Bit fields

Learn memory savings with bit fields.

Bit fields

Bit fields let you define struct members in bits rather than bytes. This is used for memory packing and flag management.

Example: struct Flags { unsigned a : 3; unsigned b : 1; };. Here a takes 3 bits, b takes 1 bit. a holds 0-7. The whole struct takes only 4 bits (half a byte); the compiler pads it to at least 1 byte. This is common in embedded systems, protocol headers and hardware registers. Bit fields save memory but require care for portability.

Bit field example

Bit fields are automatically masked when you assign a value. If you write 5 to unsigned a : 3;, 5 is stored; if you write 9, overflow happens and only the low 3 bits (1) are stored.

Example: struct Flags f; f.a = 5; f.b = 1;. Here f.a is 5 (binary 101), f.b is 1 (binary 1). If you write f.a = 9;, the low 3 bits of 9 are 1; so f.a becomes 1. This automatic masking is suitable for modeling hardware registers. Still, avoiding overflow values is a good habit for readability.

Exercise Print flags with bit fields

Define struct Flags with unsigned a : 3; and unsigned b : 1;. In main, write struct Flags f;; assign f.a = 5; and f.b = 1;. Print with printf("%d %d\n", f.a, f.b);.

Show example solution

#include <stdio.h>

struct Flags {
    unsigned a : 3;
    unsigned b : 1;
};

int main(void) {
    struct Flags f;
    f.a = 5;
    f.b = 1;
    printf("%d %d\n", f.a, f.b);
    return 0;
}

Test yourself

Match the bit field expressions

unsigned a : 3

3-bit field
unsigned b : 1

1-bit field
struct Flags

Flag struct
: 3

Bit width

Complete the bit width

struct Flags { unsigned a : ____; unsigned b : 1; };

Show answer

struct Flags { unsigned a : 3; unsigned b : 1; };

How many bits does unsigned a : 3; use?
- 1
- 3
- 32
- 0
Show answer

3 — answer: 3. The : 3 suffix means 3 bits.

Next lesson: Summary - Unions and enums

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