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.

// 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;
}

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