C Alignment Cheatsheet

struct MixedData { /* After compilation in 32-bit x86 machine */
    char Data1; // 1 byte 
    /* [1 byte padding]: for the following 'short' to be aligned on a 2 byte boundary */
    short Data2; // 2 bytes
    int Data3;  // 4 bytes - largest structure member
    char Data4; // 1 byte
    /* [3 bytes padding]: to make total size of the structure 12 bytes, which is a
       multiple of 4, the largest alignment within the struct */
};

Alignment and padding

Most important principles

• Padding is added for the next data to align with its size.
• The padding in the end makes sure that the total struct size is a multiple of the biggest alignment (size) of any member. For example, a struct has member c char c[9] and c int i, then the alignment of the whole strucy should be 4, not 9, which means, its size should be a multiple of 4, not 9, because though c has the biggest size 9, its alignment is however only 1.

The second principle ensures that, in an array of structs, every struct as well as all its members are still aligned.

┌───┬─────────┬─────────────┬───────┬──────┐┌───┬─────────┬─────────────┬───────┬──────┐┌──
│M1 │    P1   │      M2     │  M3   │  P2  ││M1 │    P1   │      M2     │  M3   │  P2  ││...
└───┴─────────┴─────────────┴───────┴──────┘└───┴─────────┴─────────────┴───────┴──────┘└──
 1B      3B          4B        2B      2B    1B      3B          4B        2B      2B   

Without the padding in the end of struct, M2 of the second struct would not be aligned at boundary of 4 byte, but 2 byte.

More detailed explanation

An array uses the same alignment as its elements, except that a local or global array variable of length at least 16 bytes or a C99 variable-length array variable always has alignment of at least 16 bytes

By default, the compiler aligns class and struct members on their size value: bool and char on 1-byte boundaries, short on 2-byte boundaries, int, long, and float on 4-byte boundaries, and long long, double, and long double on 8-byte boundaries.

The struct's first member always starts from any addresses divisible by struct's own alignment requirement which is determined by largest member's alignment requirement(here 4, alignmentof int32_t), Because the address of a struct is the same as the address of its first member. This is different from normal variables outside the struct. They can start any addresses divisible by its own alignment.

References

• Explanation from Microsoft
• Aligment of char array
• Explanation from the point of view of RAM and CPU
• Structs are created at aligned address