Structure Declaration

Basic Concepts

Structures group values of different types under a single name. Each value is called a member variable.

Declaration Syntax

struct tag {
    member_list;
} variable_list;

Example: A student structure with name, age, gender, and height:

struct Student {
    char name[20];
    int age;
    char gender[2];
    float height;
};

Anonymous Structures

Anonymous structures lack a tag and can only be used once:

struct {
    int a;
    char b;
    float c;
} x;

Self-Referential Structures

Used in linked lists, where a node points to another node:

struct Node {
    int data;
    struct Node* next;
};

Structure Variable Definition and Initialization

Variables can be defined during or after declaration:

struct Student s1 = {"Alice", 20, "F", 165.5};

Nested structures initialize inner structures with braces:

struct Point {
    int x;
    int y;
};

struct Shape {
    char type;
    struct Point center;
};

struct Shape circle = {'C', {10, 20}};

Structure Member Access

Member are accessed using . or -> for pointers:

struct Student s1 = {"Bob", 22, "M", 180.0};
struct Student* ptr = &s1;
printf("Name: %s\n", ptr->name);

Structure Memory Alignment

Memory alignment ensures efficient access. Rules:

1. First member at offset 0.
2. Subsequent members align to their size or compiler default.
3. Total size is a multiple of the largest alignment.

Example:

struct AlignTest {
    char a;
    int b;
    char c;
};
// Size: 12 bytes (due to padding)

Bit Fields

Bit fields optimize memory by specifying member sizes in bits:

struct BitField {
    unsigned int flag1 : 1;
    unsigned int flag2 : 3;
};

Enumerations

Enums define named constants:

enum Weekday {MON, TUE, WED, THU, FRI, SAT, SUN};
enum Weekday day = MON;

Unions

Unions share memory among members:

union Data {
    int i;
    float f;
    char str[20];
};

union Data data;
data.i = 10;
printf("%d\n", data.i);

Practical Applications

• Memory Alignment: Critical for performance in embedded systems.
• Bit Fields: Useful in network protocols for compact data reprseentation.
• Unions: Save memory when only one member is active at a time.

Example: Checikng Endianness

int checkEndian() {
    union {
        int i;
        char c;
    } test;
    test.i = 1;
    return test.c; // Returns 1 for little-endian
}