Why One Ampersand Instead of Two Is a Real Bug

Subtitle: Same look. Different job.

• Left column - Logical AND (&&):
• Works on truth, not bits
• Stops early if left is false
• Right side may never run
• 2 && 4 is true
• For if, while, flow control

• Right column - Bitwise AND (&):
• Works on bits, not truth
• Always runs both sides
• Pairs up the bits, one by one
• 2 & 4 is 0, so false
• For flags, masks, registers

• Simple difference:
• 2 && 4 = true (both non-zero)
• 2 & 4 = 0 (010, 100 share no bit)

Use it when - Logical AND: guards, branches, early exit

Use it when - Bitwise AND: permission masks, device flags

• The test that exposes it - 3 steps:
• if (p != NULL & p->x)
• No short-circuit, so p->x still reads
• Crash. Swap in && and it passes.

Sticky note - Common beginner mistake:

Trusting the compiler to catch it. Both forms are legal and both give a usable condition, so it compiles and ships. Only a side effect on the right side reveals the slip.