12.8 Answers to Chapter Questions

Answer 12-1:The problem is that we have a single signed-integer-bit field. A three-bit-wide signed-integer field can take on the following values:

struct foo {
    int three_bits:3;
};

Bit pattern	Decimal value
100	-4
110	-3
101	-2
111	-1
000	0
001	1
010	2
011	3

A two-bit-wide signed-integer field can take on the following values:

struct foo {
    int two_bits:3;
};

Bit pattern	Decimal value
10	-2
11	-1
000	0
001	1

A one-bit-wide signed-integer field can take on the following values:

struct foo {
    int one_bit:1;
};

Bit pattern	Decimal value
1	-1
0	0

So the two values of this bit field are 0 and -1. That means that 1 can never be stored in this field.

A unsigned bit field of width 1 can hold the values 0 and 1. Using the declaration unsigned int valid:1 makes the program work correctly.