Bit Fields in Structures

01What are Bit Fields?

Bit fields allow you to define structure members that occupy a specific number of bits. This is useful for packing multiple small values into a single word of memory.

Why Use Bit Fields?

Memory Efficiency

Store multiple flags in a single byte instead of using separate integers

Hardware Mapping

Match hardware register layouts in embedded systems

Protocol Headers

Define network packet structures with specific bit widths

Clean Code

Access individual bits using struct member syntax instead of bitwise operations

02Bit Field Syntax

bit_field_syntax.c

1struct {
2    type member_name : width;
3};
4
5// Example: Status flags
6struct Status {
7    unsigned int is_active : 1;   // 1 bit (0 or 1)
8    unsigned int priority  : 3;   // 3 bits (0-7)
9    unsigned int mode      : 4;   // 4 bits (0-15)
10};  // Total: 8 bits = 1 byte

Memory Layout Visualization

3 bits

4 bits

is_activeprioritymode

03Practical Examples

Example 1: File Permissions

file_permissions.c

1#include <stdio.h>
2
3struct FilePermissions {
4    unsigned int read    : 1;
5    unsigned int write   : 1;
6    unsigned int execute : 1;
7    unsigned int hidden  : 1;
8    unsigned int system  : 1;
9    unsigned int archive : 1;
10};
11
12int main() {
13    struct FilePermissions perm;
14    
15    // Set permissions
16    perm.read = 1;
17    perm.write = 1;
18    perm.execute = 0;
19    perm.hidden = 0;
20    perm.system = 0;
21    perm.archive = 1;
22    
23    printf("File permissions:\n");
24    printf("Read: %d\n", perm.read);
25    printf("Write: %d\n", perm.write);
26    printf("Execute: %d\n", perm.execute);
27    printf("Size of struct: %zu bytes\n", sizeof(perm));
28    
29    return 0;
30}

Expected Output:

File permissions:
Read: 1
Write: 1
Execute: 0
Size of struct: 4 bytes

Example 2: Date Structure

packed_date.c

1#include <stdio.h>
2
3// Pack a date into just 2 bytes (16 bits)
4struct PackedDate {
5    unsigned int day   : 5;   // 1-31 (needs 5 bits)
6    unsigned int month : 4;   // 1-12 (needs 4 bits)
7    unsigned int year  : 7;   // 0-127 (offset from 2000)
8};
9
10int main() {
11    struct PackedDate today;
12    today.day = 27;
13    today.month = 1;
14    today.year = 26;  // 2026
15    
16    printf("Date: %02d/%02d/%d\n", 
17           today.day, today.month, 2000 + today.year);
18    printf("Size: %zu bytes\n", sizeof(today));
19    
20    // Compare to unpacked version
21    struct UnpackedDate {
22        int day;
23        int month;
24        int year;
25    };
26    printf("Unpacked size: %zu bytes\n", sizeof(struct UnpackedDate));
27    
28    return 0;
29}

Expected Output:

Date: 27/01/2026
Size: 2 bytes
Unpacked size: 12 bytes

Example 3: Hardware Register (Embedded)

gpio_register.c

1// Typical GPIO configuration register
2struct GPIO_Config {
3    unsigned int pin_mode     : 2;  // 00=input, 01=output, 10=alternate, 11=analog
4    unsigned int output_type  : 1;  // 0=push-pull, 1=open-drain
5    unsigned int output_speed : 2;  // 00=low, 01=medium, 10=high, 11=very high
6    unsigned int pull_up_down : 2;  // 00=none, 01=pull-up, 10=pull-down
7    unsigned int reserved     : 1;  // Unused bit
8};
9
10// Usage in embedded systems
11void configure_gpio_as_output(volatile struct GPIO_Config* gpio) {
12    gpio->pin_mode = 0b01;      // Output
13    gpio->output_type = 0;       // Push-pull
14    gpio->output_speed = 0b11;   // Very high speed
15    gpio->pull_up_down = 0b00;   // No pull-up/down
16}

04Important Rules and Limitations

1. Cannot take address of bit fields

main.c

1struct S { unsigned int flag : 1; };
2struct S s;
3// int* p = &s.flag;  // ERROR! Cannot take address

2. Allowed types are implementation-defined

C standard requires int, unsigned int, and _Bool. Other types like charmay or may not work depending on compiler.

