# C bit fields

**C bit fields** pack individual bits into struct members, saving memory when many boolean or small-range fields are needed. Declare with `unsigned type : nbits` to allocate exactly N bits for the field.

Use bit fields for hardware registers, flags, or tight memory constraints.

## Example

```c
// compile: gcc -o bitfields bitfields.c
// run: ./bitfields
// description: packing bits into struct

#include <stdio.h>

struct Flags {
    unsigned enabled : 1;
    unsigned mode : 2;
    unsigned priority : 3;
    unsigned reserved : 2;
};

struct Register {
    unsigned int addr : 16;
    unsigned int flags : 8;
    unsigned int status : 8;
};

int main() {
    struct Flags f;
    f.enabled = 1;
    f.mode = 2;
    f.priority = 5;
    
    printf("Flags size: %zu bytes\n", sizeof(f));
    printf("Register size: %zu bytes\n", sizeof(struct Register));
    
    return 0;
}
```

## Common patterns

**Bit field declarations**:
- `unsigned type: nbits`: N-bit field
- Total bits in struct packed into minimal bytes
- Padding added as needed

**Portability issues**:
- Bit ordering, signedness, padding are implementation-defined
- Avoid bit fields for cross-platform data
- Use for local optimization only

**Limitations**:
- Can't take address of bit field (no `&`)
- Can't use bit fields in arrays
- Alignment and packing unpredictable

**Best uses**:
- Hardware register mapping
- Local flags struct
- Memory-constrained embedded systems