Padding, Alignment & Packing

Structure size may be bigger than expected due to padding.

Padding exists to satisfy alignment for faster CPU access.

Example

struct A {
    char c;  // 1
    int i;   // 4
};

Expected 5 bytes but actual often 8 bytes.

Layout:

| c | pad pad pad | i i i i |

Reduce padding

Reorder fields: bigger → smaller.

struct X {
    double d;
    int i;
    char c;
};

Structure Padding Example Diagram

Explains structure padding and alignment, showing why sizeof(struct) can be larger than sum of its fields due to inserted padding bytes.

Packing

Used in:

• network headers
• binary file parsing

GCC packed attribute

struct __attribute__((packed)) P {
    char c;
    int i;
};

pragma pack

#pragma pack(1)
struct P { char c; int i; };
#pragma pack()

⚠️ Packed structs may reduce performance (unaligned access).