When to use bit-fields in C?

Question

On the question 'why do we need to use bit-fields', searching on Google I found that bit fields are used for flags. Now I am curious,

1. Is it the only way bit-fields are used practically?
2. Do we need to use bit fields to save space?

Way of defining bit field from the book:

struct {     unsigned int is_keyword : 1;      unsigned int is_extern :  1;      unsigned int is_static : 1; } flags; 

3. Why do we use int?
4. How much space is occupied?

I am confused why we are using int, but not short or something smaller than an int.

5. As I understand only 1 bit is occupied in memory, but not the whole unsigned int value. Is it correct?

Answer 1

A quite good resource is Bit Fields in C.

The basic reason is to reduce the size used. For example if your write:

struct {     unsigned int is_keyword;      unsigned int is_extern;      unsigned int is_static; } flags; 

You will use at least 3 * sizeof(unsigned int) or 12 bytes to represent 3 little flags, that should only need 3 bits.

So if you write:

struct {     unsigned int is_keyword : 1;      unsigned int is_extern : 1;      unsigned int is_static : 1; } flags; 

This uses up the same space as one unsigned int, so 4 bytes. You can throw 32 one bit fields into the struct before it needs more space.

This is sort of equivalent to the classical home brew bit field:

#define IS_KEYWORD 0x01 #define IS_EXTERN  0x02 #define IS_STATIC  0x04 unsigned int flags; 

But the bit field syntax is cleaner, compare:

if (flags.is_keyword) 

against:

if (flags & IS_KEYWORD) 

and obviously less error prone.