什么时候在 C 中使用位域?

为什么要用int?占用了多少空间?

整个int的空间被占用.我们使用 int 是因为在很多情况下，这并不重要.如果对于单个值，您使用 4 个字节而不是 1 个或 2 个，您的用户可能不会注意到.对于某些平台，大小更重要，您可以使用占用较少空间的其他数据类型(char、short、uint8_t 等).

据我所知，内存中只占用了 1 位，而不是整个 unsigned int 值.正确吗?

不，这是不正确的.整个 unsigned int 将存在，即使您只使用它的 8 位.

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?

Now I am curious, [are flags] the only way bit-fields are used practically?

No, flags are not the the only way bit-fields are used. They can also be used to store values larger than one bit, although flags are more common. For instance:

typedef enum {
    NORTH = 0,
    EAST = 1,
    SOUTH = 2,
    WEST = 3
} directionValues;

struct {
    unsigned int alice_dir : 2;
    unsigned int bob_dir : 2;
} directions;

Do we need to use bit fields to save space?

Bit fields do save space. They also allow an easier way to set values that aren't byte-aligned. Rather than bit-shifting and using bitwise operations, we can use the same syntax as setting fields in a struct. This improves readability. With a bitfield, you could write

directions.alice_dir = WEST;
directions.bob_dir = SOUTH;

However, to store multiple independent values in the space of one int (or other type) without bit-fields, you would need to write something like:

#define ALICE_OFFSET 0
#define BOB_OFFSET 2
directions &= ~(3<<ALICE_OFFSET); // clear Alice's bits
directions |= WEST<<ALICE_OFFSET; // set Alice's bits to WEST
directions &= ~(3<<BOB_OFFSET);   // clear Bob's bits
directions |= SOUTH<<BOB_OFFSET;  // set Bob's bits to SOUTH

The improved readability of bit-fields is arguably more important than saving a few bytes here and there.

Why do we use int? How much space is occupied?

The space of an entire int is occupied. We use int because in many cases, it doesn't really matter. If, for a single value, you use 4 bytes instead of 1 or 2, your user probably won't notice. For some platforms, size does matter more, and you can use other data types which take up less space (char, short, uint8_t, etc).

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

No, that is not correct. The entire unsigned int will exist, even if you're only using 8 of its bits.

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