What is the difference between sign extension and zero extension?

Widening fills the new high bits: a signed value copies its sign bit (sign extension), an unsigned value fills with 0s (zero extension).

Converting 4-bit value 1011:

Sign extension:  11111011  (preserves value -5)
Zero extension:  00001011  (gives value 11)

C behavior:

• signed char → int: Sign extension
• unsigned char → unsigned int: Zero extension

Danger: Casting signed char (-5) to unsigned int first sign-extends to int (-5), then reinterprets as unsigned (huge positive number).