12.8 Answers to Chapter Questions

I l @ ve RuBoard

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
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
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

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.

I l @ ve RuBoard