# C memory alignment

**C memory alignment** refers to the byte offset where data types must start. Structs have padding to align fields, and `_Alignof` queries alignment requirements. Proper alignment improves performance (cache lines, SIMD) and is required by hardware.

Use alignment awareness to understand struct layout, optimize for cache, and meet hardware requirements.

## Example

```c
// compile: gcc -std=c11 -o alignment alignment.c
// run: ./alignment
// description: struct padding and alignment

#include <stdio.h>
#include <stdalign.h>

struct Packed {
    char a;
    int b;
    char c;
};

struct Optimized {
    int b;
    char a;
    char c;
};

int main() {
    printf("Packed layout:\n");
    printf("  size: %zu\n", sizeof(struct Packed));
    printf("  alignof: %zu\n", alignof(struct Packed));
    printf("  a offset: %zu\n", offsetof(struct Packed, a));
    printf("  b offset: %zu\n", offsetof(struct Packed, b));
    printf("  c offset: %zu\n", offsetof(struct Packed, c));
    
    printf("Optimized layout:\n");
    printf("  size: %zu\n", sizeof(struct Optimized));
    printf("  alignof: %zu\n", alignof(struct Optimized));
    
    return 0;
}
```

## Common patterns

**Alignment requirements**:
- Char: 1 byte
- Short: 2 bytes
- Int: 4 bytes
- Long: 8 bytes (on 64-bit)
- Struct: alignment of largest member

**Padding**:
- Compiler inserts padding between members for alignment
- Wasted space but improves performance
- Reorder fields to minimize padding

**Cache efficiency**:
- Keep related data together
- Minimize struct size with smart ordering
- False sharing: unrelated data on same cache line

**Hardware alignment**:
- SIMD operations require specific alignment
- Atomic operations may require alignment
- Hardware may fault on misaligned access

**`_Alignof` and `alignas`**:
- `_Alignof(type)`: required alignment
- `_Alignas(type)`: enforce alignment
- C11 feature, not in C99