3. Bit order is implementation-defined

Whether bits are allocated from LSB to MSB or vice versa depends on the compiler and architecture. Don't assume a specific layout for portability.

4. Unnamed bit fields create padding

main.c

1struct Example {
2    unsigned int a : 5;
3    unsigned int   : 3;  // 3 bits of padding (unnamed)
4    unsigned int b : 8;
5    unsigned int   : 0;  // Force alignment to next unit boundary
6    unsigned int c : 4;
7};

Portability Warning

Bit field layout is not portable across different compilers or architectures. If you need exact binary layouts (like network protocols), use explicit bitwise operations instead.

05Bit Fields vs Bitmasks

Bit Fields

main.c

1struct Flags {
2    unsigned int a : 1;
3    unsigned int b : 1;
4    unsigned int c : 1;
5};
6
7struct Flags f;
8f.a = 1;
9f.b = 0;
10if (f.c) { ... }

Cleaner syntax, but not portable

Bitmasks

main.c

1#define FLAG_A (1 << 0)
2#define FLAG_B (1 << 1)
3#define FLAG_C (1 << 2)
4
5unsigned int flags = 0;
6flags |= FLAG_A;   // Set
7flags &= ~FLAG_B;  // Clear
8if (flags & FLAG_C) { ... }

More verbose, but fully portable

06Summary

* Bit fields pack multiple values into fewer bytes
* Syntax: unsigned int name : bits;
* Cannot take the address of a bit field
* Layout is not portable - use bitmasks for protocols
* Great for memory-constrained embedded systems

01What are Bit Fields?

Bit fields allow you to define structure members that occupy a specific number of bits. This is useful for packing multiple small values into a single word of memory.

Why Use Bit Fields?

Memory Efficiency

Store multiple flags in a single byte instead of using separate integers

Hardware Mapping

Match hardware register layouts in embedded systems

Protocol Headers

Define network packet structures with specific bit widths

Clean Code

Access individual bits using struct member syntax instead of bitwise operations

02Bit Field Syntax

bit_field_syntax.c

1struct {
2    type member_name : width;
3};
4
5// Example: Status flags
6struct Status {
7    unsigned int is_active : 1;   // 1 bit (0 or 1)
8    unsigned int priority  : 3;   // 3 bits (0-7)
9    unsigned int mode      : 4;   // 4 bits (0-15)
10};  // Total: 8 bits = 1 byte

Memory Layout Visualization

3 bits

4 bits

is_activeprioritymode

03Practical Examples

Example 1: File Permissions

file_permissions.c

1#include <stdio.h>
2
3struct FilePermissions {
4    unsigned int read    : 1;
5    unsigned int write   : 1;
6    unsigned int execute : 1;
7    unsigned int hidden  : 1;
8    unsigned int system  : 1;
9    unsigned int archive : 1;
10};
11
12int main() {
13    struct FilePermissions perm;
14    
15    // Set permissions
16    perm.read = 1;
17    perm.write = 1;
18    perm.execute = 0;
19    perm.hidden = 0;
20    perm.system = 0;
21    perm.archive = 1;
22    
23    printf("File permissions:\n");
24    printf("Read: %d\n", perm.read);
25    printf("Write: %d\n", perm.write);
26    printf("Execute: %d\n", perm.execute);
27    printf("Size of struct: %zu bytes\n", sizeof(perm));
28    
29    return 0;
30}

Expected Output:

File permissions:
Read: 1
Write: 1
Execute: 0
Size of struct: 4 bytes

Example 2: Date Structure

packed_date.c

1#include <stdio.h>
2
3// Pack a date into just 2 bytes (16 bits)
4struct PackedDate {
5    unsigned int day   : 5;   // 1-31 (needs 5 bits)
6    unsigned int month : 4;   // 1-12 (needs 4 bits)
7    unsigned int year  : 7;   // 0-127 (offset from 2000)
8};
9
10int main() {
11    struct PackedDate today;
12    today.day = 27;
13    today.month = 1;
14    today.year = 26;  // 2026
15    
16    printf("Date: %02d/%02d/%d\n", 
17           today.day, today.month, 2000 + today.year);
18    printf("Size: %zu bytes\n", sizeof(today));
19    
20    // Compare to unpacked version
21    struct UnpackedDate {
22        int day;
23        int month;
24        int year;
25    };
26    printf("Unpacked size: %zu bytes\n", sizeof(struct UnpackedDate));
27    
28    return 0;
29}

