Lecture 9 - Bitfields, Unions, and Enums Bitwise Operators

Question 1

sizeof()

Answer 1

• can tell us the size (number of bytes) of any
• variable definition
• type declaration
• array

Question 2

bitfields

Answer 2

• you can create fields within struct that do not contain a round number of bits
• compacts their storage together

Question 3

why use bitfields?

Answer 3

• when you really want to store a lot of fields that contain small values
• when you want to do format conversion between different types of data
• probably won’t need them in this class

Question 4

union

Answer 4

union my_union {
// variables here
} my_var;
- declaration looks just like a struct
- all internal elements overlap (stored in same byte)

Question 5

initialization of union

Answer 5

union my_union my_var = {5.0};
- assumes you are initializing the FIRST field

Question 6

why use union?

Answer 6

• when you need to save space in your program
• DON’T USE IT in this class

Question 7

enum

Answer 7

enum color {
RED,
GREEN,
BLUE
};
enum color my_hue = GREEN;
-looks sort of like a struct declaration
- user when you want to attach a label to a value

Question 8

enums can also have values

Answer 8

enum example {
VALUE = 1
};
- you can assign exact values to the declaration’s members
- you can assign a value to an enum definition using an integer too

Question 9

endianness

Answer 9

-the order of bytes in a word or multi-byte value
- does not impact bit ordering for individual bytes

Question 10

big-endian

Answer 10

-most significant byte first when storing memory
- lowest address

Question 11

little-endian

Answer 11

least significant byte first

Question 12

bitwise operators

Answer 12

• used to perform operations on individual bits of numbers
  if (x) printf(“x = %d\n”, x);
• go back and review these

Question 13

difference between logical and bitwise operators

Answer 13

• logical operators are used to make a yes/no decision
• bitwise operators are used when you want to work on the bits of a quantity

Question 14

shift operators &laquo_space;and&raquo_space;

Answer 14

• can take a bunch of bits and shift it one way or another
  -every shift left is = to a multiplication by two
  ex. x = 10; binary 00001010
  x &laquo_space;3 binary 01010000

Question 15

bit setting/clearing

Answer 15

• user operators clear/set bits
• look at slide example

Question 16

bit checking

Answer 16

• ex. bits = 44, mask = 8
• if ((bits & mask) == mask) {
  // success
  }

Question 17

bitwise operator &