User-Defined Types

C provides three primary ways to create custom types: Structures, Unions, and Enumerations. These allow you to group related data into logical entities.

Structures: Grouping Heterogeneous Data

A struct is a block of memory that holds multiple variables (members) of different types.

struct Player {
    char name[32];
    int score;
    float health;
};

Accessing Members: . vs ->

• use the Dot Operator (.) for direct instances.
• Use the Arrow Operator (->) for pointers to instances. ptr->x is shorthand for (*ptr).x.

Memory Alignment and Padding

This is a critical systems concept. A struct’s size is not always the sum of its parts. Compilers insert “padding” bytes to ensure members are aligned with the CPU’s word boundaries for faster access.

struct Mixed {
    char c;   // 1 byte
    // 3 bytes of padding inserted here!
    int i;    // 4 bytes
};
// sizeof(struct Mixed) is likely 8, not 5.

Optimization Tip: Reorder struct members from largest to smallest to minimize padding.

Unions: Shared Memory

A union is a special type where all members share the same starting memory address. The size of the union is the size of its largest member.

Use unions for:

1. Type Punning: Interpreting the same bits in different ways.
2. Mutually Exclusive Data: When an object can be “A” or “B”, but never both at once.

union Data {
    int i;
    float f;
} u;
u.i = 42; 
// Now u.f also contains the bit pattern of 42 interpreted as a float.

Bit-fields: Power at the Bit Level

In embedded programming, we often need to packet data into specific bits to save space or match hardware registers.

struct Flags {
    unsigned int is_active : 1; // 1 bit
    unsigned int error_code : 3; // 3 bits
    unsigned int reserved  : 4; // 4 bits
};
// Total size: 1 byte (plus possible padding to int size)

union Example {
    char a;
    int b;
};
// If sizeof(char) is 1 and sizeof(int) is 4,
// what is sizeof(union Example)?
// Answer: 

Enumerations (enum)

Enums provide a way to define integer constants with human-readable names, improving code clarity.

enum Status { IDLE, RUNNING, ERROR = -1 };
enum Status current = IDLE; // 'current' is effectively 0