Can the unsigned char type have padding bits and/or unused values?

Question

All types except bit fields occupy an integral number of "bytes", and sizeof returns that number. An unsigned char occupies 1 "byte", so sizeof(unsigned char) always return 1. Right?

But does an unsigned char always fill a "byte", or can CHAR_BITS be less than the number of bits in a "byte" or UCHAR_MAX less than (2 to the power of CHAR_BITS) - 1? If an unsigned char can't hold all values that it's number of bits permit it too, how does copying other types with unsigned chars work?

int src = -1, dest;
size_t i;

for (i = 0; i < sizeof dest; i++) {
  ((unsigned char *) &dest)[i] = ((unsigned char *) &src)[i];
}

Answer 1

No, unsigned char cannot have padding bits.

(C99, 6.2.6.2p1) "For unsigned integer types other than unsigned char, the bits of the object representation shall be divided into two groups: value bits and padding bits (there need not be any of the latter)."

And yes, sizeof (unsigned char) is guaranteed to be 1.

(C99, 6.5.3.4p3) "When applied to an operand that has type char, unsigned char, or signed char, (or a qualified version thereof) the result is 1."