How do structure alignment and memory padding work in C language?

How do structure alignment and memory padding work in C language?

Memory Alignment Principles:

1. Alignment Rules

   • Structure members align to their natural boundaries
   • char: 1-byte alignment
   • short: 2-byte alignment
   • int: 4-byte alignment
   • double: 8-byte alignment (64-bit systems)

2. Structure Total Size

   • Must be a multiple of its largest member size
   • May require padding bytes at the end

Example Analysis:

c
struct Example1 {
    char c;      // 1 byte + 3 bytes padding
    int i;       // 4 bytes
};              // Total size: 8 bytes

struct Example2 {
    char c1;     // 1 byte + 1 byte padding
    short s;     // 2 bytes
    int i;       // 4 bytes
};              // Total size: 8 bytes

struct Example3 {
    char c1;     // 1 byte + 7 bytes padding
    double d;    // 8 bytes
    char c2;     // 1 byte + 7 bytes padding
};              // Total size: 24 bytes

Optimization Techniques:

1. Member Ordering Optimization

   • Arrange members in descending size order
   • Reduce padding byte waste

2. Using Compiler Directives

   c
   #pragma pack(1)  // 1-byte alignment
   struct Packed {
       char c;
       int i;
   };
   #pragma pack()   // Restore default alignment

3. Bit Field Usage

   c
   struct BitField {
       unsigned int a : 3;  // 3 bits
       unsigned int b : 5;  // 5 bits
   };

Important Notes:

• Alignment rules may vary across platforms
• Excessive packing can affect access performance
• Consider byte order and alignment for network transmission