
0x69 || 0x55  → 0x01

Contrast with bitwise operators:

// Bitwise & and | evaluate BOTH sides and operate on bits:
// Bitwise AND
0x69 & 0x55   → 0x41
// Bitwise OR
0x69 | 0x55   → 0x7D

Go deeper:

• Short-circuit evaluation — Wikipedia — how && and || stop early, and why it is safe.