C Bit Fields

Motivation

When programming close to the hardware, you often need to manipulate bits and flags efficiently. Doing this with bitwise operators and macros can get messy:

#define FLAG1 0x01
#define FLAG2 0x02
 
unsigned int flags = FLAG1 | FLAG2;
 
if (flags & FLAG1) {
    // do something for FLAG1
}

Bit fields provide a cleaner way to work with bits and flags in C structures.

Syntax

type [declarator] : width;

• The type for the declarator must be unsigned int, signed int, or int.
• The width must be less than or equal to the size of the underlying type.

Improved readability

With bit fields, the code reads more like accessing normal struct members:

struct {
    unsigned int FLAG1 : 1;
    unsigned int FLAG2 : 1;
} flags;
 
flags.FLAG1 = 1;
 
if (flags.FLAG1) {
    // do something for FLAG1
}

And it is obviously less error-prone.

Memory optimization

Bit fields also enable packing data efficiently into the smallest space needed. For example, if you write:

struct {
    unsigned int isKeyword;
    unsigned int isExtern;
    unsigned int isStatic;
} flags;

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

So if you write:

struct {
    unsigned int isKeyword : 1;
    unsigned int isExtern : 1;
    unsigned int isStatic : 1;
} flags;

This uses up the same space as one unsigned int, which is 4 bytes.

Hardware registers

Besides, bit fields are commonly used to represent hardware registers. If a 32-bit system register has a meaning for each bit, you can define it like:

struct {
    unsigned int power : 1;
    unsigned int clock : 1;
    //...
} systemRegisters;

This maps cleanly to the hardware while abstracting away the nitty-gritty of bit shifts.

Note

Bit fields can also be used to store values larger than one bit, although flags are more common.

struct {
    unsigned short icon : 8;
    unsigned short color : 4;
    unsigned short underline : 1;
    unsigned short blink : 1;
} screen[25][80]; // The size of each structure is 2 bytes.

Learn more

• C Bit Fields | Microsoft Learn