Expected Output:

Date: 27/01/2026
Size: 2 bytes
Unpacked size: 12 bytes

Example 3: Hardware Register (Embedded)

gpio_register.c

1// Typical GPIO configuration register
2struct GPIO_Config {
3    unsigned int pin_mode     : 2;  // 00=input, 01=output, 10=alternate, 11=analog
4    unsigned int output_type  : 1;  // 0=push-pull, 1=open-drain
5    unsigned int output_speed : 2;  // 00=low, 01=medium, 10=high, 11=very high
6    unsigned int pull_up_down : 2;  // 00=none, 01=pull-up, 10=pull-down
7    unsigned int reserved     : 1;  // Unused bit
8};
9
10// Usage in embedded systems
11void configure_gpio_as_output(volatile struct GPIO_Config* gpio) {
12    gpio->pin_mode = 0b01;      // Output
13    gpio->output_type = 0;       // Push-pull
14    gpio->output_speed = 0b11;   // Very high speed
15    gpio->pull_up_down = 0b00;   // No pull-up/down
16}

04Important Rules and Limitations

1. Cannot take address of bit fields

main.c

1struct S { unsigned int flag : 1; };
2struct S s;
3// int* p = &s.flag;  // ERROR! Cannot take address

2. Allowed types are implementation-defined

C standard requires int, unsigned int, and _Bool. Other types like charmay or may not work depending on compiler.

3. Bit order is implementation-defined

Whether bits are allocated from LSB to MSB or vice versa depends on the compiler and architecture. Don't assume a specific layout for portability.

4. Unnamed bit fields create padding

main.c

1struct Example {
2    unsigned int a : 5;
3    unsigned int   : 3;  // 3 bits of padding (unnamed)
4    unsigned int b : 8;
5    unsigned int   : 0;  // Force alignment to next unit boundary
6    unsigned int c : 4;
7};

Portability Warning

Bit field layout is not portable across different compilers or architectures. If you need exact binary layouts (like network protocols), use explicit bitwise operations instead.

05Bit Fields vs Bitmasks

Bit Fields

main.c

1struct Flags {
2    unsigned int a : 1;
3    unsigned int b : 1;
4    unsigned int c : 1;
5};
6
7struct Flags f;
8f.a = 1;
9f.b = 0;
10if (f.c) { ... }

Cleaner syntax, but not portable

Bitmasks

main.c

1#define FLAG_A (1 << 0)
2#define FLAG_B (1 << 1)
3#define FLAG_C (1 << 2)
4
5unsigned int flags = 0;
6flags |= FLAG_A;   // Set
7flags &= ~FLAG_B;  // Clear
8if (flags & FLAG_C) { ... }

More verbose, but fully portable

06Summary

* Bit fields pack multiple values into fewer bytes
* Syntax: unsigned int name : bits;
* Cannot take the address of a bit field
* Layout is not portable - use bitmasks for protocols
* Great for memory-constrained embedded systems

What You Will Learn

01What are Bit Fields?

Why Use Bit Fields?

02Bit Field Syntax

Memory Layout Visualization

03Practical Examples

Example 1: File Permissions

Expected Output:

Example 2: Date Structure

Expected Output:

Example 3: Hardware Register (Embedded)

04Important Rules and Limitations

1. Cannot take address of bit fields

2. Allowed types are implementation-defined

3. Bit order is implementation-defined

4. Unnamed bit fields create padding

05Bit Fields vs Bitmasks

Bit Fields

Bitmasks

06Summary

Test Your Knowledge

Related Tutorials

Bitmasks and Bit Manipulation

const and volatile Keywords

Structures in C

Have Feedback?

What You Will Learn

01What are Bit Fields?

Why Use Bit Fields?

02Bit Field Syntax

Memory Layout Visualization

03Practical Examples

Example 1: File Permissions

Expected Output:

Example 2: Date Structure

Expected Output:

Example 3: Hardware Register (Embedded)

04Important Rules and Limitations

1. Cannot take address of bit fields

2. Allowed types are implementation-defined

3. Bit order is implementation-defined

4. Unnamed bit fields create padding

05Bit Fields vs Bitmasks

Bit Fields

Bitmasks

06Summary

Test Your Knowledge

Related Tutorials

Bitmasks and Bit Manipulation

const and volatile Keywords

Structures in C

Have Feedback?